Incremental Construction as a Circular Economy Instrument in the Production of Cooperative Housing

Abstract

The Circular Economy (CE) proposal aims to minimise waste and maximise the use of resources, making it crucial for providing social housing (SH). Incorporating CE principles into SH policies makes it possible to take significant steps towards a more sustainable and equitable future. The cooperative housing movement offers a solution for growing and empowering local communities to actively develop, own, and manage housing. In some cooperative housing projects, there is the opportunity to use the incremental construction process, which offers the flexibility to adapt construction to residents’ changing circumstances and needs over time. This study aims to verify how circular economy principles applied to incremental construction in case studies of cooperative housing can contribute to more sustainable solutions in the SH sector. By conducting a systematic literature review (SLR) of scientific databases, case studies of cooperative housings that intentionally or unintentionally incorporate the incremental construction tool into their foundations were selected. The analysis and discussions in the study highlight real possibilities for incorporating CE principles into cooperative housing models by appropriating the incremental construction tool.

1. Introduction

The issue of Social Housing (SH) is a pressing concern facing urban areas around the world. Unfortunately, SH policies have not effectively addressed the challenge of the right to decent housing. Added to this context is the 2008 economic crisis, which significantly impacted housing policies, leading to a global housing crisis and the need to reorient research and housing policies. The problem is compounded by informal settlements, home to around 1.6 billion people worldwide [1]. This has resulted in declining quality of life and severe environmental degradation.

Based on [2,3], this study defines social housing as affordable housing for low-income people, families or people with difficulty finding their housing. Furthermore, social housing is managed by the government or non-profit organisations for rent or access to the property, based on values defined by rules that govern the projects. A SH urban project aims to use city spaces for promoting social and environmental sustainability; however, this faces challenges such as improving social inclusion, ensuring access to healthy housing conditions, enhancing the economy, and using natural resources optimally.

Recent efforts to integrate social housing within existing urban fabric have focused on various strategies to prevent residential segregation. These include locating social housing in central urban areas [4], repurposing abandoned buildings in historic centres [5], and implementing urban regeneration projects [6]. However, challenges persist, such as conflicting policy goals between accessibility and segregation prevention [7] and the potential for privatisation to lead to spatial fragmentation [8]. Despite these efforts, research shows that segregation remains complex and multidimensional, influenced by factors such as income, education, and migration background [9].

The circular economy concept involves designing more durable, reusable, repairable, and recyclable products to keep materials and products in circulation for as long as possible. It minimises waste and promotes sustainable use of natural resources [10,11,12].

The concept of a circular economy (CE) is gathering pace in the construction and real estate sectors as a way to address sustainability challenges and contribute to the UN Sustainable Development Goals (SDGs) [13,14].

Implementing CE practices can support multiple SDGs, mainly focusing on SDG 11, which emphasises sustainable cities and communities, and SDG 12, which focuses on responsible consumption and production [15,16]. Modular construction and innovative business models are identified as potential tools for transitioning to a circular built environment [17]. Integrating CE principles into construction practices requires systemic innovation across the value chain, including developing new technologies and management systems for construction and demolition [18].

The CE concept is increasingly gaining traction in academic and practical domains [19,20,21]; however, its implementation presents notable challenges, including consumer behaviour, financing, and skill requirements [22,23].

In the context of social housing provision, this study adheres to the following concept of a Circular Economy as defined by [12]: “an economic system that replaces the concept of “end of life” with the reduction, alternatively, by reuse, recycling and recovery of materials in production/distribution and consumption processes.”

In this regard, the challenges in progressing towards a more sustainable and equitable SHP run to the principles on which the CE is founded. Gomide et al. (2024) [24] contributes to the definition of five CE principles that can support more sustainable solutions in the SH sector. This study is based on these five principles related to CE and PCH:

• Retrofit and sustainable upgrades: increase efficiency and renewal of resources considering factors such as material quality, waste reduction, and environmental impact.
• Balancing needs: equalise tenants’ needs, preferences, and budget constraints in the modernisation process within sustainability practices.
• Policy integration: ensure national and local integration for broader sustainability objectives.
• Environmental justice and equity: promote economic development, equity, and public health, especially in disadvantaged communities, without generating environmental impact.
• Economic and social benefits: generate circular business models (material design, sharing, recycling, and reuse) to create jobs, promote equity, and reduce environmental impact.

The circular economy (CE) in social housing offers potential economic and social benefits, but its implementation faces challenges. CE principles can address social housing issues through refurbishment and targeted policies. However, Dutch social housing organisations are in the early stages of CE adoption, hindered by organisational, cultural, and financial barriers [25]. Circular demolition in social housing raises concerns about social acceptability and equity [26]. Social aspects of CE, such as labour practices and community well-being, require better integration [27]. Combining technological and social innovations is crucial for implementing CE in urban housing [28].

Research has recently explored the potential of CE principles in the built environment. However, the SH sector requires further investigation as it presents significant opportunities for establishing circular resource flows in the built environment [25].

Social housing solution models vary from country to country, changing and adapting over time. However, generally, it involves the construction of housing units subsidised by the government and intended for sale or rental. Furthermore, some programmes subsidise products such as rental assistance and construction materials. The potential of a hybrid economic model for public–private financing of social housing in developing countries has been the subject of debate and analysis [29]. This model’s fundamental challenge is establishing adequate supervision and regulation mechanisms to guarantee transparency and accountability.

The cooperative housing movement presents itself as a solution for growing and empowering local communities to actively develop, own, and manage housing. Cooperative housing is a distinctive form of residential living in which the property is collectively owned by its residents through a cooperative corporation. Rather than owning individual units outright, residents buy shares in the cooperative, allowing them to occupy a specific unit and participate in property management [30]. This approach helps create resilient, accessible communities and ensures long-term benefits. Furthermore, it operates under principles of self-help, self-responsibility, democracy, equality, equity, and solidarity [31].

Cooperative housing is an attractive model of social housing provision due to its potential to enrich individual lifestyles, local communities, and housing organisations [32]. The success of cooperative housing is attributed to its non-profit nature, private initiatives, and targeting of the middle class [33]. It also plays a crucial role in social cohesion and resident participation in urban development [34]. The La Borda cooperative housing in Barcelona is a notable case study emphasising public participation [35]. The successful model of Uruguayan cooperative housing stands out in Latin America, which involves self-management and collective ownership [36].

Incremental construction is a housing strategy that allows low-income families to gradually build and improve their homes over time based on their needs and financial capabilities [37]. These incremental housing projects, such as those in Chile, provide an essential structure for residents to expand and customise [38,39]. The process involves multiple phases, including design, construction, and customisation, often requiring collaboration between government officials, architects, and residents [40]. While incremental housing can address housing shortages, it is embedded in broader urban systems and flows, including land, finance, infrastructure, building materials, and labour [41]. Despite its potential benefits, challenges remain in urban design, climate responsiveness, and preventing over-development [42].

Recent studies [43,44] have suggested social strategies to tackle the challenges of the housing crisis. Additionally, authors [25,45,46] have emphasised the significance of integrating circular economy principles and social innovations into public social housing policies and sustainable research.

In order to face the challenge of SH-related issues, it is essential to involve user participation throughout the decision-making process. This study asks “how can the incremental construction process in cooperative housings incorporate Circular Economy principles?” Therefore, this study aims to verify which circular economy principles in the field of social housing applied to incremental construction in case studies of cooperative housing can contribute to more sustainable solutions in the SH sector.

The authors have outlined five principles of circular economy (CE) directly related to social housing policies. These principles suggest specific actions for each group involved. This study aims to explore how incremental construction—used as a tool for producing cooperative housing to address social housing needs—reflects these CE principles. The definition of the five CE principles in the field of social housing provides an analysis of a more contextualised and achievable reality.

The analysis conducted using the SLR and detailed in a Results Matrix examined documents featuring cases of (i) cooperative housing utilising (ii) incremental construction methods and incorporating (iii) techniques aligned with the principles of circular economy for social housing policies as defined by the authors. This process identified six cases meeting all the specified criteria. Notably, the six cases analysed in this study include some actions incorporating one or more of the five CE principles regarding SHPs.

In order to face the challenges of SH based on reality, it is concluded that the essence of this research is to look for practical solutions and not limit itself to theoretical solutions.

2. State of Art

Social Housing Policies (SHPs) are designed to address the housing needs of low-income demographic groups by implementing government intervention, subsidies, and partnerships with private developers to create and sustain affordable housing units [47].

These policies play a crucial role in promoting social inclusion through various components, including legislation, regulation, programme implementation, resource allocation, fiscal policies, and investments in social housing (SH) [48]. These efforts are aimed at ensuring the availability of suitable and long-term social housing for those in need.

Within this universe, collaboration strategies between local governments and higher authorities are crucial to meeting the housing needs of communities. Local governments are crucial in providing financing, regulating land use, and connecting with other partners to increase the supply of social housing [49].

Nations have traditionally restricted investment in subsidised social housing, and disadvantaged groups have faced financial constraints in accessing market housing, contributing to the crisis in the SH sector. As states have political obligations to improve access to housing for all, particular European Union (EU) countries and developing nations have sought to provide alternative housing options through private investors, who receive financial support from government entities [50,51].

Currently 1.6 billion people, corresponding to 20% of the world’s population, live in inadequate, overcrowded, and unsafe housing [1], highlighting the urgent need for sustainable housing solutions. Most of this population is located in developing countries, with estimates of 30–70% of the urban population [52]. Over half of this population lives in East, Southeast, Central, and Southern Asia, and 23% live in Sub-Saharan Africa [1].

In Latin America, almost a quarter of the urban population lives informally, with some countries, such as Bolivia, having informal housing rates above 40% [53].

Notably, France has 2 million pending applications for social housing, while in England, the waiting lists for social housing now encompass 1.1 million households. In Italy, the proportion of renter households with rent arrears in the private rental sector has surged from under 10% to 24% within a year from the onset of the pandemic [54].

The availability of affordable housing in Europe decreased since the 1990s, leading to a growing concern. This reduction is a more significant trend of decreased state intervention in various social areas, particularly in developed economies [55]. Many developed countries’ governments have implemented austerity measures in response to economic crises, resulting in significant cuts in public spending, including funds allocated for social housing. Consequently, there has been a decrease in the construction of new social housing units and reduced housing subsidies for low-income families, leading to increasing housing insecurity [56,57]. Privatisation of the real estate market has further exacerbated the situation by shifting responsibility from the State to the private sector [56]. This trend has substantial implications for social equality and access to affordable housing, underscoring the urgency of addressing this issue. The issue is expected to worsen across Europe, particularly given the adverse impacts of the COVID-19 pandemic [54].

2.1. Cooperative Housing Experiences

Within this context of such distinct and complex conditions, alternative solutions for providing dignified housing that are ecologically, socially, economically, and culturally sustainable require space. For this space to be made viable, it must be legally backed and supported by the government, aiming at productivity, quality, and new forms of ownership and financing.

Cooperative housing can be an exciting solution in social housing policies along this path for several reasons; they are adaptable and flexible models that can work in various circumstances. Different cooperatives allow for customisation based on members’ needs, preferences, and local context [30]. Also, this solution can serve the interests of groups with different economic profiles. The great attraction of this housing solution model is the possibility of obtaining housing at a cost price.

The cooperative housing model has a long history, dating back to the 1920s in the United States, and has been a fundamental part of housing-delivery systems in many parts of the world. Cooperative housing emerged in the early 19th century, inspired by pre-Marxian communitarian socialist ideas of Charles Fourier and Robert Owen [58]. It gained traction in various countries, with notable examples in Great Britain in the 1860s and Sweden and Norway post-1945 [59]. The movement evolved from self-help, democratic, and non-profit organisations to market-oriented ones. Cooperative housing has faced challenges in scaling up, often remaining a niche solution [60]. However, successful implementations exist, such as in Barcelona’s La Borda project [35] and Australia’s growing sector [61]. The development of cooperative housing has been influenced by policy mechanisms, historical contexts, and social movements [62]. Recent projects in Barcelona demonstrate how cooperatives challenge existing design standards, regulations, and cultural norms of ownership and management [63]. Despite the challenges, cooperative housing remains a promising model for affordable housing, with potential for widespread development. The role of the State in enabling the growth of cooperative housing is crucial, as seen in Sweden, Chile, and India [60].

Cooperative housing is a broad term encompassing various housing models where residents collectively own and manage the property. There are several models of cooperative housing with different objectives. Ganapati (2014) [64] classifies these as cooperatives in ownership, construction, and financing. While not mutually exclusive, tenure cooperatives serve collective ownership and management, construction cooperatives focus on land development and housing construction, and financial cooperatives lend money to members for housing purposes. Cooperative housing schemes worldwide take three primary forms—rental, limited capital, and market value. They manage various types of buildings, from high-rise to single-family homes, located in both urban and rural areas [65].

Regarding popular cooperative housing, self-management goes beyond self-construction, which can happen dynamically using incremental construction. These definitions must be discussed and agreed upon with the members’ profiles, and the ownership model, financing, maintenance, purchase, and sale are essential in contracts of this type of solution.

The cooperative housing model plays a crucial role in achieving the overall goal of providing adequate shelter. This is possible by addressing three functions in this model: families can collectively pool their resources to acquire and develop land and housing; access to financial resources is facilitated; and fundamental construction costs are reduced [66]. According to [66], this model serves as an essential means of providing housing for low- and medium-income groups, in which residents of urban areas with medium and low incomes come together; they form cooperatives, pooling their resources and labour with banks and local government to provide land and housing financing.

Regarding Africa’s experience, cooperative housing members in Kenya are responsible for saving 30% of housing development, including land costs. Then, the remaining 70% is covered by the bank’s long-term minimum-interest loan [67]. Solid and experienced institutions in Kenya believe in various housing types in the country’s cooperative housing model. Homes range from initially small, incremental single-story homes to apartments, which makes the programme an incremental project defined by the users’ necessities and possibilities [68]. In Zimbabwe, cooperative housing is a crucial solution to urban housing challenges. With a significant housing backlog and the government’s social housing programme, cooperative housing provides a practical approach to tackling this issue by organising low-income populations into cooperatives and providing land with services, ensuring safe and affordable shelter for all. Zimbabwe’s ambitious Vision 2030 prioritises housing delivery in cities and settlements, and cooperative housing can contribute to achieving this goal. In line with the United Nations Sustainable Development Goal (SDG) 11, Zimbabwe aims to create inclusive, safe, resilient, and sustainable cities and human settlements [66,69].

Cooperative housing is an integral part of the housing market in many European countries. For instance, cooperative housing manages about 9.4% of the total housing stock in the Czech Republic and Sweden, and 14% in Norway [70].

With experience in this model, Spain has entered a new phase in producing cooperative housing. What distinguishes the current phase from the 20th-century phase in Spain is that property remains collectively owned and is self-managed due to a fundamental change in its social aims and objectives [63]. Spain brings a recent case of a cooperative housing called La Borda in Barcelona, in which the use concession model is based on collective ownership to separate ownership from use so that these common assets are protected from the processes of commodification [71]. La Borba’s case raises an interesting aspect related to the renewal of housing stock. The government provided the land and building, intending to make the inhabitants’ cooperative responsible for renovating it to improve its energy efficiency and quality. Social activism and greater community participation in urban governance and housing policy have been relevant drivers of this housing solution.

The Norwegian government is currently addressing the issue of housing accessibility, particularly for young individuals with relatively low incomes. Various factors, including the criteria for mortgage lending in Norway in recent years, have influenced this challenge. As a potential resolution, the government is endorsing a “rent to buy” model, which is being implemented through the cooperative housing sector [70].

The history of cooperative housing in Uruguay is extensive and sets a standard for other countries in South America. In Uruguay, financing is unique, meaning that the cooperative commits to single financing on the housing project rather than individual mortgages for each unit or member. This means that cooperative members only sometimes have ownership of the housing they live in but, rather, they have the so-called right to use the space (private and common). Ownership of the units is exclusive to the cooperative. Thus, this process in Uruguay has avoided sales speculation if a partner leaves; it promotes solidarity and collaboration within the cooperative, as all members share financial responsibility and project management [72,73].

Understanding that this model is in tune with the Uruguayan context, the contemporary cooperative housing model in the country has evolved towards sustainability in the sector in the last decade. Despite the tireless fight for land accessible to the model, Santos (2020) [74] outlines the new premises of this model, as follows: accessibility and flexibility of projects; use of prefabricated elements in constructions and vertical construction systems; incorporation of contemporary life into the model; thermal comfort; and rehabilitation of degraded areas.

The experience of Uruguayan cooperative housing has had a significant influence on the southern states of Brazil. Although many countries have already adopted this practice, Brazil still needs to work on prejudice, fear that they will compete with construction companies, and high bureaucratisation. In this model, there is a path to follow in the country in which the protagonist must be those who will live there and not the government, the technicians, or the coordinators of the movement [75].

However, the country is about to approve a bill that creates the National Self-Managed Housing Program and defines guidelines for housing associations and cooperatives to encourage these practices. This includes several measures, such as changing existing laws to facilitate the registration of properties in the name of members and the organising entity, and promoting collective ownership [76].

2.2. Incremental Construction

In some cooperative housing projects, there is the opportunity to use the self-construction process, which involves the cooperative committee already at the design stage. They continue to make decisions about the incremental construction process in the following phases for execution and maintenance—both homes and shared living spaces [77].

Self-construction in the expansion of housing delivered within a country’s social housing programmes happens, and it is scientifically known that self-construction makes housing viable for many low-income people. Of the 50 million Brazilians who have conducted renovation or construction work, 82% chose to refrain from engaging the services of certified technical professionals, such as architects or engineers. Consequently, these projects are deemed irregular, and more design and execution registrations are needed with competent regulatory bodies [78].

Government-supported incremental housing programmes have been established in several countries, enabling low-income households to gradually modify their dwellings through a self-construction process [40,79]. Despite these efforts, challenges persist in reconciling technical recommendations with builders’ perspectives and addressing sociocultural needs [80,81].

Marinovic [40] advocates for increased accountability among government officials and technicians involved in self-construction programmes, positing that this approach fosters greater motivation among low-income families to engage in self-building and customising their homes. Indeed, the involvement of government and technicians in self-construction programmes improves housing quality and significantly enhances the satisfaction and sense of belonging among low-income families. This collaborative approach empowers families, fosters community ties, and contributes to their well-being [82,83].

However, Artemisia [84] notes that self-construction and self-management do not guarantee the quality of the house and still result in a cost of up to 30% higher in the total value of the renovation. Nascimento (2015) [85] emphasises that this approach can lead to quality challenges without proper technical assistance and skilled labour, though it also allows families to participate actively in urban space development. Also, ref. [86] recognises that the need for more technical assistance and hiring unqualified labour is responsible for the low performance of a large portion of self-built housing.

As per [87], incremental self-construction is noticeable, especially in the Global South, where low-income households often participate in self-help housing. Ref. [36] is convinced that with guaranteed conditions such as adequate housing financing and legal frameworks, this model can aid in the social production of housing through mutual self-construction because residents prefer cooperative housing. This approach is seen as a sustainable and empowering solution to the housing problem; however, larger urban and regional systems also influence it [41], and policies can limit its implementation and conventional housing practices [83].

In addition to these authors, the experiences of countries such as Chile and Namibia exemplify how SH design guidance can be provided for future expansions that will be carried out by self-construction. Chilean architect Alejandro Aravena brought to light Incremental Architecture (IA) as a solution to SH projects [88,89]. In Namibia, architect Chima Mangaliso [90] worked with the community to develop projects primarily aimed at future expansions through self-construction. Through the SH spectrum, architecture unfolds incrementally in construction planning, within each person’s possibilities, and even into a possible social business.

Within this context, incremental construction can be defined as a gradual process by which the owners attach or improve the components of an embryo project depending on the availability of financing, time, labour, materials, and functionality. According to [91], this is a catalyst for engaging people and promoting local businesses from low-income families and communities.

Marinovic [83] uses the term “unfinished house” when he analyses that this unfinished condition of a house, which initially seems to devalue the project, generates a commitment among residents to invest in expanding and improving houses. Furthermore, incremental construction in housing offers the flexibility to adapt construction to the changing circumstances and needs of residents over time.

This approach allows individuals to gain practical skills, problem-solving skills, and a deeper understanding of the construction process. This solution can be a catalyst for engaging people (social development) and promoting local businesses (economic development) for low-income families and communities [92].

Incremental construction must be based on the incremental architecture proposal to guarantee safety and quality and meet the context’s legal parameters. Incremental architecture proposes self-management and self-construction by the residents of expansions and renovations but on embryonic house projects designed for incremental construction. Thus, co-design decisions refer to architectural and social issues [77].

This is not a new concept; it was already created in the middle of the last century. However, considering the urgency of social inclusion, the optimisation of infrastructure and urban services, and residents’ satisfaction with their homes, incremental construction is once again discussed as a possible sustainable solution to the problem of SH on the planet.

However, cooperative housing does not provide a unitary model, and its principles are adapted to each specific context of regulations, financial framework, and cultural values. Regarding the size of cooperative housing, larger organisations have their professional technical departments. In contrast, smaller cooperatives are typically organised voluntarily and supported by external interdisciplinary teams [63].

Avilla-Royo et al. [63] highlight some barriers still hindering the viability of cooperative housing projects. Financing options are limited, and the lack of government guarantees makes the process unfeasible. Furthermore, political support must be coherent at the local and district levels. More rigorous views on the interpretation of district regulatory compliance end up ruling out the possibility of projects with minimum quality, thus increasing project costs.

3. Materials and Methods

The first part of this research concerns a bibliographical review of the topic in which white and grey literature was assessed.

Subsequently, through a systematic literature review in scientific databases, this investigation sought case studies of cooperative housing that, intentionally or not, incorporate the incremental construction tool into their foundations.

Finally, seeking to answer this study’s original question, the authors extract CE principles for social housing policies that were applied in the incremental construction process in case studies of cooperative housing.

3.1. Literature Review Approach

Conducting Systematic Literature Reviews (SLRs) is imperative for gathering evidence that meets specific eligibility criteria to address research inquiries. SLRs employ explicit and systematic methods to minimise bias, establishing them as the gold standard in research methodology.

Table 1. Systematic literature review protocol for this study.

Steps	Stages	Research Aspects
1. Planning	Background to review	Problem: Solve the social housing challenges based on CE principles. Rationale: Incorporating an incremental construction process in cooperative housing can contribute to sustainable SHPs. QP: How can the incremental construction process (co-design, co-execution, and co-maintenance) in cooperative housings incorporate Circular Economy principles?
	Objectives Statement	Primary objective: Verify which Circular Economy principles, applied to incremental construction in case studies of cooperative housings, can contribute to more sustainable solutions in the SH sector.
2. Processing	Criteria for selecting studies	Context: Social housing policies, social housing, circular economy, cooperative housing interventions, mechanisms, and outcomes: strategies, theories, practical examples, concepts, principles, guidelines, and recommendations. Types of studies: Both qualitative and quantitative.
	Search strategy for identification of studies	Databases: ISI Web of Science, Scopus Timeframe: 2000 to the present time of the study Keywords: “Cooperative housing” and “housing cooperative” Language: English only Article type: Indexed journal papers, conference proceedings, books, book chapters Grey literature: Included
3. Analysis	Eligibility	Inclusion/Exclusion criteria: Open Access only Journal papers, conferences, proceedings, book chapters, editorials, abstracts. Online full-text availability or obtained by requesting full texts from authors. cooperative housing 2000 to time of study. Number of reviewers screening the articles: three.
	Quality appraisal	Reviewers assess the quality of papers, and the paper is included when approved by at least two.
4. Extraction and Reporting	Data collection	During this stage, the eligible articles are thoroughly examined and analysed. Additionally, more sources and studies may be included to ensure a comprehensive analysis. The data extraction corresponds to themes, environmental aims, proposals, and countries.
	Results synthesis	Type of synthesis: Interpreting results from bibliographic analysis.

presents a protocol that synthesises the systematic literature review process developed in this study.

In addition to mapping and synthesising existing literature to advance knowledge in a specific field, SLRs can also identify gaps and stimulate new research directions. With these qualifications, SLRs enable researchers to obtain reliable results that can be invested in policy-making and decision-making processes.

While this study acknowledges its limitations, such as reliance on specific databases and exclusion criteria, the authors are confident that it provides comprehensive coverage of the literature.

This study utilised the SLR to generate initial findings, serving as a foundation for a more comprehensive analysis later. The authors applied the five principles of the CE to assess social housing policies. Each document obtained from the SLR search underwent thorough analysis, resulting in the development of finalised documents. The authors meticulously examined these documents to denote actions aligned with the five principles of the circular economy within public housing systems. Subsequently, a Results Matrix was formulated to categorise techniques, actions, and methods identified in the case studies. This structured approach facilitated the comprehensive analysis and resolution of the primary research question.

3.2. Stages of Systematic Review Protocol

3.2.1. Planning

A meticulous and unbiased review protocol was established to fulfil the research objectives. This protocol was crafted following the Cochrane Handbook for Systematic Reviews of Interventions guidelines, Version 6.4, 2023 [93], to ensure rigorous methodology. Utilising a Systematic Literature Review (SLR) approach is crucial for bolstering the authenticity of evidence and the credibility of outcomes. This method involves a comprehensive elucidation and justification of the review goals, planned research methods, criteria for study inclusion, and techniques for data extraction, processing, and synthesis.

It is essential to use robust data-handling practices, effective project management methods, and reliable quality control systems to conduct a successful systematic review. These components ensure review are comprehensive, accurate, and trustworthy. By incorporating these strategies, researchers can confidently navigate the review process and produce high-quality results. The first step involved identifying the issues to be investigated and formulating a rationale for the chosen topic, setting the stage for the rest of the review process.

Transitioning to CE in social housing policies involves the development of alternative solutions and the integration of social value as a critical element. As such, exploring “how can the incremental construction process in cooperative housings incorporate Circular Economy principles?” is an essential step towards ecologically correct, socially fair, economically viable, and culturally appropriate solutions.

In order to ensure a comprehensive review, the research period was defined as 2000 to the present study time. From the 2000s onwards, there has been a growing urgency to address affordable housing concerns through innovative approaches and new legislation supported by community involvement [94,95]. Therefore, this research concentrates on current housing issues that reflect present-day dynamics.

Peer review is a crucial process in academic research for assessing the quality of scientific work. Several studies have explored criteria and methods for evaluating research papers. This study’s quality appraisal is supported by [96], which proposes that a common approach involving multiple reviewers independently assessing manuscripts, with acceptance typically requiring approval from at least two reviewers.

Only documents written in English were considered. Qualitative and quantitative studies within the scope of cooperative housing were included in this review. Open access was also a requirement in the SLR.

3.2.2. Proceedings

The researchers used information from two academic databases, Clarivate Analytics’ Web of Science and Elsevier’s Scopus. The Web of Science database was selected because it covers all indexed journals with an impact factor calculated in the Journal Citation Report (JCR), thus ensuring high-quality research results. The strength of the Scopus database is its broad coverage of scientific journals, conference proceedings, and books on a global and regional scale, ensuring a SLR with comprehensive coverage and access to documents [97]. The filter applied to the databases was “document type”, selecting all documents.

3.2.3. Analysis

The research methodology consisted of three distinct phases. Phase 01 aimed to understand the current state of research on social housing policies, the circular economy, cooperative housing, and incremental construction.

Advancing to Phase 02, the focus was on conducting a systematic literature review (SLR). It was essential to have clear inclusion and exclusion criteria for the documents in order to carry out this process. Specific criteria were adopted to ensure a thorough search, including all document types published from 2000 to the present, availability in English, and complete text accessibility.

The database search terms were “cooperative housing” (CH) and “housing cooperative” (HC) as a previous bibliographic review showed that some authors alternated between the two terms for the same purpose.

Two selection lists were created from this initial selection, with 127 documents from Scopus and 68 from Web of Science. These two lists were exported to Microsoft Excel version 16.77.1 for data processing, resulting in 195 sources. In Excel, the lists were unified, and duplicate articles and those without free access to the full text were excluded, bringing the total to 107 documents. In this phase, 27 documents were added that were cited in the documents’ bibliographic references and met the pre-defined criteria for the initial research, giving a final selection of 134 valid documents (see Figure 1).

The 134 documents were listed, and a thorough analysis of each document identified which cooperative housing studies also contained incremental construction as a tool, resulting in 6 documents. After selecting the six documents [35,63,77,87,98,99], they were organised into a Result Matrix to facilitate the organisation of the findings. This method utilises a tabular structure to effectively and succinctly display research findings. A Results Matrix is especially valuable when managing substantial amounts of information, as it facilitates straightforward visualisation and arrangement of the data [100]. It can also be interpreted as an analytical tool, as the categorisation and extraction of information aid in recognising connections among the results [100].

In Phase 03, the Results Matrix (see Figure 2) was produced as an Excel document listing the six documents in rows and the five CE principles for the SHP in columns. From this organisation, the authors identified CE principles involved in the incremental construction process based on an exhaustive analysis and noted them in the respective Excel cells.

4. Results

Despite starting with a selection of 195 scientific documents on cooperative housing, only 6 documents presented cases that used the incremental construction tool, intentionally or not. The other cases discussed user participation in design and planning; however, they did not include the incremental construction process in the project. Documents that analysed cases of incremental construction were also accessed, but not in cooperative housing models.

The authors of these six documents [35,63,77,87,98,99] are recognised as references in studies related to the topics discussed here.

In Figure 2, it is possible to see that more than one document addressed the case of La Borda in Barcelona, Spain, with other cases in Spain, Argentina, and Nicaragua.

Figure 2 shows that only some of the five CE principles had practices identified in all six documents. However, all documents brought some data sought with the applied methodology, making it possible to analyse the results.

5. Analysis and Discussions

With the results presented in Section 4, the path to answering this research’s problem question follows: QP—“How did the incremental construction process, in the cases of cooperative housing, incorporate principles of the circular economy?”

The six documents analysed in Figure 1 contributed material to enrich this research. Therefore, it was possible to align practices identified in the documents with the five CE principles for SH.

According to the organisation of the Results Matrix (Figure 1), the following points emerge:

• To achieve sustainable renovation, it is crucial to prioritise factors such as material quality, waste reduction, and environmental impact. The revitalisation of underused and neglected old industrial neighbourhoods with the participation of cooperative housing committees and the possibility of incremental construction could breathe new life into an area. Furthermore, openness to using innovative and ecological construction systems in the basic project and its expansions and renovations proved to be a solution that residents supported. The vision that the product and the process are inseparable from achieving sustainable retrofits and upgrades brought a holistic approach that improved resource efficiency, reduced waste, and paved the way for a more sustainable future in the community.
• It is crucial to balance residents’ needs and budget constraints. Designing the “base unit” to be customised, making varying investments over different timeframes, encouraging co-design and self-organisation, and combining local building traditions with new, low-cost technologies proved to be solutions that promote sustainability and profitability while meeting the residents’ needs.
  Testimonies indicate that practical construction approaches can contribute to increasing accessibility. Nevertheless, it is important for them to be organised collectively, and the time spent should be viewed as a shared responsibility rather than a burdensome task [100].
  The findings also indicate a need to redefine minimum quality standards, incorporating various factors such as (i) decreased surface area and infrastructure in private spaces, (ii) increased tolerance for incomplete spaces or surfaces, (iii) scrutiny of certain construction norms, and (iv) emphasis on principles like sustainability and enhanced energy efficiency. Embracing smaller living quarters, fewer amenities, and unfinished areas may also stem from greater resident involvement [100].
  The results showed that residents’ redefinition of minimum housing standards defines their own set of socially acceptable standards.
• Policy integration involves aligning national and local systems with broader sustainability goals, which promotes resource efficiency. Examples of this practice showed alternatives to the harm caused by the existing gaps between national and local powers. Residents could access land through a surface right at an affordable price, giving them security of tenure and the ability to invest more in their homes. Public–private partnerships and public subsidies also provide security of tenure for residents and make it possible to invest in homes in the long term. Ensuring technical assistance to families throughout the incremental process depends on a long-term arrangement by local government, third-sector organisations, partnerships, and cooperative housing committees focused on ensuring quality and safety.
  The government plays a fundamental role as a facilitator and protector in ensuring the success of housing initiatives. Their unwavering support and willingness to adapt to new realities, including adjusted technical standards, streamlined bureaucratic requirements, and accessible ownership models, position local and national governments as key stakeholders in this endeavour.
• Environmental justice and equity are crucial in combating pollution and prioritising public health, economic development, and equity, especially in disadvantaged communities. Studies have detected that encouraging social interaction at all stages of life and resocialising domestic work is crucial to promoting autonomy and appropriation of space. The case studies showed that providing personal security through social support networks is possible. Since women mainly occupied management and administration positions and committees, this brought a high degree of feminisation to the communities. What stands out here is the case in which self-management prevailed, but only under the condition that gender equality was guaranteed.
• Circular business models are fundamental to achieving sustainable and equitable development. These models prioritise the design, sharing, recycling, and reuse of materials, resulting in the creation of resources, the reduction of environmental impact, and contribution to the circular economy. Studies have shown that mutual support, active involvement in construction activities, and the creation of work cooperatives are essential for promoting a sense of belonging and shared responsibility among community members. In terms of physical participation in construction, this experience enabled the development of practical problem-solving skills and a deeper understanding of the construction process. Even in cases where residents contracted the services, the exchange of experiences created training that brought autonomy to residents and the locality.
  Furthermore, using locally sourced construction materials was fundamental in supporting local economies, thus reducing transport emissions. Local producers and sellers of building materials play a vital role in promoting sustainable development. It is crucial to recognise the importance of these approaches and implement them in communities to build a more equitable and sustainable future. Promoting community engagement, hands-on learning, and resource efficiency can create a better world for future generations.

Regarding the State of the Art, Uruguay’s experience in cooperative housing is working on a sustainability proposal for cooperative housing. The incentives to prioritise prefabricated construction systems are an openness to innovative and ecological construction systems.

The traditional construction industry encounters obstacles in embracing circular economy (CE) principles and sustainable practices. These barriers encompass economic factors such as high costs, limited awareness, regulatory hurdles, and technical challenges, including inadequate recycling processes and the absence of design standards [101,102]. Moreover, cultural and behavioural impediments hinder CE adoption, including resistance to change and negative perceptions of second-hand materials [102,103]. Effective mitigation of these barriers necessitates collaborative efforts among stakeholders, the establishment of robust regulatory frameworks, and the integration of technological advancements [101,103].

In this regard, the acceptance of innovative and ecological construction systems in basic projects and their extensions and refurbishments has proved to be a viable solution supported by residents. The allocation of buildings to cooperative housing for renovation, utilising innovative construction systems with government backing, as observed in case studies like La Borda, La Balma and Cirerers Florits, Sotrac, La Regadora, and Quinta Força, serves as an exemplar of how prefabricated construction systems can be integrated into this evolving construction industry.

The Spanish cooperative housings analysed in this study demonstrate how they adapt the concept of housing solutions to their specific national contexts and needs. They form partnerships with public authorities, who may provide financial support or donate land for development in addition to the cooperative’s own efforts. The cooperatives emphasise self-management and residents’ active participation in all acquisition phases, including community-led design, collective ownership, and housing management. These cooperatives are private autonomous organisations that share physical space, legal structure, economic responsibility, governance and management, maintenance, quality assurance of buildings, collective identity and values, and mutual care.

The 2030 Agenda includes SDGs directly related to habitat and the right to place, particularly Goal 11, which aims to make cities and human settlements inclusive, safe, resilient, and sustainable. Achieving this goal requires considering the importance of territorial identity and place in urban planning and development. Policymakers, urban planners, and community development organisations are crucial in creating environments that foster community belonging and engagement by prioritising local contexts and identities.

In this sense, the cases studied provided some solutions such as (i) using the entrepreneurial spirit and social capital of the community to advance the project and cultivate a sense of belonging and shared responsibility among its members; (ii) mutual and voluntary support such as solidarity days as a strategy to save money and promote group integration; and (iii) skills development and job creation with the hiring and creation of work cooperatives and contracts with small or family construction companies, promoting group integration and developing or awakening skills, thus stimulating the creation of new jobs and promoting group integration among producers and the local construction materials market.

The contemporaneity of this model necessarily permeates the institutional, legal support that brings a solution to the issue of land ownership and security to the cooperative housing model. This is a real challenge and a complex process led by social housing movements and supported by technical organisations that depend on updated regulatory support to advance this model.

At least in the countries involved in the six documents analysed, there has been a significant change in the conception of existing housing policies and programmes, particularly about “what” should be achieved by a housing policy. Governments are paying more attention to informal housing construction processes, although the incorporation of support for incremental housing construction is still pending. This is a significant challenge involving adding solutions based on household savings, demand-side subsidies, and the development of housing finance markets. It is necessary to provide families with services they cannot obtain independently, such as access to lots suitable for residential use and technical assistance for the more complex tasks involved in incremental home construction [37].

The role of academia and universities in legitimising citizens’ contributions regarding social housing issues and providing expertise to governments and urban movements is multifaceted. Ramirez et al. [104] emphasise that universities can foster coalitions between social movements and scientists, producing new knowledge and urban change. Furthermore, Miotto et al. [105] highlight that universities should use social responsibility strategies to legitimise their actions and engage with stakeholders. Gorostidi et al. [106] suggest that universities are instrumental in training students in active research to support community processes.

It is worth mentioning that local social housing movements can facilitate unique, heterogeneous alliances with crucial actors in the science and technology system. For example, this process can lead to bricolage in which the scientific community’s technical knowledge supports social movements [104].

It is crucial to note that the agendas of social movements influence the priorities of scientists. This influence produces new forms of knowledge, highlighting the collaborative nature of addressing social housing issues.

6. Conclusions

Returning to the objective of this study, which was to verify which Circular Economy principles were applied in incremental construction in case studies of cooperative housings, the results pointed to several relevant conclusions. With the analysis and discussions aligned, the documents selected through the SLR show real possibilities for incorporating CE principles into cooperative housing models by appropriating the incremental construction tool.

The case studies utilise various practices related to CE principles. These include incorporating innovative and ecological construction systems, retrofits, and co-design; redefining minimum standards of quality, size, and functionality; ensuring the security of tenure; enabling long-term investment; and fostering autonomy and local economies.

To successfully implement these actions in social housing policies, it is essential to have institutional and legal support. This support should address the land issue and provide security for the housing cooperative model. This is a significant challenge, involving a complex process with limitations and difficulties to overcome. It is crucial to ensure that gender equality, local economic development, the use of circular materials, and the rehabilitation of vacant buildings are all prioritised within this movement. This will create opportunities to integrate the principles of the circular economy into the housing cooperative model.

This work delves into the intersection of social housing and environmental sustainability, highlighting the potential of targeted public policies to address social housing challenges. The integration of community, government, and CE principles is identified as pivotal for developing effective social housing policies. By fostering synergy among these stakeholders, this study advocates for solutions directly involving the community backed by contemporary legislation, contextualised subsidies, efficient financing, and government arrangements. Ultimately, this study emphasises the collaborative role of such research in shaping innovative public policies for social housing.

The analysis conducted using the SLR and detailed in the Results Matrix (Figure 2) examined documents featuring cases of (i) cooperative housing utilising, (ii) incremental construction methods and incorporating, and (iii) techniques aligned with the principles of circular economy for social housing policies as defined by the authors. This process identified six cases meeting all the specified criteria. While alternative analysis approaches could have yielded different results, this work focused on documents with the complexity the authors perceived, providing comprehensive insights related to the question problem challenge. However, aside from the scenarios analysed in the six cases studied in this research, there is potential to develop other scenarios that were not covered in these cases.

This research is part of a more complete study on the topic. It was possible to verify that the production of scientific and published analyses involving incremental construction as part of the cooperative housing model still needs to be improved. The authors hope this research will motivate more interest in studying this topic.

It should be noted that the research presented here has its focus defined on the incremental construction process, which does not exclude future studies focusing on CE principles related to other aspects of SH projects, such as design, financial arrangements, regulations, maintenance, and infrastructure among others, as these need to be studied and improved in the search for social housing construction with a smaller ecological footprint and greater accessibility.

The research started by identifying the need for SH solutions that are more efficient, accessible, and with less environmental impact and showed that this is possible through practices that incorporate CE principles. Resuming the model of cooperative housing from the last century and adapting it to current contexts, by incorporating the tool of incremental construction, promises to contribute to the solution of decent housing in the world in line with the principles of a circular economy.