**1. Background**

*1.1. The New Urbanization "Beyond Mere Growth" in China*

Since 2014, the Chinese central government has promoted new-type urbanization in the *National New-type Urbanization Plan (2014–2020)* [1–3]. The direction of the plan "beyond mere growth" supports green and low-carbon development and requires avoiding urban sprawl, low efficiency in land use, the proliferation of useless urban districts, and waste of urban space in over-dimensioned infrastructures [4,5]. Consistent with the goals of new-type urbanization, the national guidelines on urban planning released in 2016 prohibit expanding cities beyond what their natural resources can support, enforcing urban growth boundaries [6]. In other words, to be sustainable urban development should not expand beyond a planned threshold, as also required by the environmental protection laws issued since 2015. This limit to expansion is also known as the "red line of China's cultivated land", and was first drafted by the National Land Planning Outline issued by the Ministry of Land and Natural Resources in 2009, which also specifies that the impermeabilization of the ground should be diminished [7].

These policies for preserving agricultural land are combined with the regulations for controlling land supply and the real estate market. Since 2003, the central government established year-by-year construction land development quotas for every local government according to the assessment of the local demand–supply situation [8–10].

**Citation:** Chen, J.; Pellegrini, P.; Yang, Z.; Wang, H. Strategies for Sustainable Urban Renewal: Community-Scale GIS-Based Analysis for Densification Decision Making. *Sustainability* **2023**, *15*, 7901. https://doi.org/10.3390/su15107901

Academic Editor: Miguel Amado

Received: 6 April 2023 Revised: 8 May 2023 Accepted: 9 May 2023 Published: 11 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Together with the sustainability goals, the increase in the urbanization rate is a key national priority in China. It has been one of the most significant drivers of economic, political, and social progress since the reform was promoted at the end of the 1970s [11–14]. So far, urban population growth has followed economic growth because economic development and city growth are strongly related to Chinese policies. The massive urbanization process has transformed millions of farmers into city dwellers, attracting an essential workforce into the expanding city [15,16].

These recent policies promoted by the Chinese government—on one side the goal of a growing economy and on the other side the limits to urbanization to protect fertile land and natural environments—can produce a contradiction and urban concentration: probable newcomers in growing cities and industrial districts versus a not expanding city.

#### *1.2. The Solution of High Density for Growing Cities and Sustainability*

Multiple actions should be considered for promoting urban sustainability, such as limits to new urbanizations and conservation of agricultural land, investments in public transportation [17], limits to car-oriented projects [18], reduction in energy consumption and greenhouse gases emission in all sectors of production, water management [19], and improving water, soil, and air quality [20]. Of the multiple elements that impact the sustainability of urban development, this paper focuses on the quantitative parameter of population density on urbanized land, more specifically on the opportunity to increase the dwelling units in existing residential neighborhoods. The research presented in this paper applies urban-scale GIS analysis to make some hypotheses on densification at the community scale.

Density is neither a good nor a bad indicator: too much can result in overcrowding and eventual urban decay; too little can deprive the dwelling environment of the chance of social interaction and public transport efficiency [21]. The definition of density is, nevertheless, closely connected to sustainability because land is a limited resource; urbanization is "sustainable" when valuable agricultural land is saved, and the ecosystem is not jeopardized.

High density has both positive and negative effects. Many scholars have presented evidence that higher density in urban areas is associated with a variety of desirable outcomes, including increased use of public transportation, improved financial stability for local governments, walkable and healthy living environments, housing diversity and affordability, enhanced community character, and cultural vitality [9–13,22–28]. Other scholars, on the contrary, have noted how some high-density neighborhoods produce excessive concentration and congestion, difficult management, and inefficient facilities. The issue cannot be assessed in general terms because the output of high-density conditions depends on several context-related factors, such as urban planning strategies, technical capacity, building regulations, infrastructure, public utilities, regional services, and the economic conditions of the residents [29]. For example, the interplay between density and building types must be considered because the same FAR realized by different architectural forms and open spaces produces different urban environments. In China, the different ranges of FAR—usually classified as low, medium, and high in building regulations and master plans—often correspond to specific building types; for example, medium density is realized with multilayer condos [30–33].

Over the last two decades, the idea of urban densification has been explored worldwide in several large cities and also in medium or high-density cases [34–38]. These densification projects are promoted mainly for the following reasons:


Both reasons justify the actions of densification promoted in dense cities, such as Paris [39,40], Seoul [41–43], Rotterdam [44–46], or London [47–49]; policies and projects differ among cities because they adapt to local conditions, including the socio-cultural approach to dwelling.

Regarding China's concerns, often urban land expands faster than population growth. In recent years, this trend has impacted several Chinese cities, which have experienced declining densities, very likely due to the rising average income leading to the demand for more residential space per capita, and for the restrictions due to the hukou system (a household registration which prevents the change of residence in order to limit the number of permanent residents in cities) [50,51]. According to the World Bank, even though China underwent massive urban population growth, its urban population density in 2010 was lower than the average of the rest of the East Asia region [52].
