**1. Introduction**

With the rapid development of economy, China overtook the US as the world's biggest energy consumer and greenhouse gas (GHG) emitter. About 1.6–2.0 billion m2 of buildings are constructed every year in China [1], accounting for about 40% of the world's total new buildings [2]. A large amount of GHG will be emitted during the life cycle of buildings, especially in construction and operation stages. In order to achieve the sustainable development of construction, there is a great need to clearly know both the costs and the environment costs of buildings.

At present, there is no common understanding of the concept of environmental cost in the academic circle, and there are still some differences among different research fields. According to United States Environmental Protection Agency (USEPA) [3], how environmental costs are defined depends on how the information is used. Whether a cost can be defined as environmental cost is not absolute but needs to be considered according to specific research purpose. The definition of environmental costs is more representative in the System of Integrated Environmental and Economic Accounting (SEEA) published by the United Nations Statistics Division (UNSD) in 1993 [4]. According to the definition, environmental costs consist of two levels: (1) the use and loss value of natural resources in output and final consumption; (2) the impact value of pollution generated by output and consumption activities on environment. In addition, the United States Council on Environmental Quality divides environmental costs into four parts: environmental loss costs, environmental protection costs, environmental affairs costs and environmental pollution elimination costs.

In China, the Research Group on Integrated Environmental and Economic Accounting (Green GDP) proposed in its technical guidance that environmental costs are composed of pollution control costs and environmental degradation costs, among which pollution control costs can be divided into actual pollution control costs and virtual pollution control costs. Based on the definition, the Research Group has conducted a study on China's green national economic accounts and published a number of studies on China's environmental economic accounts [5–7].

The relationship between environmental performance and economic performance is critical for environmental cost analysis. Several methodologies have been proposed to reveal the relationship, such as life cycle cost analysis (LCCA), whole life cost, eco-cost and eco-efficiency. Usually, the LCCA term implies that environmental costs are not included, as is the case in the similar whole life cost. Eco-efficiency has been proposed as one of the main tools to facilitate the transformation from unsustainable developments to sustainable developments [8]. It is based on the concept of increasing productivity and reducing economic and environmental performance at the same time [9,10]. Eco-efficiency refers to the ratio between the added value of a product (e.g., GDP) and the environmental impacts of the product or service (e.g., SO2 emissions) [9,11]. It has significant implications for environmental management accounting (EMA) system as well as environmental accounting [10,12].

Additionally, the environmental costs or eco-cost indicators are used to assess the environmental costs. Eco-costs are a measure to prevent the burden of products by expressing the amount of environmental burden. Vogtländer et al. [13] used "eco-costs 2007", an indicator for assessing ecosystem deterioration and human health problems, to compare the environmental impact of bamboo materials with commonly used materials such as timber. Baeza-Brotons et al. [14] applied eco-costs to evaluate the environmental impacts of cement with and without addition of sewage sludge ash. Kravanja and Cuˇ ˇ cek [11] presented a novel indicator called eco-profit, which was defined as the sum of eco-benefit (positive impact of environmental unburdens) and eco-cost (negative impact of environmental burdens).

For the application of environmental cost in civil engineering, only handful of studies can be found. Kendall et al. [15] proposed an integrated life cycle assessment (LCA) and LCCA model to assess and compare traditional concrete bridges with cement-based composite bridges. The LCCA they calculated includes construction, consumer and environmental costs, reflecting the loss caused by air pollution. Chen [16] established a life cycle environmental impact cost analysis index system of bridges based on LCCA, calculating life cycle environmental costs of bridges at different stages. The results show that, among all stages, the environmental cost of the material production stage is higher than that of any other stages. A method translating the environmental impact into monetary units was composed by Carreras et al. [17]. The approach used eco-cost indicators to quantify the cost to prevent a given amount of environmental burden. However, the eco-costs only considered the material consumption and energy consumption. Chou and Yeh [18] developed a CO2 emissions evaluation system and an environmental cost calculation method to compare the difference of environmental performance between fully prefabricated and cast-in-situ construction. In their study, CO2 emissions were simply converted into environmental costs by referencing the profit-seeking enterprise income tax in Taiwan, and the progressive tax rate was used to transform the simulated total CO2 emissions into environmental cost.

Through literature review, studies of building environmental costs, especially the life cycle environmental costs, are still quite insufficient. At present, several existing issues could complicate these efforts in research on environmental cost in civil engineering. For instance, environmental costs are always underestimated. Additionally, lack of adequate measuring and managing systems of environmental costs is another obstacle [19]. To overcome this gap, a calculating model for environmental costs of a building throughout life cycle is presented in this paper to obtain total energy consumption and pollutant emission costs of buildings.

The aim of this paper is to establish a single-objective optimization model by converting environmental impact into environmental cost, with the same unit of direct cost. The following investigations are conducted: (1) Firstly, this study builds an LCA model with all processes; (2) A virtual abatement cost of pollutants and environmental degradation cost according to macroscopic data of environmental economic accounting in China is calculated; (3) The green construction measures fee is incorporated into the environmental cost for the characteristics of building construction; (4) In order to analyze the differences in northern and southern parts of China, two residential buildings, one located in Beijing and the other in Xiamen, China, are taken as case studies; (5) Uncertainty analysis is carried out, including model and data uncertainties to evaluate how these sources of uncertainty may affect the environmental cost results; (6) Finally, sensitivity analysis of the environmental costs is conducted to identify major input variables, including the discount rate and the unit virtual abatement costs of pollutants.
