## 1 Background
### The Solution of High Density for Growing Cities and Sustainability  
Multiple actions should be considered for promoting urban sustainability, such as limits to new urbanization 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; 10; 11; 12; 22; 23; 24; 25; 26; 27; 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; 31; 32; 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; 35; 36; 37; 38]. These densification projects are promoted mainly for the following reasons:  
1. The high value of the land combined with the increased demand for housing is a potential profit for the developer if what exists is demolished and rebuilt at a higher density. 2. The available buildable land is either scarce due to geographical conditions or limited by the local regulations for protecting agricultural land. Both reasons justify the actions of densification promoted in dense cities, such as Paris [39; 40], Seoul [41; 42; 43], Rotterdam [44; 45; 46], or London [47; 48; 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.