The social space of the city is distributed with groups of residents of different age structures, different ethnicities, and different socio-economic conditions. As different social groups are integrated into the social macrostructure to varying degrees, they settle in neighborhoods with distinctive attribute characteristics and establish unique relationships within their communities [67]. This gives rise to a series of social problems, such as social exclusion and residential segregation, which pose a great challenge to the sustainable development of cities.

Since the 1990s, “social integration” has gradually replaced “equality” as one of the central concepts in social policy practice and research on community composition [68]. The concept of “social integration” originated from European scholars’ research on social exclusion and was originally proposed by Emile Durkheim, who defined social integration as the process by which individuals develop a collective consciousness based on the division of labor in society, thereby maintaining social order [69]. The concept of social integration has been used in many studies since then, but because of its complex, multidimensional, and dynamic nature, scholars have interpreted it differently in different contexts [70], resulting in definitions such as social inclusion [71] and social interaction [72]. However, these definitions have largely failed to cover multiple aspects of social integration [73]. Although there is no consensus on a formal definition of integration, most scholars consider social integration as a two-way process of mutual adaptation of different groups [74]. According to Esser’s classification, social integration includes four basic forms: structural integration, cultural integration, interactive integration, and identificational integration [75]. Structural integration means that people have equal access to public resources and social benefits such as education, the labor market, and health; cultural integration is the acquisition of knowledge and competencies about cultural aspects, such as language or general rules of behavior, which enable individuals to avoid cultural exclusion in society; friendships, partnerships, and other social aspects are interactive integration, meaning that people are included in the main networks and relationships of the local society; and finally, identificational integration is the sense of belonging to the society in which people live, that is, the emotional ties people have to the place or other local groups [76].

In the study of social integration, social space is a factor that cannot be separated from consideration. On the one hand, social integration occurs in social space, which provides the place and context for social integration. The socio-spatial location of residents can reflect the extent to which individuals have access to the resources and opportunities provided by the social system and can also influence the process by which individuals form new social relationships and create new opportunities for themselves [67]. On the other hand, spatial integration is an extremely crucial part of the process of social integration [77] and is a central element in many theories of social integration.

The theory of spatial assimilation, which emerged from the theory of urban ecology combined with the theory of social status acquisition, has been central to the interpretation of social integration/exclusion in the American academy [31]. Most of the series of other theories that have emerged since then are the product of a critique or rethinking of this dominant theory [77]. The central idea of the spatial assimilation theory is that as social status increases, non-mainstream/disadvantaged groups will achieve spatial integration with the dominant race by moving into mainstream communities that offer better superiority in terms of public services and healthcare [78]. This theory views the transition from social exclusion to social integration of non-mainstream/disadvantaged groups as a natural process of resource acquisition and transformation, in which socio-economics is the most important consideration [77]. Subsequent scholars have made a series of modifications based on this theory, resulting in the contemporary spatial assimilation theory, which emphasizes the proximity and integration of non-mainstream/underprivileged groups with the majority rather than the unidimensional assimilation of non-mainstream/underprivileged groups by the majority [77,79], and the delayed spatial assimilation theory, which emphasizes that spatial assimilation may not continue to strengthen with generations [77,80] and so on.

The segmented assimilation theory, which builds on but differs from the spatial assimilation theory, focuses on the differences in the social integration of disadvantaged groups [31]. This theory does not deny the transformation of economic and social capital into residential space, as described in the spatial assimilation theory, but it further states that while the non-mainstream/disadvantaged try to enhance residential space, the mainstream group that holds power will isolate the non-mainstream/disadvantaged from themselves on the physical and social levels by influencing the allocation of space [81]. So, it can be seen that the spatial assimilation theory is concerned with the comparison between non-mainstream/underprivileged groups, that is, whether members of higher socio-economic status within groups live in communities with better conditions and more mainstream groups. The segmented assimilation theory, on the other hand, is more concerned with the comparison, or game, between non-mainstream/underprivileged groups and mainstream groups. In other words, while the spatial assimilation theory assumes that residential segregation between mainstream and non-mainstream/underprivileged groups will inevitably be gradually eliminated over time, the segmented assimilation theory assumes that for some groups, social integration will not occur due to the influence of mainstream social groups.

Some scholars have also questioned the assumption of the voluntary and inevitable integration of non-mainstream/underprivileged groups into the mainstream in the spatial assimilation theory. The residential preference theory argues that each group exhibits preferences based on culture and similarity and that residential preferences are closely related to their residential patterns, so the essence of social segregation may be the self-segregation of residents [82]. The pluralism theory argues for a pluralistic social and economic order because it sees assimilation theory as distinctly dominant group-centered and discriminatory against non-dominant/underprivileged groups [83]. In the exploration of assimilation and diversity theories, the group threat theory suggests that frequent interactions with out-group members threaten in-group members’ identity, while the contact hypothesis suggests that interactions with out-group members reduce in-group members’ prejudice [84]. Thus, the group threat theory supports pluralism, while the contact hypothesis supports assimilation.

There are a large number of empirical studies on social integration with more diverse research objects and concerns. In terms of research subjects, since those who are socially excluded are often non-mainstream/vulnerable groups in society [68], social integration research, which originated from social exclusion studies, also focuses on these groups, such as immigrants, ethnic minorities, the elderly, and the poor. One group that has received much attention is immigrants. Most social integration theories are developed in Western countries to understand and explain their immigrants [85], before being widely applied to social integration studies of other non-mainstream/vulnerable groups. Most residents may have multiple roles at the same time, and therefore, most studies consider multiple socio-economic characteristics of the study population simultaneously, such as occupation type and immigration status [86,87], age group and refugee status [88], etc.

Studies on immigrant integration have focused on different scales in different countries, with developed Western countries focusing on international migration [86,89] and developing countries such as China focusing more on domestic migration [90,91,92] studies. However, in terms of research topics, different countries are concerned with the characteristics of immigrant integration, measures of immigrant integration [93,94], factors affecting immigrant integration [87,89,95], and effects arising from immigrant integration [91,96].

Scholars have different opinions on the system of indicators to measure the degree of immigrant integration. Hao Zhou included economic integration, cultural adaptation, social adaptation, structural integration, and identity in the measurement system [94], while Acikalin et al. measured immigrants’ social integration from the perspectives of education, law, social inclusion, and employment [93].

With the rise of humanism, individual and microscopic studies have received increasing attention from scholars [110]. Human orientation has become the core of urban development in many countries/regions [111,112]. As seen in SDG 11, the livelihood of residents is a very important aspect of measuring urban sustainability; for example, ensuring that everyone has access to services such as housing, transportation, green public spaces, and inclusive and safe community environments are emphasized [4]. Therefore, when exploring the issue of urban sustainability, it is important to focus not only on macroscopic urban and community issues but also on the daily lives of residents. By focusing on the daily lives of residents, people can, on the one hand, see the overall and macroscopic urban socio-spatial development issues from the perspective of individual and micro-residents, and, on the other hand, discover the diverse needs of different social groups, effectively improve the quality of residents’ lives, and thus promote the sustainable development of cities.

The study of residents’ daily life can start with the residents’ activity space and living space. The concepts of activity space and living space originate from behavioral geography and time geography. Activity space refers to the spatial area involved in the daily activities carried out by individuals to satisfy their needs and the movement between activities, while the latter refers to the spatial area involved in the series of activities that people maintain in their daily lives, which is a kind of home-centered activity space [110]. On this level, the activity space has a broader scope; in other words, the living space can be understood as a special kind of activity space. However, different scholars have different understandings. For example, Wang Li et al. deconstructed living spaces and argued that the constituents of living spaces at the level of the spatial system include urban resources, community types, and community resources, and those at the level of the socio-spatial system include social equity, spatial public, cultural equality, and value respect [113]. Regardless of the specific definition, the study of activity spaces and living spaces revolves around the daily life of residents to improve the quality of life of residents. Therefore, this Section combines the existing research on activity spaces and living spaces to examine them together.

Doxiadis, a Greek scholar, first gave the connotation of urban living space science, and he proposed the concept of “habitat science” based on the study of urban life and living environment [114], which set off a wave of urban living space research and planning. Since then, many theories around activity space and living space have gradually emerged, among which two theoretical foundations, behavioral geography and temporal geography, occupy the core position.

Behavioral geography began with the behavioral revolution and is a new paradigm of human geography research emphasizing the microscopic perspective, behavioral processes, and the combination of subjectivity and objectivity [110]. Its core is to explore the interaction between human behavior and space in different geographical environments and pay attention to the thinking of “people” [115]. In the process of development, behavioral geography has absorbed closely related theories and methods such as time geography and the activity-based approach, integrating with neighboring disciplines (e.g., psychology) [116] and gradually diversifying, becoming an important theoretical paradigm in the study of residents’ daily lives.

Time geography is a methodology proposed and developed by the Swedish geographer Hagerstrand and his Lund School in the late 1960s [117,118]. This methodology emphasizes the holistic nature of space–time, the constraints that individuals are subject to in space–time, etc. [110,118] and provides a human-based perspective and methodology by emphasizing the focus on individual activities from space–time [119]. Time geography and the activity-based approach are often combined as the core of space–time behavior research theory [120]. The theoretical framework of Ping Cai and Hagerstrand is the source of the theoretical foundation of the activity-based approach, with the former’s research pointing to time and space as influential factors behind behavioral patterns and the latter providing a framework for activity-based decision making under spatio-temporal constraints [117,121,122]. Therefore, the activity-based approach provides a decision model of everyday life, focusing on and explaining differences in residents’ behavior across different lifestyles [120]. In conclusion, theories based on the study of spatio-temporal behavior, which can portray the daily lives of individuals or groups in detail at the microlevel [123], are regarded as the core theories for the study of daily life [124] and have received widespread attention and application.

Empirical studies on residential activity spaces and living spaces have been conducted from several aspects, and the topics explored include the identification of spatial boundaries of different residential activity spaces and living spaces [125,126], evaluation of spatial quality [127,128,129], characteristics of activity spaces and living spaces [130,131], and identification of influencing factors [132,133]. Some studies analyze all of the main daily activity characteristics of residents at the spatial level [131], whilst other studies summarize the corresponding spatial characteristics from specific behaviors [134,135], and the summary of characteristics is usually combined with the analysis of influencing factors [130,136]. In terms of research data, in addition to traditional data such as in-depth interview data [137] and activity diary survey data, new types of data such as GPS data [138,139] and cell phone signaling data [140,141] have been used. An in-depth interview is a detailed and targeted conversation conducted by the researcher or research team to understand the respondent’s viewpoints, experiences, attitudes, feelings, etc. The questionnaire data are collected by asking a series of questions to the respondents and recording their responses. GPS data and mobile phone signaling data contain a large amount of geo-spatial information, both of which are different in accuracy but can dynamically record individual movement characteristics and vividly display the living and activity space of residents [141].

The quality of activity spaces and living spaces is one of the important indicators reflecting people’s happiness in life and one of the key concerns in academic studies. Scholars have evaluated the quality of the living space of different people based on different data and from different dimensions [113,127,128,129,142]. Duan Zhaowen et al. collected data through a questionnaire survey to evaluate the quality of the living space for residents of public rental housing in terms of resource accessibility (educational resources, shopping facilities, medical resources, and leisure resources), satisfaction (housing conditions and community environment), and residents’ sense of belonging [128]. Zhaozhong Li, on the other hand, evaluated the quality of the living space in Nanjing in terms of five dimensions, including comfort, convenience, health and safety, and sociality, by combining multiple sources of data such as community attribute data, poi data, and environmental monitoring data [127]. Closely linked to the evaluation of living space quality is the analysis of residents’ satisfaction with their living space and activity space. For example, using a questionnaire survey method to study workers’ satisfaction with activity spaces in office areas, problems in office areas in terms of commercial support facilities and greening conditions can be identified [143]. The study of living space quality and satisfaction can provide a reference basis for urban planning, thus making it possible to effectively improve the well-being of residents.

The characteristics of residents’ activity spaces and living spaces and the factors which influence them are central research topics that have received much scholarly attention. For example, a Nanjing-specific study based on activity diary survey data found that low-income participants who are highly dependent on the central city for their daily activities (especially work) are disadvantaged in terms of access to the central city, compared to non-low-income people who have more diverse activity spaces and can more fully utilize various types of urban areas; the factors influencing this difference include activity characteristics and the built environment, in addition to income [130]. A survey study of community residents in the Beijing Economic and Technological Development Area based on questionnaire data points out that distance, business preferences, requirements for quality of service facilities, and changes in residents’ demand due to improved transportation modes, together with the spatial distribution of market supply and the relative shortage of government supply, shape the daily activity space of community residents [131]. In addition to these socio-economic conditions and built environment factors, the rapid development of ICTs has led scholars to gradually focus their attention on the impact of ICTs [128,132,144]. Nowadays, more and more individuals can access the Internet, and people’s daily life has changed considerably with the emergence of some new characteristics. These features can be summarized as fragmentation, multitasking, and space–time substitution, where fragmentation refers to the original activity being broken up into multiple sub-activities then widely and discontinuously distributed in time and space, multitasking refers to the phenomenon of people carrying out multiple activities at the same time with the help of ICTs, and space–time substitution refers to the substitution of online activities for offline activities [128]. A study of female residents in Beijing’s urban villages finds that young women not in a marital relationship have a high level of education and are in a better economic situation than the majority of those who engage in Internet activities, and ICTs are not an influential factor in their choice of living space but can weaken their time and space constraints on leisure activities [144]. These studies of ICTs help urban planners to respond positively to the changes that digitalization brings to the lives of residents.

The study of living spaces and activity spaces provides important insights into the problems of non-mainstream/vulnerable groups such as low-income groups, the elderly, and immigrants. Studies of low-income population clusters, such as urban villages, have found that the daily activities of the population have certain regularities [136] but also exhibit individualized and diverse characteristics [145], which are the result of multiple factors such as the macro-environment and their characteristics. Some scholars have also focused on children in urban villages. Through in-depth interviews, they have found that children’s outdoor activity space is mainly concentrated in places with spacious grounds and high store density, but there are differences between children of different ages [146]. A study of older adults based on household travel survey data found that there are two main activity patterns of older adults, the recreation–shopping-oriented (RS-oriented) pattern and the schooling–drop-off/pick-up-oriented (SDP-oriented) pattern, with RS-oriented older adults facing spatial constraints and SDP-oriented older adults being more likely to experience time constraints when making daily trips [147]. Mobility in living spaces is a representation of an individual’s ability to move in the context of daily life [148]. A study on the mobility of elderly people’s living space divides the influencing factors into non-physical environment elements and physical environment elements: non-physical environment elements include home environment and social culture; physical environment elements are divided into objective environment elements and subjective perception elements, where objective environment elements refer to physical indicators such as density, diversity, and street connectivity, while subjective perception elements refer to perception indicators such as safety, accessibility, comfort, and pleasure [149]. A study of refugees based on in-depth interviews found that daily life in neighborhood spaces provides opportunities for refugees to develop and maintain social relationships, but that integration with residents is difficult because some potential social spaces are legally or economically inaccessible [150]. It can be seen that activity spaces and living spaces are effective entry points to analyze the issues of social equity and social inclusion faced by non-mainstream/vulnerable groups.

The study of living spaces and activity spaces starts from the micro- and individual level, studying the daily life of residents to summarize the characteristics of residents’ daily lives and discover the problems and influencing factors. It can both summarize the daily life patterns of mainstream resident groups and discover the social injustice problems faced by non-mainstream/underprivileged groups. These findings emphasize the importance of optimizing resource allocation to ensure equitable access to essential services, enhancing housing and community design to create safe and comfortable environments, promoting digital inclusion for all residents, addressing spatial inequalities through flexible zoning and mixed-use development, actively engaging the community in the planning process, adopting an adaptive approach to accommodate evolving trends, and, above all, adopting a holistic and inclusive approach to urban planning. In conclusion, empirical studies on activity and living spaces offer a wealth of information that can be invaluable for urban planning and policymaking. By incorporating these perspectives, policymakers can consider various viewpoints and communities while developing urban policies that enhance the quality of life for residents and facilitate sustainable urban growth.

Cities are protagonists in the sustainable development of society, and urban planning offers a variety of solutions to the complex problems and challenges of sustainable urban development. In an era of public skepticism toward science, sustainability science offers a way to increase the application of science in planning and policy [151,152]. Urban planning can develop solutions to a range of urban problems at the national, city, and community levels and can also involve residents through bottom-up planning. It is an important tool for achieving sustainable development. Therefore, this Section will review the urban planning concepts and practices that have received much attention in the existing studies from three dimensions: city, community, and individual.

The idea of planning at the city level plays a guiding role in planning at the community and individual levels. Modern urban planning emerged over 100 years ago and has been divided into three stages: the first stage was concerned with scientific rationality and attempted to solve the problem of material planning, represented by the Athens Charter in the 1930s; the second stage was advocacy planning, which shifted the focus to the equity of the planning process, with special attention paid to disadvantaged groups, and attempted to solve the problem of sociological planning; the third stage belonged to collaborative planning, concerned with the collective rationality of the planning process, and emphasized the concept of sustainable development, and the theories of this stage included New Urbanism and Smart Growth, which emerged after the 1980s and 1990s [153]. In the 21st century, frontier urban planning concepts can be divided into six categories: sustainable development, information and digital technology, urban social and community life improvement, political or economic, conservation and renewal as the core, and other planning concepts (such as Infrastructural Urbanism, Stereoscopic Urbanism, etc.), among which, sustainable development and information and communication technology are the core drivers for creating new urban forms in the 21st century [154]. As can be seen, sustainable development has become a central goal in current urban planning. Based on cutting-edge urban planning concepts, new urban concepts such as sustainable cities [155,156,157,158,159,160,161], resilient cities [162,163,164,165,166], smart cities [162,167,168,169], and healthy cities [170,171,172] have been derived in academic and political circles. Sustainability is a multidimensional concept, and sustainable cities focus not only on sustainability in the physical spatial realm of cities but also in the socio-spatial realm of cities, involving multiple dimensions such as environmental, economic, social, and cultural dimensions [173]. The term resilience originated in physics to describe the ability of an object to recover from deformation under the action of external forces [166]. After being introduced to urban planning, it was defined as the ability of individuals, communities, institutions, etc., within a city to survive, adapt, and grow after experiencing shocks [174]. The definition of smart cities is not unique and has understandings based on technology, knowledge, and governance perspectives, with the technology-centric perspective dominating [175,176]. Healthy cities focus on healthy people, a healthy environment, and a healthy society and hope to enhance the health of residents in urban societies through the implementation of planning programs such as urban health projects [170]. These new urban planning concepts are not only widely discussed in academic circles but also applied to actual urban planning processes in different countries [165,174,177,178]. For example, Oswiecim realizes the sustainable development of towns by enriching their functions, designing town spaces based on the concept of balance, and the main measures include the provision of jobs and housing to meet the basic needs of Oswiecim’s residents, increasing the accessibility of local sports and recreational venues, good safety monitoring, taking care of the needs of each social group, and encouraging social participation [177]. Japan focuses on public participatory resilience building with the community as the basic unit, and about 30% or more of Kyoto’s resilience strategy emphasizes the importance of community and public participation in reducing the risk of future shocks. The Kyoto government encourages citizens to participate in improving the safety and security of their neighborhoods and cities and builds communication channels between the government and citizens in the community through smart programs to maintain and strengthen the assets of urban facilities while also forming a good government–society partnership [165]. The construction of smart cities has also received attention from many countries, and smart governance, smart people’s livelihood, and solving problems such as traffic congestion, environmental pollution, and resource constraints are the development goals of smart cities. Shanghai is one of the earliest pilot cities in China to build a smart city, and Shanghai has made great progress in information infrastructure construction, information perception and intelligent application, a new generation of the information technology industry, information security assurance, etc. [178]. The sixth phase of the European Healthy Cities Program supports cities to strengthen linkages, bring key stakeholders together for health and well-being, and use leadership, innovation, and change to increase the potential to address local public health challenges [171]. It can be seen that many countries are applying these concepts to urban development planning to create more livable, sustainable, safe, and healthy urban environments. However, there are limitations to the application of these urban planning concepts, such as the need for large-scale investment in the construction of resilient cities; the construction of smart cities, which may bring about issues of privacy and security, digital divide, and technological dependency; the construction of healthy cities, which may also lead to issues of health inequality; and the planning of sustainable cities, which requires weighing conflicting interests, such as those of the environment, society, and the economy. Therefore, by applying these cutting-edge concepts, urban planners and governments need to develop integrated policies that encourage innovative, diverse solutions and ensure equitable, inclusive, and sustainable urban development.

In addition to city-level planning concepts and programs, there are also many community-level planning concepts and programs that aim to improve social integration and enhance the well-being of residents, such as the “15-min Community Life Circles” planning and urban housing projects. The concept of life circles originated in Asian countries and cities and was first introduced in Japan in 1965 in the Second National Comprehensive Development Plan as a planning strategy to promote social equity through the rational arrangement of basic services and facilities [179]. The concept was proposed and combined with the later concept of 15-minute cities as a new solution for community-level planning in many countries [180,181,182]. The concept of 15-minute cities was proposed by Carlos Moreno in 2016 and defines a highly flexible urban model [150,183] with several characteristics such as proximity, density, diversity, mixed-use, modularity, and adaptability [184]. The concept was introduced and sparked many controversies. Firstly, there were disagreements on the definition of time as 20 min, 30 min, or even longer [181], and secondly, the concept followed the philosophy of physical determinism, setting goals but not specifying how to achieve them [184]. However, it is undeniable that this planning concept focuses on the basic needs of residents and communication and interaction and is extremely humanistic. Empirical studies have shown that such planning solutions can help increase urban resilience, especially in the context of climate change, pandemics, and other issues, while also reducing inequalities between different parts of the city [185]. In addition to community-level life circle planning, housing projects are also planning programs which have been adopted by most countries to promote social integration. In Chile, housing programs have been implemented to promote social integration [186,187]. In Albania, housing production is seen as key to promote the integration of various social groups into the urban development process [188], and in China, the government has implemented policies to relocate poor people and thus improve their living conditions [189]. A study based on China’s affordable housing program finds that the implementation of the program contributes to the integration of migrants into urban society [190]. In conclusion, positive planning at the community level follows a humanistic ideology and aims to enhance community inclusiveness and promote sustainable community development.

When urban planning focuses its target on individuals, the conversation revolves around public participation, that is, bottom-up planning. The controversy over whether public participation can contribute to urban planning has existed for many years, with some scholars arguing that public participation does not play a great role [191] and others pointing out that public participation is not only an important manifestation of government democracy but also an important initiative to enhance the harmonious relationship between the government, the planning department, and the people [192]. Some scholars have also shown that the combination of bottom-up and top-down planning approaches helps to match the needs of residents with the urban socio-spatial development vision of decision makers [193]. For example, the UK spatial planning guidance recognizes the importance of involving stakeholders in the assessment process at an early stage of the planning process [194]. Since the 1970s, European countries have adopted public hearings and discussion groups to promote public participation in urban planning [195]. In Chicago, Illinois, USA, São Paulo, Brazil, and Delhi, India, public participation is a key tool used in the development of their planning documents and can play a role in achieving social equity and justice [196]. In short, resident participation in planning can be a complement to top-down planning, while at the same time giving individuals a way to express their wishes and needs, thus promoting social equity to a certain extent. But how to combine the two properly is a question that needs to be explored.

Urban planning is the field of study that focuses most explicitly and specifically on urban areas and is able to be applied at multiple levels—city, community, and individual—to address a variety of issues that arise in urban development. However, there is controversy about the role that urban planning can play, and in the United States, for example, the planning department has very limited powers, making it difficult for urban planning programs to achieve the appropriate goals [197]. A study based on the city of Changchun, China, also finds that urban planning, although it plays a role in guiding the employment activities of residents, is unable to play other roles originally planned. This shows that although urban planning can provide many good visions, cities, communities, and residents do not always develop as planners expect, and this is a problem that needs more attention in future urban planning studies.