1. Social Sustainability Barriers and Strategies
The construction industry greatly contributes to civilization’s environmental footprint, thereby influencing the achievement of economic standards and quality of life (social well-being). Therefore, sustainable construction represents a key social sustainability topic/objective, which concerns the well-being of the community with regard to environmental, social, and economic aspects 
. In contrast to the conventional modes of design and construction, which only focus on cost, performance, and quality objectives, sustainable construction considers additional aspects/objectives: minimizing resource depletion and environmental degradation as well as the creation of healthy built environments 
. Furthermore, the key features of sustainable construction presented in the literature are a whole building project’s lifecycle, environmental protection, technological and non-technological solutions related to social and economic sustainability, and addressing the needs of present and future stakeholders 
It is noteworthy that a growing amount of social sustainability research is focusing on sustainable construction and accompanied barriers and developmental strategies. The low levels of knowledge, awareness, and governmental support/incentives with respect to sustainable construction have been recognized as the main barriers for the implementation of green construction practices 
. Hence, educational programs and financial support (e.g., economic incentives, awards, etc.), along with the introduction of standards, regulations, and legislations on sustainable construction, have been identified as relevant driving factors. Furthermore, the basic understanding of the sustainable development formula, namely, ecology, economy, and social equity, is quite utopian, requiring ideal conditions 
. However, a holistic approach to sustainability encompasses at least three more relevant factors: technology, politics, and culture, an intertwined chain of causes and consequences perceived through quantitative and qualitative parameters, wherein culture represents a corrective, fundamental parameter constituting the root of all social processes 
Furthermore, some authors have conducted comprehensive, interdisciplinary surveys encompassing multiple key barriers and drivers of sustainable construction, including those socio-cultural, economic, stakeholder-related, political, technological, and environmental 
. In this regard, socio-cultural drivers encompass increased awareness and education regarding sustainable construction among stakeholders in the building industry and enhanced indoor environmental quality (i.e., ensuring the health, comfort, and well-being of building occupants). It is noteworthy that the socio-cultural drivers in the study additionally relate to economic factors (e.g., the reduced lifecycle cost of a building, high return on investment, etc.), thereby demonstrating the symbiotic and intertwined relationship of the social and economic factors in sustainable construction. In addition, the environmental drivers concern environmental protection and waste reduction; the stakeholder drivers concern integrated building design approaches with multiple project stakeholders; the technological drivers concern product and material innovation; and the political drivers concern governmental support for sustainable construction through upgraded regulations and urban-planning policies as well as financial (tax relief) and other market-based incentives 
. This theoretical, online-based study on social sustainability analyzed through the aspects/objectives of sustainable construction demonstrates the relevance of the three-pillar, integrative research approach, which, in this research, is implemented with respect to social sustainability strategies.
Collective housing, originating in the 1850s, has been constantly evolving due to the mass-housing developments that occurred during the second half of the 20th century. Changes in the conservative housing politics and the housing market started in the 1980s and 1990s, impacting the transformations within this sector in the following years 
. Over the last few decades, the old housing models have been questioned, and numerous attempts have been made to discover new, more sustainable policies and construction models of residential buildings, for which ‘sharing models’ have been prioritized, i.e., housing evolution has developed ‘’from collective housing to co-housing’’ 
. Furthermore, ecological (green) concepts became more popularized in the housing domain and in residential politics, wherein the social aspects remain dominant, which underlines the necessity of developing an integral, holistic approach to social sustainability research. Additionally, within the housing sector, social sustainability principles prevail in the new, similar housing models, e.g., eco villages, social and affordable housing, collaborative housing, and cooperative housing/co-housing. Eco-villages represent specific neighborhood and international community concepts designed to be environmentally and socially and economically sustainable 
. Although more related to rural zones, these significantly affected communities are raising awareness of environmental impacts and social sustainability issues both practically (e.g., by using locally grown and managed food sources; renewable energy and waste treatment systems; shared spaces, resources, and businesses; and engaging in community gatherings) and through educational programs 
Notably, the revitalization of residential quarters/blocks/buildings (constructed in the second half of the 20th century) plays a vital role in the implementation of sustainability strategies within developed countries, especially those in the European Union, wherein the social sustainability aspect represents a crucial factor of sustainable development. In this regard, the dominant contemporary principle of social and affordable housing becomes achieving a ‘’social mix’’ (mixed-income and mixed-tenure) residential community on different levels (building, street, block, and neighborhood). This social sustainability strategy, which has been implemented in many urban regeneration and neighborhood revitalization projects throughout Europe, Canada, and the USA, contributes to the realization of favorable urban diversity and the balanced urban growth of residential quarters and entire cities 
. Along with flexibility (the possibility of changing residential units) and participation, the ‘social mix’ strategy is considered the key factor in achieving social equity, inclusion, resilience, and security and thus improving quality of life 
. Additionally, common (public) spaces play a vital role by promoting interaction between residents and easing the adjustments to changed living conditions within local communities. Urban revitalization is a crucial aspect of systematic housing politics with the objective of achieving more human-scale and sustainable urban models and thus enhanced well-being and quality of life, with the latter two representing crucial social sustainability indicators 
. However, multiple stakeholders and their complex interrelations and impacts on communities during construction additionally complicate ongoing processes. Therefore, it is essential to expand the sustainability assessment and management framework and develop more complex, interdisciplinary, and integrative multileveled strategies (e.g., with respect to corporate, business, and functional aspects) 
. The strategies represented in this research address these complex, diverse, and multileveled relations and interdependences by proposing a theoretical framework encompassing several interdisciplinary categories relevant to sustainable construction.
2. 2030 Agenda’s Sustainable Development Goals (SDGs) in Reference to Social Sustainability Topics
The UN’s 2030 Agenda: “Transforming our World: The 2030 Agenda for Sustainable Development”, which was adopted in 2015, comprises 17 Sustainable Development Goals (SDGs) accompanied by 169 targets measurable by 232 indicators 
. The agenda represents an integrative approach to sustainability that encompasses all three pillars (ecological, economic, and social) equally. Additionally, social sustainability aspects are to a major extent included within the eight SDGs: (1) No poverty, (3) Good health and well-being, (4) Quality education, (5) Gender equality, (8) Decent work and economic growth, (10) Reduced inequalities, (11) Sustainable cities and communities, and (17) Partnerships for the Goals. In terms of urbanity (urban planning) and the built environment, SDG eleven (make cities and human settlements inclusive, safe, resilient, and sustainable) is highly relevant, prioritizing adequate, safe, and affordable housing; accessible and sustainable transport systems; inclusive and sustainable urbanization; participatory, inclusive, and accessible green and public spaces; the development of resilient buildings utilizing local materials; and multi-stakeholder partnerships. The agenda underlines the vital role of global and, to a greater extent, national and regional sustainability policies and politics as well their mutual hierarchies.
Furthermore, the European Union carried out a study on social sustainability, “Social Sustainability—Concepts and Benchmarks”, in 2020 
, exploring the definitions, quantitative parameters, and benchmarks of social sustainability and the means of the concept’s integration into EU policy-making processes at the national, regional, and local level. This relevant document relies on the 2030 Agenda, recognizing the complexity of the aspects of social sustainability and identifying indicators and a statistical basis for their assessment and better integration into national and local politics. Moreover, analysis has shown that the concept of social sustainability is still underdeveloped in the EU in comparison to sustainability’s ecological and economic aspects. Additionally, the implementation of the 2030 Agenda’s SDGs is noticeable, but the focus mainly remains on specific social sustainability aspects (e.g., social rights, guaranteeing the rights of children, inclusion, social economy, gender equality, etc.), while the relevant social sustainability topics of urban planning and housing (residential buildings) maintain underrepresented. However, the research in this field has provided valuable examples of social sustainability practices that demonstrate the implementation of green and inclusive housing models accessible to all, thereby promoting social cohesion and supporting social life in deprived neighborhoods 
. Accordingly, the participative planning of urban quarters has been recognized by researchers, e.g., Sweden’s Vallastaden; the E-Co-Housing/public–private co-creation of a regenerative housing project together with the local community in Budapest; the Belgian project of Brussels Capital Region—CALICO—Care and Living in Community; and the Paris OASIS Project, concerning the renovation of urban space via the creation of ecological ‘islands of freshness’.
In conclusion, the 2030 Agenda sets relevant SDGs that are equally related to all three sustainability pillars, thus having an impact on national sustainability strategies. The European Union’s sustainability policies and politics follow this globally relevant agenda. Despite this supposedly integrative approach, the implementation of social sustainability concepts and strategies remains underdeveloped and insufficiently present in comparison to the other two sustainable development aspects (ecological and economic).
3. Implementation of the SDGs through Social Sustainability Strategies: Case Study of Freiburg, Germany
The 2030 Agenda and its specified Sustainable Development Goals (SDGs) represent a globally relevant, crown-strategic document on sustainability that has been adopted and disseminated at the national, regional, and local level worldwide. The involvement of local governments is highly relevant for the implementation of the social and ecological strategies leading to the realization of the three pillars of sustainable development. Sustainability strategies provide horizontal integration at the local level and link municipal achievements to national and global strategies vertically 
. As a highly regulated country that has been dealing with sustainability processes and concepts for a long time, Germany represents a valuable case study of the national and local implementation of sustainability strategies derived from the 2030 Agenda, e.g., SDG-related budgeting, sustainability controlling and reporting, indicator-based sustainability strategies, and local government financing 
The first German Sustainable Development Strategy (GSDS) was proposed in 2002 and presented at the United Nations World Summit on Sustainable Development in Johannesburg. Furthermore, starting from 2004, the GSDSs have been updated every four years (i.e., 2004, 2008, and 2012), followed by the release of progress reports devised by the German government. Additionally, in 2015, the German Sustainable Development Strategy began following the 2030 Agenda’s Sustainable Development Goals, thereby addressing the 17 SDGs at the national level. Since then, all federal subsidies must be assessed in terms of their sustainability impact, comprising their long-term economic, environmental, and social impacts in reference to the National Sustainability Strategy. In this regard, the attention paid to sustainable budgeting is growing, and the first steps remain to be taken with respect to the promotion of a systemic and holistic approach. Finally, the most recent Sustainable Development Strategy for Germany was adopted in 2021 
. Therein, the construction industry and the transport sector are addressed, which are recognized as relevant parts of the sustainable building and mobility transition–transformation sectors. These sectors are related to the SDGs number 7, 8, 9, 11, 12 and 13.
Moreover, the so-called “Club of Agenda 2030 Municipalities” represents a German network of municipalities, cities, and districts dedicated to the implementation of the 2030 Agenda’s Sustainable Development Goals proclaimed in the resolution “2030-Agenda for Sustainable Development: Shaping Sustainability at the Municipal Level” signed by 190 German cities in September 2021 
. The participating municipalities are engaged to various degrees in socio-political activities relating to the SDGs’ implementation on the local (municipal) level, where citizens’ and multi-stakeholders’ engagement, the fostering of interdepartmental processes, and governmental regulatory mechanisms represent the crucial sustainability strategies leading to a socio-ecological transformation and the achievement of the SDGs.
Freiburg has a long tradition of dealing with sustainability topics. It was the first city in Germany that introduced sustainability-related budgeting in order to align the municipal financial resources with sustainability objectives (i.e., localized SDGs) 
. In reference to the social sustainability strategies’ implementation in Freiburg, the key topics are social equity, sustainable community, and the social mix, which were implemented in the so-called ‘fresh cell strategy’ wherein young families with children became residents of newly erected buildings constructed in dwelling quarters with a predominantly elderly population (built in the 1960s and 1970s). This example represents a proactive social sustainability model contributing to achieving a more balanced residential neighborhood in terms of its demographic structure, services, and community infrastructure, thus encouraging more intensive face-to-face contact between neighbors 
. The ‘fresh cell strategy’ is based on the analysis of the new city quarters encompassing smaller residential units inhabited by the younger population (Rieselfeld, Vauban), which have been successfully developed through ‘building group’ (Baugruppen) models 
. Moreover, it represents a continuation of the already-developed, well-known sustainable strategies implemented in Freiburg, which are defined according to the following 12 principles within “The Freiburg Charter for Sustainable Urbanism”: diversity; safety and tolerance; neighborhoods; short distances; urban development along public transport routes/high-density model; education, science, and culture; commerce, economy, and employment; nature and environment; quality design and long-term planning; communication, reliability, obligation, and fairness; and cooperation, participation, and partnership 
. The strategies are implemented through diverse levels, e.g., regional/local; neighborhood/quarter; group of buildings/public space; or house/building.
In conclusion, owing to the well-planned nature of the local strategies, the principles of social sustainability have been implemented in cities, especially in relation to residential zones (e.g., mix of uses; communication/social spaces; accessibility; identity/neighborhood culture; nature in the immediate residential environment; quality of design/designing residential areas, streets, and/or squares; traffic; citizen participation; and innovative forms of accommodation). In addition, traffic and public-space-related policies have been specifically developed, prioritizing car reduction measures, the social use of street space and child-friendly environments, non-car travel networks and services, home-zone streets, shared-surface streets, etc. The authors of