The assessment of the sustainability of buildings and cities is at the forefront of the environmental analysis of the built environment. Buildings are essential components of cities, where people spend a substantial part of their lives living, working, studying or relaxing. However, the same buildings and the whole construction industry are also responsible for considerable energy consumption, greenhouse gas emissions and waste generation. Sustainable development seeks to find “a balance between economic development, environmental protection and social improvement”, and the main aim of sustainable urban development is to create “beautiful, distinctive, secure, healthy and high-quality places for people to live and work in that foster a strong sense of community, pride, social equity, integration and identity”.
The assessment of the sustainability of buildings and cities is at the forefront of the environmental analysis of the built environment. At this point, at the European and World level, a significant number of research projects, standards and methods have been developed, to assess the sustainability aspects. At the same time, all techniques to assess buildings are digitized, as we are moving fast to the Industry 4.0 era, where all buildings-related information should be easily communicated among designers, users and stakeholders.
Buildings are essential components of cities, where people spend a substantial part of their lives living, working, studying or relaxing. However, the same buildings and the whole construction industry are also responsible for considerable energy consumption, greenhouse gas emissions and waste generation. Therefore, how the buildings and entire cities are designed, constructed and configured to operate is crucial for reducing the environmental impact of urbanization, improving the health and well-being of the population and achieving the Sustainable Development Goals 
adopted by 193 countries.
The concept of sustainable cities has evolved over time and has been defined in different ways by different scholars and organizations. According to one of the initial explanations of sustainable development, it is “a development that meets the needs of the present without compromising the ability of future generations to meet their own needs” 
. In other words, sustainable development seeks to find “a balance between economic development, environmental protection and social improvement” 
, and the main aim of sustainable urban development is to create “beautiful, distinctive, secure, healthy and high-quality places for people to live and work in that foster a strong sense of community, pride, social equity, integration and identity” 
. Recently, there has been a growing recognition that achieving this certain objective is not solely reliant on tangible measures but “based on the principles of democracy, gender equality, solidarity, the rule of law and respect for fundamental rights, including freedom and equal opportunities for all” 
2. Approaches to Sustainable Buildings and Cities
Several approaches towards sustainable buildings and cities were identified throughout the analyzed literature, depending on the context, priorities and resources available. The most common approaches are presented below.
it is one of the leading paradigms of both sustainable development 
and the New Urbanism movement 
. The compactness of the city can be defined in the following three aspects 
. Firstly, the urban form should be defined by high-density settlements, fewer dependences on automobiles and clear boundaries from the surrounding areas. Then spatial features should encourage mixed land use, diversity of life as well as clear and unique identities. Finally, social functions should aim for social equality, self-sufficiency in daily life and independence of government. The main critique of this approach is that compact cities, in order to reduce sprawl and minimize environmental impact, promote densification, whereas low-density urban forms are often considered to be more livable. However, that critique has been denied 
as residents from compact cities are more satisfied with their neighborhood because, despite high density, this model also provides a better public transport network, accessibility and a variety of land uses.
the concept of eco-city is also broad; there are many overlaps with other approaches. Still, the following ten critical eco-city dimensions can be distinguished 
: compact and mixed-use urban form, an abundance of the natural environment, walking and cycling infrastructure, extensive environmental technologies for water, energy and waste management, the central city with subcenters, high-quality public realm, human scale physical environment, innovation and driven economy, visionary—“debate and decide” and sustainability-based decision making. Essentially, eco-cities focus on environmental sustainability, promoting green infrastructure, renewable energy and zero-waste strategies while addressing social and economic issues 
resilience, in terms of cities, refers to the ability to absorb, adapt and respond to changes in an urban system. For this reason, cities should be conceptualized as complex adaptive systems and divided into components and analytical elements. Hence, this systematic approach allows a better understanding of how urban system design, planning and management work towards resiliency enhancements 
. Overall, resilient cities focus on the adaptation to the challenges posed by climate change, such as extreme weather, natural disasters or sea-level rise, ensuring the continuity of essential services and minimizing the impact on the population 
offer innovative services based on broadband communication and service-oriented computing 
. Digital cities were built and made operational throughout the developed countries between the 1990s and the 2000s. Digital cities are distinguished by activities based on online services 
. The fast spread of developing digital technologies, digital service creation and delivery necessitate new and more organized approaches to service design, development and management. There are numerous perspectives and strategic elements for developing digital services from diverse stakeholders, with different solutions stating how to design the digital service inside IT infrastructures or how to reuse design techniques learnt from prior Digital City initiatives 
According to a literature review, Digital City and Smart City are the most commonly used terms to describe a city’s smartness. Smart cities appear to be the inevitable successors of digital cities.
there are many definitions of smart cities. The reason for that is the application of the term for two different kinds of domains: “hard” and “soft” 
. The “hard” one includes buildings, energy grids, natural resources, water and waste management and mobility, while the “soft one” covers culture, education, policy innovations, social inclusion and governments. Depending on the domain, the role of information and communication technologies (ICTs) is also different—decisive for the “hard” and not so much in the “soft” domain. Anyway, the common conception of smart cities is the use of digital technologies to optimize urban systems, such as transport, energy, water and waste management, improving efficiency, reducing costs and enhancing citizens’ quality of life 
the World Health Organization (WHO) evolved the concept of “Healthy Cities” to improve city-based public health and environmental hygiene with a special focus on marginalized urban areas 
. “Healthy Cities Project” was launched in Europe in the 1980s and has spread globally. The main principle of the initiative was that “health can be improved by modifying living conditions, namely, the physical environment and the social and economic conditions of everyday life” 
. Eventually, the tools to measure the index of healthy cities were developed 
, and the index’s indicators fall into four main sectors: health, health services, socioeconomic indicators and environmental indicators. The latter is strongly dependent on urban development strategies. To succeed in the creation of healthy and sustainable cities, urban development has to promote access to green spaces, sports and leisure facilities, mitigation measures of air, water and noise pollution, walkability and cycling and public transportation modes. The observed spatial inequality can also reveal the existence of social inequality 
Differences in various approaches to sustainable buildings and cities are presented in Table 1.
Quantitative literature analysis and graphical visualization and analysis—knowledge mapping—were applied to understand better the current research situation and research frontiers 
in the mentioned areas. According to Chen 
, the frontier of research reveals the emergence of theoretical trends and new topics. According to Price 
, the research frontier is the dynamic nature and ideological status of the research field; generally, the research frontier consists of approximately 40 or 50 recently published scientific papers. Knowledge mapping is part of the broader field of science metrology and is defined as a cross-disciplinary field of applied mathematics, information science and computer science; the purpose of it is to extract and visually reorganize the knowledge from a large number of previously published scientific research documents and to carry out knowledge discovery