## 1 Introduction
Thereby, the study conducted in reference [13] should be emphasized, whose probabilistic net present value (NPV) analysis shows that there is a low financial risk for installing any type of green roof. Additionally, many other investigations [14; 15; 16] demonstrate that at the end of its lifetime, the NPV for the green roof is significantly lower when compared with conventional roofs. Determination of NPV of green roofs is usually analyzed through private or social (and environmental) benefits or taking into account both economic impacts, which often implies considering diversified cost and benefit effects. This paper deals with the analysis of the impact of green roofs on the energy performance of the existing building in the center of Belgrade. The potential private benefits of installing a green roof on such a flat roof are indicated. It can be said that green roofs are not widely represented in Serbia and that there is great potential for its application. This includes, among other things, the conversion of flat roofs to green rooftops on the existing buildings. According to a recent investigation [17], the distribution of the flat roof in the building stock in the Republic of Serbia is about 20%. However, in the central urban area of the capital city of Belgrade, that number reaches as much as 40%. Therefore, the economic feasibility of applying of a green roof system to this type of building is presented in this paper. Commonly, governments have a leading role in the implementation of green roofs. Therefore, the lack of institutional incentives can be a potential barrier to the implementation of a green roof system, especially for existing buildings. For the greenery project implementation, the barriers are similar to standard renovation projects. They are categorized as market, organizational, and economic related, and even in developed countries, policies are different for overcoming these issues [18]. In order to decrease the energy efficiency gap, reference [19] emphasizes that barriers' inner complex relations and transforming nature must be approached dynamically. Additionally, these obstacles could be further amplified in green roof implementation in countries with an undeveloped green industry. Fortunately, there are many examples of good practices in the world in terms of policies, programs, and projects that encourage the development and implementation of green roofing [20; 21; 22]. In Belgrade, the policy of supporting the use of green roofs is reflected through the strategic goals of the Development Strategy of the City of Belgrade 2021 and the pilot project \"Green roof of public buildings in the city of Belgrade\". In green roof implementation, policies are categorized as direct and indirect financial incentives but also ecological compensation measures with the aim to be integrated into standard development strategies, as presented in ref. [23]. In ref. [22], the incentive policies (mainly concentrated in Europe and North America) were classified into six different categories: tax reductions, financing, construction permits, sustainability certification, obligations by law, and agile administrative processes. In the distinct analysis for southeast Europe [24], state building typology, climate, period of construction, building service, and user behavior are determined as the main considerations for building retrofit packages. In this paper, the focus is on existing residential buildings with a flat roof in the urbanized area, with the presumption that the conclusions could be applied to some extent to other categories of residential buildings with a flat roof. The purpose of this type of research is also to provide background for the possible large-scale implementation as the cost-benefit analysis is regarded as helpful in raising awareness of environmental goods and in increasing the transparency of policymaking [25].