The aim of this thesis is to get acquainted with flat green roofs, their characteristics, development, application possibilities and construction technology. For the implementation of energy, economic and environmental evaluation of the implementation of the green roof, the existing public building with a flat roof was selected. Extensive roof greening has been proposed as an adequate solution. An extensive flat green roof is the simplest example of a green roof. It involves planting low and resistant vegetation that does not require special care. At the beginning of the thesis, the basic concepts related to energy consumption in buildings, energy efficiency and energy certification are described, with an emphasis on green roofs. Also in the first part of the thesis are described the types of green roofs and their characteristics, advantages and disadvantages of green roofs and the technology of performing the basic layers and performance details of flat green roofs. The second part presents and analyzes the existing condition of a public building with a flat roof and the projected condition after the construction of an extensive flat green roof. At the end, a cost estimate for the construction of a green roof was prepared and an analysis of the impact of the costs of building a green roof on the total costs of the building was performed. Energy evaluation of the implementation of the extensive flat green roof was carried out by preparing energy certificates for the existing condition of the building and for the situation after the construction of the extensive flat green roof. A double-legged steel staircase has also been designed to provide access to the flat green roof.