The aim of this paper is to create an energy certificate of the designed state of energy renovation of a public building. The building in question is an accompanying building within the sports and recreational purpose in Osijek. Six measures have been proposed to improve the energy performanc of the building in order to reduce energy consumption, reduce costs and reduce CO2 emissions, namely: replacement of part of the existing carpentry, thermal insulation of external walls, thermal insulation of flat roof, extensive green roof, thermal insulation of the floor on the ground and the installation of photovoltaic modules on a flat roof. In the implementation of measures to improve energy efficiency, it is necessary to adhere to applicable regulations and regulations, while paying attention to the economic viability of individual measures. The first part of the paper describes the basic concepts related to energy, energy consumption, systematic energy management, building systems that consume energy and typical examples of good practice in increasing the energy efficiency of buildings. The legislative framework for the implementation of energy efficiency and the history of the introduction of the legislative framework related to energy efficiency in the Republic of Croatia are also presented. The second part of the paper analyzes the current condition of the public building in Osijek and energy consumption and energy consumption, energy prices and CO2 emissions in the application of individual measures to improve energy efficiency, as well as all measures together. Finally, the energy consumption in the projected state was compared with the actual energy consumption according to the collected data, and it can be seen that the application of measures reduces heat consumption, energy prices and CO2 emissions by more than 80 percent compared to current actual consumption.