The project concerns the energy upgrade of the municipal indoor gym of the Municipal District of Polygyros, the Municipality of Polygyros. It is a prefabricated reinforced concrete building, built around the end of the 1980s. The building currently exhibits considerably reduced thermal comfort conditions throughout the year. The energy inspection of the building showed that energy consumption in the building is significant, placing the building in category D of the Building Energy Efficiency Regulation (KENAK).This project provides for the necessary interventions for the energy upgrading of the building. These include interventions in the building shell (change of frames, thermal insulation of faces and roof), modification of the building’s heating installation (application of a hybrid system with boiler and heat pump), installation of solar panels for the production of hot water for use (ZNX), installation of photovoltaic system and interconnection with PPC for energy netting, replacement of luminaires and installation of automatic control system.