In this diploma thesis I have carried out an energy renovation of a new extension at elementary school Loka in Črnomelj. Currently, the basic legislative document regulating the area of energy efficiency in newly constructed as also renovated buildings in Slovenia is PURES 2010 (Regulations on energy efficiency of buildings). In compliance with its requirements, calculations are made in TOST computer programme, in which I have checked the energy efficiency balance of the building in consideration. Final results have confirmed the assumption that the building does not meet the minimum requirements of PURES 2010. To improve the existing state, I have applied certain measures alone and in combination. The chosen combination of measures which lowered existing critical values to those still allowed involved installing windows with lower heat transfer coefficient, applying extra thermal insulation on external walls and installing a mechanical ventilation system. Since, after 31st December 2018, every newly built public facility will need to comply with nearly zero-energy building (nZEB) requirements, I additionally checked if the building in consideration which now meets PURES 2010 requirements also meets those of nZEB. The nZEB action plan states the annual maximum permitted energy use for heating, annual primary energy necessary for the functioning of the building and the minimum share of renewable energy sources. Although implementing the measure, the building does not meet all nZEB requirements in the action plan due to the reason that, for public buildings, PURES 2010 does not set forth the annual maximums permitted neither for primary energy use nor for cooling systems.