In graduation thesis I investigated the parameters that play a key role in the classification of a building in energy class and carried out a survey on the educational facility Osnovna šola Šmarjeta. Mainly due to changes in legislation, the general protection of the environment and expensive energy, construction and renovation trend of buildings focuses on low-energy buildings and that is why the thesis topic is very relevant. In this case, I was wondering which parameters are most crucial for energy renovation and how they affect on energy consumption for space heating, hot water, etc. In order to assess the existing and renovated state I used a computer program TEDI and TOST, developed by the Department of Building and construction elements UL FGG. With TOST and TEDI I calculated the heat transfer factor of construction elements, specific heat loss coefficient, the annual primary energy consumption, annual heat required for heating and annual energy required for cooling – for existing situation and the situation after renovation of the building. I found that the various measures, which are carried out at renovation, are effective differently. With the combination of these measures, I managed to lower the annual heat required for heating by almost half, but the building still does not meet all requirements to be met by low-energy buildings. I came to conclusion that each building at energy renovation, can not be rationally transformed into low-energy facility and rigid law is not lenient to such cases.