Over the last few decades the climate on our planet has changed and the awareness of the consequences of climate changes caused by increased concentration of greenhouse gases in the atmosphere is becoming more developed. Priority in the field of energy and construction becomes energy efficiency in buildings, sustainable construction and application of renewable energy sources because the building sector is the largest individual energy consumer. The energy refurbishment of existing buildings and the construction of new energy-efficient buildings is the main topic in the field of construction, environmental protection and sustainable development due to the possibility of large energy savings in this sector. A large amount of energy can be saved by changing the behavior of the building user. The selection of measures to improve energy efficiency depends on the energy status and type of building, the way the building is used and the location of the building. Significant energy savings are achieved by implementing measures to improve energy efficiency. Also, the operation and greenhouse gases emissions are reduced, the environment is protected, the durability of the building increases, a more comfortable indoor climate in the building is achieved, the quality of life is increased and the health of the people is preserved. The subject of this graduate work is the one-story apartment building built in 1957. Measures to improve energy efficiency have been suggested on the basis of analysis and calculating of the current building state. The evaluation was also made after the implementation of the each measure. The work compares the results of the evaluation of the existing building energy performance and the situation after the implementation of the measures. This evaluation of building energy performance was done in respect to the Ordinance on energy audits and energy certification of buildings ( Official Gazette 48/14, 150/14, 133/15, 22/16, 49/16, 87/16, 17/17) and the Tehnical regulation on energy economy and heat retention in buildings (Official Gazette 128/15).