Abstract. The paper emphasizes possibilities for substantially reducing energy consumption with modern ecofriendly buildings. Passive building construction is a sector of the construction industry that has extremely low demand for the energy for heating house interiors. A passive house requires a small amount of energy to provide thermal comfort, but it requires proper systems (HRU, heat pumps, solar collectors) to accomplish that effect. The modification proposal presented in the paper has dramatically reduced the demand for heat capacity of the building. Unfortunately the passive standard has not been reached, but thermomodernisation would allow for a significant reduction of heating costs. The demand per 1 m2 of heated surface area in a traditional building was 41.9 W/m2, while after the modification -only 15.01 W/m2. The tested building, if located in a warmer climate, with an average outside temperature of ≥ -10ºC would certainly perform its function. The use of modern technology ensures high energy savings. Such solutions, however, are not the cheapest on the market, and therefore are less available. The construction of a passive solar buildings is a "green" investment with tangible environmental results. By ensuring the thermomodernisation of a traditional building we can enjoy real environmental and economic benefits.