Abstract— Retrofitting of existing buildings can result in energy savings, although it is difficult to predict real benefits. As showed on the example of the Polish school theoretical effect of improvements (59-71%) could differ significantly from the achieved reduction in energy consumption (33%). Situation is much more complicated if we consider ancient buildings. They are often high energy consuming structures, however some technical and legal constraints could hinder a process of modernization. This paper discusses issues connected with retrofitting of an old factory building that has been adopted for a school, apartments and offices several years ago. Possible to apply improvements regarding the structure, HVAC systems were discusses, with emphasizing main technical problems as well as legal ones resulting from a heritage conservator’s recommendations. Part of requirements leads to a significant increase of investment costs, whereas others make few improvements not possible, for example isolation of external walls. Ecological effect of retrofitting was estimated as 51% of CO2 emission reduction..