The research concerns renovation variants for modernist housing estates built in Szczecin, Poland during 1918−1925 and in Bialystok, Poland during 1950−1990. These buildings are now substandard in many ways; functionally, aesthetically, technically, and ecologically they do not fulfil the current energy efficiency standards. Some of them have architectural heritage, so not all energy-saving technologies can be applied. Renovations must include energy-saving improvements and the use of renewable energy sources. Equally important is the well-being of residents, meaning the quality of the apartments should be increased. The aim of this research was to analyze the renovation options in terms of energy efficiency and well-being criteria, as well as in relation to the cultural value of the buildings. The simplified energy calculation method was used to check the present buildings’ energy demands to compare them with retrofitting results. Three retrofitting possibilities were considered: low-cost, current standards, and near-zero energy. The results show that without EU financial aid, which will soon be introduced under the “Renovation Wave” program, such modernization projects will be difficult, making the target of 55% CO2 emission reductions compared to 1990 levels by 2030 impossible.