For the energy transition to be successful, it will also be important to secure heat supply of the housing stock by sustainable technologies. About half of the apartments in Germany is located in apartment buildings. However, backfitting with modern building shells or heat pumps lags behind. The cooperation project “LowEx-Bestand” coordinated by KIT and Fraunhofer ISE is aimed at developing commercially viable concepts and demonstrating their operation quality. It is funded with about EUR 5.3 million by the Federal Ministry for Economic Affairs and Energy.