Heat pumps are a key technology in bringing renewable shares into heat supply of buildings; especially their combination with onsite renewable electricity production e.g. by PV allows to bring high renewable shares. Their current installation in existing multi-apartment buildings is however still marginal. The proposed technological solution is based on decentralized heat pumps, in such a way that it results an easy-to-install solution for installers, low-intrusive for the occupants and easily adaptable to a large number of different building contexts. This is supported by developing planning, implementation and operation processes, in order to facilitate the work during the planning phase, ensure a high-quality installation, effective operation and reduce the effort and costs along the whole retrofit project.  Through 3 demo sites (in Spain, Italy and Austria), the project will demonstrate a highly versatile, scalable and replicable solution package for buildings retrofit, reaching 70-75% of renewable energy fraction, 30-50% of PE and GHG emissions savings, reduction of planning time by 50% and installation/operation time by 30%.