Compared with newly constructed buildings, in retrofit the conditions of the existing building have to be particularly regarded and the focus has to be on the reduction of the energy consumption of the building. Therefore the aim is the improvement of the thermal quality of the building envelope and the implementation of an innovative energy production and energy supply system. The improvement of the thermal quality includes the reduction of the thermal losses (transmission and ventilation) and the consideration of the necessary measures, which are based on an in-depth investigation. So the thermal envelope has to be optimized regarding the geometry, heat losses and thermal bridges. This will be done by different studies. At the same time the requirements of an optimized solar energy production (solar thermal and photovoltaic) have to be considered as well: a comparison of different measures should deliver the advantages and disadvantages of each solution. With this information the decision will be eased.