During the last years research on the design of cost-efficient lowest energy houses had been finalized and in September 2006 the occupation of the first multifamily house started. The paper describes the general rules of the design strategies, the tools used to calculate energy performance and indoor comfort. Especially the question, if a flat can be heated only by heating the ventilated air, is discussed. Regarding the energy performance, different methods for calculating the energy demand for heating and hot water are discussed. Using a whole building simulation tool with 300 zones, the monthly energy balances for each of the 39 flats and the monthly energy balance for the whole house are compared. Finaly the assumptions for calculating the energy performance according the Austrian rules for the Energy Performance Certificate are discussed.