Among architects the question is being discussed in how far new buildings can be designed in a way to make more economical use of energy by architectural means. Solar houses in the USA are often taken as a model. As yet it is unclear how such measures will affect heat demand in the central European climate and with domestic building materials being used. A computer simulation program is introduced by which these questions can be answered as early as in the stage of planning. The program can be run on a common microcomputersystem.