Nowadays a significant part of energy is consumed in buildings. In order to reduce the energy consumption different retrofit technologies are applied. As every building and every energy consumer is different the same modernization technic can not be used for all buildings. This paper shows a computer aided opportunity which is able to simulate different modernization technic for the same building in order to the best opportunity can be selected. The paper highlights the advantages of computeraided building design and the opportunities of optimization techniques in a new under design building case. This paper also contains examples in order to introduce this modelling method.