In current architectural practice some important changes are taking place because of the development of numerous Building Performance Simulations (BPS) tools to support design decisions during early phases of the design process. Many difficulties still persist, however, not necessarily due to the limitations of the available technology, but to the lack of appropriate methodologies to use the existing tools to improve the decision making process, particularly at the early design stages. In this work we present an application of performance-based design in early design phases, with the purpose to take better-informed decisions which would ultimately contribute to improve the energy performance of buildings.