A computer-aided design tool for assisting the designer to set appropriate passive solar systems for heating and cooling is presented. The system is based on a knowledge base oriented design process (KBDP). The knowledge base stores design guidelines and procedural methods for determining the passive systems that best suit the local climatic conditions. This tool is aimed to be used already at the very early stages of the design process, the pre-conceptual and the conceptual, with the purpose of achieving a passive solar architecture from the energy point of view that will better fit local climatic conditions. At the pre-conceptual design stage the system determined the bio-climatic strategies and at the conceptual design stage the recommended passive systems are presented according to previously selected design strategies. This paper focuses mainly on the later one i.e. the conceptual stage, in which the geometry, as well as the building orientation are determined. The geometrical considerations include the determination of the type and size of the passive systems that fit the requirements of both climatic conditions in winter and in summer at the given location. In addition, the design tool that was developed includes knowledge bases that contain examples and descriptive explanations. The knowledge base may be retrieved automatically by the system or upon request. Thus the system can support the designer as an expert that provides advice when needed. (C) 2008 Elsevier Ltd. All rights reserved.