The research work presented here is concerned with the development of a knowledge based system to support preliminary design. Preliminary design is the first stage in the design of buildings during which a number of structural schemes are formulated and accessed. It is a task that requires experience, knowledge and creativity as much is unknown and ill-defined, and it involves activities and decisions that are heuristic in nature. The system is currently being developed within KappaPC, the chosen expert system toolkit, using object oriented methodology. This paper discusses the approach taken in modeling preliminary design, the KappaPC environment, and the implementation of the design model within that environment. The approach presented here could be applied in other areas of design where the problem is ill-structured with many unknowns and a variety of constraints of varying importance.