This paper describes a usability-based building model we developed for behaviour simulation in design phase, and a systematic approach to implement it. Our model possesses not only geometric information of design elements but also non-geometric information of these elements, specifically, the relationships between the elements and the intended users. This relationship, which was implicitly understood by the designer, now becomes explicit to the simulated users. It utilizes CAD models and automates the conversion from CAD models to usability models and therefore requires minimal additional effort in preparing building data for behaviour simulation, which has proven to be efficient and effective.