This paper addresses the problem of predicting and evaluating the impacts of the built environment on its human inhabitants. It presents a simulation system comprising a usability-based building model and an agent-based virtual user model. The building model represents both geometric information and usability properties of design elements, and is generated automatically from a standard CAD model. Virtual users are modelled as autonomous agents that emulate the appearance, perception, social traits and physical behaviour of real users (walking, sitting, meeting other virtual users, etc.). Their behaviour model is based upon theoretical and practical environment-behaviour studies, real world data from a field study, and Artificial Life research. By inserting the virtual users in the usability-enabled building model, and letting them “explorei it on their own volition, the system reveals the interrelationship between the environment and its users. The environment can then be modified, to see how different arrangements affect user behaviours.