In this paper, we propose a new approach to simulating users’ behavior in yet un-built buildings. For this purpose, we have developed a model that connects two different representations: a formal representation of the building use, by means of a method borrowed from Business Process Modeling and Notation (BPMN) approach; and a game-engine based 3D virtual environment, where this process is effectively simulated and integrated with some autonomous behaviour of users/agents. The model has been applied to two test cases, where the activities of doctors, nurses, patients, and visitors in different hospitals were tracked, simulated, and reviewed by medical professionals for validation.