We introduce the outline of a multi-agent model that can be used for visualizing simulated user behavior to support the assessment of design performance. We will consider various performance indicators of building environments, which are related to user reaction to design decisions. This system may serve as a media tool in the design process for a better understanding of what the design will look like, especially for those cases where design or planning decisions will affect the behavior of individuals. The system is based on cellular automata and multi-agent simulation technology. The system simulates how agents move around in a particular 3D (or 2D) environment, in which space is represented as a lattice of cells. Agents represent objects or people with their own behavior, moving over the network. Each agent will be located in a simulated space, based on the cellular automata grid. Each iteration of the simulation is based on a parallel update of the agents conforming local rules. Agents positioned within an environment will need sensors to perceive their local neighborhood and some means with which to affect the environment. In this way, autonomous individuals and the interaction between them can be simulated by the system. As a result, designers can use the system to assess the likely consequences of their design decisions on user behavior. We think that the system provides a potentially valuable tool to support design and decision-making processes, related to user behavior in architecture and urban planning.