Firm location is an important component in integrated land use and transport models. However, representation of firm behavior and firm locations is often too aggregated to model firm behavior adequately. A calibrated simulation approach that models dynamics in the firm population at the level of individual firms is presented. A number of firm demographic events and transitions in the state of individual firms are simulated: firm migration, growth, formation, and dissolution. This provides the opportunity to account for firm-specific behavior and allows a large variety in responses to changes in urban environment. The model quantifies the effects of different spatial and transport planning scenarios on firm population and mobility. Moreover, the firm-level simulation output provides improved possibilities to evaluate the impact of spatial scenarios. It can be linked to an urban transport model to obtain a dynamic simulation of urban development and mobility. The model specification is presented, as well as the calibration of the individual firm demographic processes. The behavior of the calibrated model is verified with multiple test runs.