Urban planning and urban design are separated disciplines. As a consequence, there is hardly any feedback from the urban design process to the urban planning process. To improve interaction between these two, an interactive urban design (IUD) tool has been developed. The tool is implemented in a Desk-Cave allowing for direct manipulation of masses and with immediate feedback on the urban planning programme performance. In a small-scale experiment, the tool proved to improve compliance of the urban design with the planning requirements without obstructing the design process.