Urban planning and city modeling, once demanding tasks in terms of required skills and supporting information are now supported by efficient, widely available applications and have become more popular and accessible than ever. Many GIS systems offer nowadays a freely navigable three dimensional environment. The evolution of GIS systems has lead to the creation of virtual 3D models of the ground and buildings. Online internet communities have been created and libraries with georeferenced 3D models have become well known. People are encouraged to participate in the design of a 3D scene and many commercial companies offer free tools for the design of 3D models, specialized in buildings. In this paper, we present a collaborative approach for constructing a city model, and its implementation as a prototype environment in cooperating with free designing tools. The prototype system comprises a collaborative database, supported by a website. Users are able to create and upload their models to the collaborative database. They may collaborate in order to construct a realistic 3D city model in a given area.