The paper proposes a novel approach for a scale-dependent geometric simplification of 3D building models that are an integral part of virtual cities. In contrast to real-time photorealistic visualisations, map-like presentations emphasize the specific cartographic properties of objects. For buildings objects, such properties are e.g. the parallel and right-angled arrangements of facade walls and the symmetries of the roof structure. To a map, a clear visual percept ion of the spatial situation is more important than a detailed reflection of reality. Therefore, the simplification of a 3D building model must be the transformation of the object into its global shape. We present a two-stage algorithm for such an object-specific simplification, which combines primitive instancing and cell decomposition to recreate a basic building model that best fits the objects original shape.