The reconstruction of 3D city models has matured in recent years from a research topic and niche market to commercial products and services. When constructing models on a large scale, it is inevitable to have reconstruction tools available that offer a high level of automation and reliably produce valid models within the required accuracy. In this paper, we present a 3D building reconstruction approach, which produces LOD2 models from existing ground plans and airborne LIDAR data. As well-formed roof structures are of high priority to us, we developed an approach that constructs models by assembling building blocks from a library of parameterized standard shapes. The basis of our work is a 2D partitioning algorithm that splits a building’s footprint into nonintersecting, mostly quadrangular sections. A particular challenge thereby is to generate a partitioning of the footprint that approximates the general shape of the outline with as few pieces as possible. Once at hand, each piece is given a roof shape that best fits the LIDAR points in its area and integrates well with the neighbouring pieces. An implementation of the approach is used now for quite some time in a production environment and many commercial projects have been successfully completed. The second part of this paper reflects the experiences that we have made with this approach working on the 3D reconstruction of the entire cities of East Berlin and Cologne.