ARMILLA5 is a generic computer aided design system, which supports the cooperative design of complex buildings (such as labs, offices or schools) over multiple levels of abstraction. It follows the metaphor of a virtual building site. The designers and engineers meet at a spatial location on the Internet and prepare the building construction by simulating the building site. This article describes the three essential components of the ARMILLA5-model: the geometric model which describes the spatial and physical aspects of the building site, the semantic model which implements passive building components as objects and active building components as applets or applications, and the planning model, which organizes the work steps of the individual engineers and their cooperation. The model is described using different software prototypes written in Objective C, CAD systems and HTML/JAVA.