At the CABD LAB at the University of Rome, an interactive expert system for architecture is being implemented to supervise building design at every stage of development. The system operates by checking the consistency of design choices against given sets of constraints, and by automatically checking the design process. It is therefore an innovation with respect to current architectural software developed as specific design aids. The system is based on a general representation of building objects (from components to the whole building) by means of semantic nets and a set of inferential procedures. The general representation is developed by making explicit the relational structures according to which architects organize their knowledge about building objects. To do this, the `Frame'formalism is used: this is a knowledge representation technique used in the field of artificial intelligence. It is then shown that such an expert CAAD system is a general purpose tool for architectural design, enabling architects to assess any constraint and/or building attribute by means of a declarative method, which in no way affects their own specific design methodologies