The process of designing, constructing and managing buildings is fragmented, and involves many participants interacting in complex ways over a prolonged period of time. Currently, sequential communication among the participants is the norm. Consequently, while individual parts of the project may be optimized, the optimality of the overall project suffers. It is our view that the quality of the overall project can be significantly improved (in terms of time, money, and quality of design) if there was a tighter, non-sequential collaboration among the participants. Additional improvements will accrue if the participants were provided with discipline-specific design and evaluation tools, which assist them in performing their tasks. This paper describes the development of an integrated design environment, which is intended to facilitate such collaboration. It comprises a semantically-rich, object-oriented database, which forms the basis for shared design decisions. The database is augmented by knowledge-based query and update operators. Geometric and semantic editing tools round out the environment.