For more than a quarter of a century, building simulation programs have been developed to undertake non-trivial performance appraisals. In general these programs deal only with a small sub-set of the overall problem. However, advanced architectural developments require an integrated approach to design. The domains of heating, lighting, ventilation and acoustics, for example, are often closely related and it is only by taking into account their interactions that a complete understanding of building behaviour can be obtained. This paper describes some recent work to further the development of a multiple-domain approach.