This paper reviews the cooperating solver approach to building simulation as encapsulated within the ESP-r system. The application of the approach to the core technical domains underpinning building simulation is discussed along with its extension to the additional, diverse domains needed to support broader and/or more detailed analysis. The implications for computational overhead through expanding the repertoire of ESP-r are also considered. The paper concludes with an appraisal of the ability of the cooperating solver approach to cater for anticipated future application demands.