This paper describes a generative evolutionary design system that aims to fulfil two key requirements: customisability and scalability. Customisability is required in order to allow the design team to incorporate personalised and idiosyncratic rules and representations. Scalability is required in order to allow large complex designs to be generated and evolved without performance being adversely affected. In order to fulfil these requirements, a computational architecture has been developed that differs significantly from existing evolutionary systems. In order to verify the feasibility of the this architecture, the generative process capable of creating three-dimensional building models has been implemented and demonstrated.