The Semantic Modelling Extension (SME) prototype implements a unique approach to integrated architectural CAD that places the drawing act first in the design process. After drawing a design idea using a computer graphic system, the designer interprets the design, providing semantic content to the graphic entities. An interpretation expresses the meaning of the design with respect to a particular issue, such as structural sufficiency, energy consumption, or requirements for egress, and provides reasoning to evaluate the design addressing that issue. A design may have many interpretations to express the multiple issues that are relevant in a design project. The designer may add or delete interpretations of the design as issues change during the course of the project. Underlying the SME prototype are the concepts of form, function and behaviour. In the prototype, evaluation of a design is done by deriving behaviour from the graphically represented forms and relating the behaviour to stated functions or requirements. The concepts of interpretations and form, function and behaviour together establish a virtual product model for design. In contrast to component based approaches to product modelling that tightly bind form representations to their behaviour and function, a virtual product model allows the designer to manipulate the relations among these three descriptors of a design, and thus manipulate the semantics of the design entities. By distinguishing between the act of proposing a design by drawing the conceived form and the act of assigning meaning to the form, the virtual product model approach supports both graphic thinking for design synthesis and symbolic reasoning for design evaluation. This paper presents a scenario of the use of the SME prototype in building design, provides an analysis of the design process and computational support described in the scenario, contrasts a virtual product model approach with a component-oriented product model approach, describes the software implementation of SME, and presents implications and conclusions regarding design process and technical integration.