In this paper, the FunctioneBehavioureStructure model of design previously proposed (Gero & Kannengiesser, 2004) is critically analysed and extended. Because the original FBS model does not properly take into consideration the cognitive processes occurring when a designer addresses needs and requirements, the most relevant contribution of the extended model is the capability to represent in detail reasoning about Needs Identification and Requirements Definition. While maintaining the ‘situatedness’ formalism adopted by Gero, the Needs Identification and the Requirements Definition phases are explicitly defined and modelled through the introduction of two new classes of variables, namely Needs (N) and Requirements (R). By introducing these two phases, the Formulation phase proposed in the original FBS framework must be reviewed in terms of those processes, which definitively substitute the direct processes from the external world of R variables to the interpreted world of Fi, Si,and Bi variables.
This revision allows eliminating an irregularity that appears in the original FBS model: the reference world and the variable changes occurring during the first step are made in a different way when compared with the rest of the model