An approach for designing and developing decision aiding systems is presented through the design and definition of the architecture of a system, corresponding to a real problem: that of providing aid to designers. The purpose of the decision aid is to provide assistance to computer system designers tackling interface usability problems and needing to know which modelling techniques out of an array would be more appropriate to apply. Different aspects of the design problem may call for different modelling techniques to be recruited. The design of the DDAS accommodates that reality. The architecture of DDAS is based on principles of systems thinking, in particular soft systems methodology, for eliciting and structuring knowledge relevant to a generic design space, and on fuzzy sets through test score semantics for representing and evaluating the meaning of relationships between components of the design problem and the modelling techniques. The advantage of this architecture is that it could be used for other problems where the decision maker, who is not necessarily an expert, wants to know what tools and techniques are appropriate for him and his particular problem.