This paper deals with using knowledge in order to support the conceptual stage of the design process. Both a visual language composed of design drawings and the internal graph-based structures representing these drawings are specified on the basis of the specified conceptualization of the design domain. The defined ontological commitment enables to transform knowledge about drawings encoded in their internal representations into logic formulas. The extracted facts concerning drawings together with axioms describing general domain-oriented design knowledge allow the design support system to reason about validity of created design solutions. The approach is illustrated on examples of designing floor layouts.