This paper concludes a series of three, in which the formalization of “soft” architectural design knowledge is studied in the context of making it accessible to knowledge-based design systems for support of early sketch design. A wide range of design ideas with the common theme of schematic “plaza design” is studied, including design from scratch and the reconstruction of existing plazas. Formal conceptual models in Horn clause logic are suggested. The additional evidence thus provided, that “soft” design knowledge may be accessible to design support systems, is the main contribution of the paper, which is rounded off by a brief discussion of the feasibility and future development of such systems.