The goal of the described research is an interactive computer system for the design of customized mass housing. Shape grammars are the formalism proposed to systematize the design rules required for such a system. A shape grammar for Alvaro Siza's patio houses at Malagueira, a 1200-unit development still being designed and constructed today, is presented. The grammar is based on the corpus of thirty-five houses designed between 1977 and 1996. The generation of houses in the grammar proceeds by the recursive dissection of rectangles locating four different functional zones (patio, living, services, and sleeping) and the key placement of the staircase. The schematic generations of two existing houses and the detailed generation of a novel one illustrate the grammar.