Prefabricated, steel framed, rectangular building modules are joined in groups to form multi-unit residences, or commercial or institutional buildings. The modules are disposed in rectangular groups abutting one another so as to form a central rectangular core bounded by the units. Utilities are connected to each module through the core. Multiple story buildings are formed by similar groups having cores aligned vertically. Alternating stories are rotated relative to one another about the central core axis to produce cantilevered structures.