Abstract: Three dimensional pre-fabricated self-supporting elements, each being one-quarter portion of a room, are formed with a floor section, a ceiling section and at least two interposed wall sections.These elements are integrally pre-formed with stiffening structural ribs or ribs shaped to provide support means for differently shaped pieces of furniture; with door and window apertures; with channel passages, pipe fittings and the like for receiving pipes and electrical wiring for utilities; and interengaging fitting means formed integrally on the outside of those floor, ceiling, and wall sections to fasten together the elements forming a room, or a number of rooms together to form residences of one or more floors.