The specification discloses a modular sun porch which can be easily and quickly assembled. The wall assembly includes a plurality of identical modular window panels whose lateral edges interfit in male/female relationship to provide weather seals therebetween. Modular door units, which have a width equal to one or more window panels, can be substituted for the window panels in the wall assembly and include frames which interfit with the male/female scheme. The base for the wall assembly is levelable and hides both the base tie-downs and the leveling mechanism. The roof includes rafters and purlins which slide-lock into position between beams and rafters, respectively. Roof panels rest on gaskets carried by the rafters and purlins, and retainers are secured to the rafters to retain the panels in position and to improve the weather seal.