Abstract: A structural building module is provided as a generally rectangular, hollow, extruded, interlocking tubular element formed of a plastic material. The module has two opposed first sides and two opposed second sides extending along a longitudinal axis. Two flanges project from one of the second sides of the element parallel to the first sides and extend along the longitudinal axis for the length of the element. Two recessed areas extend along the longitudinal axis for the length of the element from another of the second sides of the element and aligned with the two flanges. The flanges of one module interlock with the recessed areas of an adjacent module. Structures made with the modules may be assembled easily and quickly. Individual modules have a high stiffness to weight ratio, rendering the modules particularly appropriate for low cost residential housing, auxiliary buildings such as garden sheds or garages, or emergency huts or shelters.