A set of prefabricated, interlocking plastics components for construction of a building of any shape is described. The components are preferably rotationally moulded or extruded as regular shapes (rectangles, triangles or hexagons) from thermoplastics materials. Moulding allows each component to be comprised of more than one region or layer of material, or to include a void. The building can be carried to a site as individual or aggregated components as dictated by the transport facilities available, then erected at the site. Disassembly is facilitated if non-adhesive fasteners are used to secure the tongue and groove joints.