A prefabricated foldable building module including a central core comprising first and second opposing fixed walls with roof and floor beams interposed therebetween; a plurality of vertically foldably connected walls mounted upon the first and second walls foldable out therefrom; a pair of roof panels, each foldably connected adjacent said roof beam; a pair of floor panels each foldably connected adjacent floor beam: characterized in that the wall panels, roof panels and floor panels may be folded out from their supporting structures in a defined sequence to form an enclosed/partly enclosed building structure, and may be re-folded in reverse sequence to original configuration. Standard modules can be stretched, or joined side by side, end to end or one on top of the other to form larger buildings.