A prefabricated building module includes a rectangular floor structure, three walls and a trussed roof structure enclosing a ceiling space with two end walls pivoted about the short sides of the floor structure, the third wall pivoted at its lower end to a long side of the floor structure and the roof structure pivoted at the upper end of said side wall such that said walls and roof structure fold onto the floor structure to form a flat pack with the same footprint as the floor structure. The roof structure comprises a ceiling and upright roof trusses pivoted to the ceiling which fold down within the plan of the roof and the walls have provision for doors and windows.