Abstract: A collapsible building including a semi-trailer chassis. At each side of the chassis a pair of sidewalls are foldable between a collapsed position on the chassis for transport and an outwardly extended in-use position. Ground-engaging support legs are provided at an outer end of each sidewall. In the extended in-use position, a collapsible floor extends between each pair of sidewalls, the floor having a number of hinged panels which collapse onto the chassis sliding along the sidewalls. A folding roof is formed by a number of rafters movable from a nested position on the chassis to extend out along a top of the sidewalls, slidably engaging the sidewalls. An end wall extends between free ends of the sidewalls.