Abstract: A trailer, such as a conventional full trailer or semi trailer used for hauling freight, is convertible into an assembly hall by folding its sidewalls upwardly to define ceiling extensions extending from the trailer roof. Additional floor extensions which are pivotally affixed to the sides of the trailer floor may then be folded downwardly in substantially parallel relation to the trailer floor to define an assembly hall floor. The floor extensions fold in nested relation between the folding trailer sidewalls/ceiling extensions such that the entire arrangement fits within the same space envelope as the conventional trailer. Once unfolded, the trailer may be used as a shelter/assembly hall for meetings and other events, and may be provided with interior seating, safety railings, and other enhancements.