Abstract: A building unit for providing shelter, storage space, and the like generally includes a floor panel, two side wall panels, two end panels, and a roof panel. The floor panel has a pair of floor supports extending along its exterior surface. The floor supports define a passageway therethrough for receiving air. Side brackets releasably mount the two side wall panels to the floor panel and roof panel. Roof brackets matingly engage upwardly extending flanges of the roof panel to stabilize the building structure and seal the interior of the building unit against leaks. The roof brackets also interlock with the side brackets to stabilize the building unit. A building unit may be erected adjacent to other building units to form a modular building assembly.