Abstract: Prefabricated building units for constructing a small building such as a caravan annexe include a mounting rail which may be supported at roof height, for example on a caravan side, and a number of rectangular roofing panels, each having across its head an attachment piece releasably engageable pivotally with the mounting rail when the roofing panel is at an angle to horizontal greater than the pitch of the roof to be installed, sealing means being brought into effect when the roofing panel is raised pivotally to the required pitch. The units include rectangular wall panels each with an attachment piece across its head engageable, as set out, with a mounting rail across the foot of each roofing panel. Opposite sides of each roofing panel and wall panel have respectively, a weather capping, and a weather flange engageable in the weather capping of an adjacent like panel.