No accessory building shall be built upon any lot upon which there is not a principal building or structure. On any lot used for residential purposes, which such lot be located in a single-family, townhouse, or multifamily district, no accessory building shall exceed 200 square feet in floor or ground area for each residential unit that is located on the same lot except as hereinafter provided. Swimming pools and garages structures designed for the storage of motor vehicles shall not be restricted by the foregoing floor area limitation. Other structures, such as tool sheds, barns, greenhouses, cabanas, etc., which are larger than 200 square feet may be allowed upon approval of a site plan or waiver of site plan by the Planning Board.