On open land, all facilities and structures for accessory purposes (such as swimming pools, tennis courts, garages, carports, parking areas, lamp posts, small sheds for tools or sports equipment, fences, including the kind enclosing a tennis court or swimming pool, bath houses, and other accessory structures for accessory purposes) shall be subject to the approval of the Planning Board as to their number, design, locations, uses, and sizes; provided, however, that all such facilities and structures, including roadways and driveways, shall not involve the use of more than 20% of all of the open land on the parcel.