Every accessory building, except an accessory building constructed as an integral part of a main building, shall be located on the lot to the rear of the front line or extended front line of the main building, and no accessory building shall be located in or encroach upon any required yard, except that a detached accessory building may be located on the rear yard of the lot but not within three feet of any property line, and except that a detached common or joint garage may be erected on adjoining lots. No accessory building shall be located in any required side yard.