Attached accessory structures shall be considered to be a part of the main building and the total ground floor and elevation of the combined structure shall be subject to all regulations governing the main building, except that an attached garage constructed on a lot existing at the time of passage of this chapter having less than the required area, width or depth shall be permitted to extend into the rear yard for its full length or width but not to exceed 22 feet, or 12 feet respectively, and shall be permitted to extend three feet into a required side yard, except on a corner lot, provided that it does not exceed 14 feet in height and has no windows in its side wall.