In all districts, no building and no part of a building shall be erected within or shall project into the side yard prescribed hereunder, except cornices, steps, bay windows, eaves, gutters or chimneys projecting not more than 24 inches. In all districts, except in those as hereinafter provided, one accessory building may be built or constructed within one of the side yards if entirely separated from the main building and located at least 10 feet farther back from the front street line than the rearmost portion of the main building and at least five feet from any property line, and provided that the placement of such accessory building, in the opinion of the Township Engineer, does not aggravate existing water runoff or drainage. In all districts, except in those as hereinafter provided, if there exists a common wall between buildings, an accessory building may be built within two feet of the common property line of such, and provided that the placement of such accessory building, in the opinion of the Township Engineer, does not aggravate existing water runoff or drainage.