A minimum of 120 cubic feet of enclosed, ground-floor-accessible, private, lockable storage space shall be provided for each dwelling unit. Such storage space may be located in a storage shed or closet within, or affixed to and extending from, any side (end) or rear building wall that does not face a front lot line. A storage shed or closet affixed to a building wall shall not be deemed to encroach on the required minimum distance between buildings, provided that a minimum twenty-foot-wide clear corridor is maintained in the area between the buildings, that the heights of any storage extension does not exceed the height of the first floor of the building and that no storage extension protrudes from its building wall a distance of more than six feet.