Usable open space. For all permanent residential uses involving three or more housing units there shall be provided a usable open space area equal to the lessor of 500 square feet per housing unit or 10% of the gross lot area. The usable open space shall have no dimension less than 20 feet. The required open space shall be exclusive of driveways, drainage swales or other drainage facilities, parking lots or storage areas. The following additional regulations shall apply in determining the required usable open space:
(a) Swimming pools, gardens, patios and paved recreation areas may be developed in the required usable open space.
(b) An accessory building may not occupy any part of the required open space.
(c) The slope of any usable open space shall not exceed 10%. The open space may be provided on a deck or balcony.