Where the district regulations specify a minimum of useable open space for each dwelling unit or a building site, the calculation of useable open space shall be made by deducting from the total area of the building site (A) all the area included as coverage pursuant to Section 17.52.380; (B) all areas paved to provide parking spaces, required driveways and maneuvering areas; (C) any remaining area having a ground slope in excess of twenty (20) percent; and (D) any open space less than ten feet in its least dimension. To the remainder may be added any roof top or outside deck spaces more than seven feet in least dimension which are directly accessible to and safely useable by occupants of the dwelling. (Prior gen. code § 8-60.51)