There is established a supplemental district designated as the H hillside modifying district which may be attached to any existing zone in the City. Within the boundaries of any hillside modifying district, the permitted density and extent of grading for residential uses shall be determined by the average natural slope of the portion of the site to be placed in residential use in accordance with the hillside development graph (Figure 1). (Ord. 1512 § 2, 1973; Ord. 1212 § 1, 1969; prior code § 33.601 (A)(7)).