Where earth fill of twelve inches (12") or more is placed on property having a natural slope of five percent (5%) or less, or is abutting a street which has a slope of five percent (5%) or less, a retaining wall not to exceed thirty inches (30") in height shall be placed adjacent to the property line when the grading permit boundary abuts a side yard of adjacent properties, excepting the rear twenty five feet (25') thereof when the fill slope exceeds ten percent (10%) in any area within the side property line. The treatment of the rear twenty five feet (25') of a lot upon a permit boundary which abuts an adjacent side yard may comply with section 18.212.150 of this chapter. (Prior code § 86204(A))