Where earth fill of twelve inches (12") or more is placed on property having a natural average slope of five percent (5%) or less, a retaining wall not to exceed five feet (5') above the original natural grade shall be placed adjacent to the property lines at locations where a grading permit boundary line abuts vacant property, or the rear yard area of adjacent properties when the slope of the finish grade exceeds ten percent (10%) within twenty feet (20') of the permit boundary line. (Prior code § 86203(A))