There shall be a rear yard upon each lot in an R-4 zone behind every main building of not less than fifteen feet; provided, however, except where the rear property line separates a lot in an R-4 zone from an adjacent lot zoned R-1, R-S, R-S-1A, MH, or a PUD project of single-family character with private rear and/or side yards, the rear yard may be reduced to five feet if not more than sixty percent of the lot is covered by buildings or structures. Whenever the rear property line separates the lot from a lot described in the above exception, the rear yard shall not be less than ten feet for the first story and twenty-five feet for any second and additional stories. (Ord. 2980 § 3, 1985; prior code § 17.19.060)