There shall be a rear yard upon each lot in an R-3 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-3 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 fifty-five 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 be not less than ten feet for the first story and twenty-five feet for any second and additional stories. (Ord. 2980 § 2, 1985; prior code § 17.17.060)