A principal structure shall be set back at least ten (10) feet from a rear lot line, except where the rear lot line abuts a dedicated public alley or public easement which has a minimum right-of-way width of eighteen (18) feet, then a principal structure shall be set back at least five (5) feet from a rear lot line. Except as provided in Section 17.120.020, an accessory building shall be set back from a rear lot line at least five (5) feet.