Walks, drives, paved terraces, mechanical appurtenances for all single-family and two-family structures (such as air conditioners, venting, and service panels), and purely decorative garden accessories (such as pools, fountains, statuary, flagpoles, etc.), where subject to "permanent structure" classification, shall be permitted in setback and offset areas but not closer than three feet to an abutting property line other than a street line.