When in the establishment of zones certain properties are included which cannot enjoy the same possibilities for usage that other parcels in the same zone do, thus causing practical difficulties and unusual hardships because of some physical problems, such as size of lot, shape of lot, slope of terrain or condition of the soil, a variance may be issued to permit the subject properties the rights and privileges generally allowed in the zone. The variances shall be limited to modifications in property development standards such as height, yards, setbacks and coverage. A variance may not be issued to permit a use not specifically listed in any zone.
(`78 Code, § 17.96.010.)