In addition to those uses permitted in the district with which it is combined, one secondary dwelling unit per building site shall be permitted only if approved by the board of zoning adjustments as provided in Section 17.54.130 on parcels zoned for not more than one dwelling and having one but no more than one existing dwelling unit on the parcel subject to the minimum requirements specified in Section 17.30.110. (Ord. 2002-60 § 1 (part): Prior gen. code § 8-44.10.2)