One ADU within the portions of existing multifamily dwelling structures that are not used as livable space, including, but not limited to, storage rooms, boiler rooms, passageways, attics, basements, or garages, if each unit complies with state building standards for dwellings. If requested, multiple ADUs shall be allowed, up to the number of ADUs that equals twenty-five percent of the existing multifamily dwelling units in the structure.