ADU within non-livable space in existing multifamily structure. 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, more than one ADU shall be allowed, up to the number of ADUs that equals 25% of the existing multifamily dwelling units in the structure.