Multiple ADUs 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. At least one ADU within an existing multifamily dwelling shall be permitted, with the maximum number of ADUs limited to 25% of the existing multifamily dwelling units. All interior ADUs must be converted from non-livable space.