A minimum of one ADU and a maximum of up to 25% of the number of existing multi-family dwelling units are permitted within existing multi-family dwelling structures. The ADUs must be within the portions the existing multi-family dwelling structure 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.