Call your county planning and development office to find out what the rules are for multi-unit properties. In my area it depends on the county. They have certain areas designated for multi-units. If you are having two separate entrances to the building that is considered multi-unit here. For related living, you can have anyone
stay with you. You also can rent a room to someone other than family and not be considered a multi-unit as long as they do not have a separate entrance.
The specifics do vary locally. Check with your county office.