If it was built by her it could be more complicated as this apartment could have been meant to accomodate step-mom's relatives or something. But it was built before so it's pretty simple - she has no business going there.

Of course she has business there -- either getting clean and/or whatever women do in showers (the OP may have an especially stimulating detachable shower head ;) )

In california, my uncle converted a large shed (330sq ft) to a 1-floor tenancy and rented it out. The person who rented it got free electricity, free internet, free water and free gas (my uncle is a plumber by trade) which has its own full bath, small kitchen, large sliding-door closet and its own heat/AC system. I imagine this is what he’s referring to is a unit like this. Its almost completely stand-alone. Has its own path from front driveway to its door.