Wiktionary
mother-in-law apartment
n. A small apartment attached to or carved out of a nominally single-family house, ostensibly intended for occupancy by a mother-in-law or other relative, but potentially also rented out to a stranger.
Usage examples of "mother-in-law apartment".
Her old beat-up Datsun, parked on four flat tires in front of the mobile home, was like a little annex, a mother-in-law apartment.