mother-in-law apartment

Noun

 * 1) 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.
 * 2) * "Accessory dwelling units are commonly referred to as mother-in-law apartments. Accessory dwelling units (ADUs) are a second dwelling on a piece of property where a primary residence already exists." —King County (Washington) Online Directory