mother-in-law apartment
noun /ˈmʌðər ɪn lɔː əpɑːtmənt/
/ˈmʌðər ɪn lɔː əpɑːrtmənt/
(also in-law apartment, in-law suite)
(all North American English) (British English granny flat)
(informal)- a set of rooms for an old person, especially in a relative’s house