accessory apartment

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Also called in-law apartment; especially British, granny flat.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for granny flat

granny flat

noun
  1. self-contained accommodation within or built onto a house, suitable for an elderly parentAlso called: granny annexe
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012