ADUs, or accessory dwelling units, are separate, supplementary dwellings that are often located on single-family property. An ADU can be a separate area on the same property as the primary residence, or it can be an apartment inside the main house. The terms accessory apartments, garage apartments, carriage houses, and backyard cottages are also sometimes used to refer to accessory dwelling units, which are also known as granny or mother-in-law flats.