What is an Accessory Dwelling Unit?

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According to the San Francisco Planning Department, "an Accessory Dwelling Unit (ADU) is a residential unit added to an existing building.  ADUs are subordinate to the primary residential unit(s), generally due to their location on the lot and/or the size of the unit.  Also known as Secondary Units, In-Law Units, Cottages, or Granny Flats, ADUs are generally developed using under utilized spaces within a lot, whether a garage, storage, rear yard, or an attic."