An Accessory Dwelling Unit (ADU) is a room or set of rooms in a single-family home in a single-family zone that has been designed or configured to be used as a separate dwelling unit and has been established by permit.
ADUs generally include living, sleeping, kitchen and bathroom facilities and have a lockable entrance door. A detached ADU is the same concept, but the living area is in a separate structure, normally in a backyard cottage of some sort.
ADUs are different than a legal duplex. A legal duplex is a home with two dwelling units in a multi-family zone. A legal duplex will not have restriction on rentals, while an ADU often requires the property owner to live in one of the dwelling units. The City of Seattle has procedures in place to establish an attached ADU or a detached ADU.