Any county fully planning under the Growth Management Act (GMA) or with a population of over 125,000, and any city with a population of over 20,000, must have accessory dwelling unit (ADU) provisions incorporated in their development regulations, zoning regulations, or official controls.  These provisions must be consistent with a 1993 report from the Department of Community, Trade, and Economic Development that provided recommendations to encourage the development and placement of ADUs in areas zoned for single-family residential use.  Local communities have some flexibility to adapt these recommendations to local needs and preferences. 
 
As of July 1, 2021, fully planning cities under the GMA may not require the provision of off-street parking for ADUs within a quarter mile of a major transit stop, such as a high capacity transportation system stop, a rail stop, or certain bus stops, unless the city determines that on-street parking is infeasible for the ADU.
 
An ADU is a residential living unit providing independent living facilities and permanent provisions for sleeping, cooking, sanitation, and living on the same lot as a single-family home, duplex, triplex, townhome, or other housing unit.  A short-term rental is a lodging use, outside of a hotel, motel, or bed and breakfast, in which a room is offered for a fee for fewer than 30 consecutive nights.

As of July 1, 2024, any fully planning county or city under the GMA may not impose or enforce an owner-occupancy requirement on any housing unit on a lot containing an ADU, unless the ADU is being offered for short-term rental.  If such a county or city amended or updated its comprehensive plan regulations relating to ADUs after July 1, 2021, the deadline is extended to the county or city's next comprehensive plan update.  After the applicable deadline has passed, the prohibition on imposing an owner-occupancy requirement on lots with ADUs takes effect in any city or county that has failed to take action as required.