Growth Management Act.  The Growth Management Act (GMA) is the comprehensive land use planning framework for county and city governments in Washington.  Enacted in 1990 and 1991, the GMA establishes numerous planning requirements for counties and cities obligated by mandate or choice to fully plan under the GMA, and a reduced number of directives for all other counties and cities. Twenty-eight of Washington's 39 counties, and the cities within those counties, are planning jurisdictions.
 
Urban Growth Areas.  Counties that fully plan under the GMA must designate urban growth areas (UGAs), areas within which urban growth must be encouraged and outside of which growth may occur only if it is not urban in nature.  Planning jurisdictions must include within their UGAs sufficient areas and densities to accommodate projected urban growth for the succeeding 20-year period.  In addition, cities must include sufficient areas to accommodate the broad range of needs and uses that will accompany the projected urban growth, including, as appropriate, medical, governmental, institutional, commercial, service, retail, and other nonresidential uses.
 
Local Planning for Accessory Apartments.  Local governments are required to have accessory apartments, commonly referred to as ADUs, 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 (CTED) that provided recommendations to the Legislature designed to encourage the development and placement of accessory apartments in areas zoned for single-family residential use.  CTED recommendations include standards and criteria regarding size, parking, design, and quantity of accessory apartments. Local communities have some flexibility to adapt these recommendations to local needs and preferences.  "Local government" means a county planning under the GMA, a city with a population over 20,000, and a county with a population over 125,000.
 
Accessory Dwelling Unit Regulation.  In 2019, the Legislature encouraged fully planning cities to take an array of specified planning actions to increase residential building capacity.  Specified planning actions include authorizing attached and detached ADUs on all parcels containing single-family homes on lots of a certain size.
 
In 2020, the Legislature required any city within a county planning under the GMA that has not adopted or substantively amended its ADU regulations within the previous four years to adopt or amend ordinances, regulations, or other official controls that do not require the provision of off-street parking for ADUs within 0.25 mile of a major transit stop, with exceptions.

Any detached ADU located outside of an urban growth area will not be included in the calculation of housing density in that area if the detached ADU is subject to local development regulations that impose the following limitations:

• a parcel must not have more than one detached ADU;
• the location, size, and appearance of detached ADUs must be consistent with rural character and not interfere with any agricultural or other natural resource use of the property;
• detached ADUs must be in close proximity to the primary dwelling unit; and
• the square footage of a detached ADU may not exceed 50 percent of the square footage of the primary dwelling unit.

The provisions do not apply to any detached ADU in existence before the effective date of the act.