Growth Management Act. The Growth Management Act (GMA) is the comprehensive land use planning framework for counties and cities in Washington. The GMA establishes land-use designation and environmental protection requirements for all Washington counties and cities. The GMA also establishes a significantly wider array of planning duties for 28 counties, and the cities within those counties, obligated to satisfy all planning requirements of the GMA. These jurisdictions are sometimes said to be fully planning under the GMA.
 
Counties that fully plan under the GMA must designate urban growth areas (UGAs), within which urban growth must be encouraged, and outside of which growth may occur only if it is not urban in nature. Each city in a county must be included in an urban growth area. Planning jurisdictions must include within their UGAs sufficient areas and densities to accommodate projected urban growth for the succeeding 20-year period.
 
The GMA also directs fully planning jurisdictions to adopt internally consistent comprehensive land use plans. Comprehensive plans are implemented through locally adopted development regulations, and both the plans and the local regulations are subject to review and revision requirements prescribed in the GMA. Comprehensive plans must be reviewed and, if necessary, revised every ten years to ensure it complies with the GMA. When developing their comprehensive plans, counties and cities must consider various goals set forth in statute.
 
Each comprehensive plan must include a plan, scheme, or design for certain enumerated elements, including a mandatory housing element. The housing element must ensure the vitality and character of established residential neighborhoods and among other requirements consider the role of accessory dwelling units in meeting housing needs.
 
Urban Growth Areas. Counties that fully plan under the GMA are required to designate UGAs within their boundaries sufficient to accommodate a planned 20-year population projection range. Urban growth must be encouraged within the UGAs and only growth not urban in nature can occur outside of UGAs. Cities must include sufficient areas to accommodate the broad range of needs and uses that will accompany the projected urban growth. Urban governmental services cannot generally be extended to rural areas except in limited circumstances necessary to protect basic public health and safety and the environment.
 
Accessory Dwelling Units. Any county fully planning under the GMA, city with a population of over 20,000, or county with a population of over 125,000 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. However, 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 detached ADU is a dwelling unit that is separate and detached from another housing unit.

Counties fully planning under the GMA may allow detached ADUs outside of UGAs if the units are subject to development regulations that include the following limitations:

• a parcel must not have more than one detached ADU;
• the detached ADU is subject to water supply requirements established in the state building code;
• the applicant must provide documentation that the existing or proposed sewage or septic system is capable of handling the additional demand placed on the system by the detached ADU;
• the floor area of a detached ADU must not exceed the floor area of what would otherwise be authorized by the county as an expansion of the primary dwelling to create an attached ADU and is not greater than 1296 square feet;
• the detached ADU must be constructed with exterior materials, roof form, window spacing, and proportions approximate to those of the primary dwelling, unless the ADU is a mobile or manufactured home; and
• the detached ADU must use the same driveway as the primary dwelling.

 
The authority of a county to allow detached ADUs outside UGAs is in addition to existing county authority under the GMA and does not:

• affect or modify the validity of any county ordinance authorizing ADUs adopted prior to the effective date of the act;
• exclude other means of authorizing ADUs in urban or rural areas, if consistent with the act; and
• exclude other innovative techniques authorized under other provisions of the GMA, if consistent with the act of:
  1. clustering;
  2. planned unit developments; and
  3. other innovative zoning techniques.