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, that are required or have chosen to plan under the GMA.

 
Counties that 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 planning county must be included in a UGA, and UGAs must include sufficient areas and densities to accommodate projected urban growth for the succeeding 20-year period. 

Comprehensive Plans.
The GMA also directs cities and counties planning under the GMA 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 in the GMA.  A comprehensive plan must be reviewed and, if necessary, revised every 10 years to ensure that it complies with the GMA.  

When developing comprehensive plans, counties and cities must consider specific planning goals related to certain subjects, such as urban growth, reduction of sprawl, transportation, and housing.  Each comprehensive plan must include certain mandatory elements, including elements related to land use, housing, capital facilities, utilities, rural areas, transportation, economic development, parks and recreation, and climate change and resiliency.

Countywide Planning Policies and Multicounty Planning Policies.
Counties that plan under the GMA must adopt a countywide planning policy in cooperation with the cities located within the county.  A countywide planning policy is a written policy statement used for establishing a countywide framework for developing and adopting comprehensive plans to ensure that city and county comprehensive plans are consistent.  Counties that each have a population of 450,000 or more and have contiguous urban areas must adopt multicounty planning policies.  Other counties may choose to adopt multicounty planning policies.

Counties planning under the GMA may authorize development of detached accessory dwelling units (DADUs) in rural areas on lots of any size, even where otherwise prohibited by a comprehensive plan, countywide planning policy, or multicounty planning policy, if the DADUs are subject to the following development regulations:

• A parcel may not have more than one accessory dwelling unit (ADU), whether attached or detached.
• The DADU must be subject to certain water supply requirements and may not be located on a parcel that uses a water source that is closed to further appropriation, must use water that is part of the water right for the primary dwelling, and withdrawals of water must be metered.
• The DADU may not be located within or encroach upon any existing critical area buffers.
• The building permit applicant must demonstrate that the sewage, septic, or on-site sewage system can handle the additional demand from the DADU.  If the DADU will be connected to an existing system, the system must be inspected by a licensed contractor prior to issuance of the building permit.
• The floor area of the DADU may not exceed the lesser of 1,296 square feet, or the square footage that could be authorized by the county as an expansion of the primary dwelling to create an attached ADU.
• The DADU must be constructed so that exterior material, roof form, window spacing, and proportions approximate those of the primary dwelling, unless the DADU is a manufactured/mobile home.
• The DADU must use the same driveway as the primary dwelling.
• The DADU must be sited to prevent loss of agricultural land or forestland.
• A parcel may not be subdivided for the purposes of avoiding these development regulations.

The comprehensive plan, countywide planning policy, or multicounty planning policy for any county that authorizes development of DADUs under the specified development regulations must be amended, at its next regularly scheduled update, to allow development of DADUs consistent with the specified development regulations.

Population growth from the development of DADUs that comply with the specified development regulations may not be counted for the purpose of determining whether a county is achieving rural or urban growth targets contained in a comprehensive plan, countywide planning policy, or multicounty planning policy.

The section in the GMA that requires cities and counties to protect agricultural lands of long-term commercial significance may not be interpreted to limit the development of DADUs that comply with the specified development regulations.