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 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. Planning jurisdictions must include within their UGAs sufficient areas and densities to accommodate projected urban growth for the succeeding 20-year period.

Accessory Dwelling Units. Any city or county fully planning under the GMA must ensure that within a UGA, local development regulations allow for the construction of accessory dwelling units (ADUs).  Cities and counties may apply certain regulations to ADUs, including:

• generally applicable development regulations;
• public health, safety, building code, and environmental permitting requirements that would be applicable to the principal unit, including regulations to protect ground and surface waters from on-site wastewater;
• a prohibition on the construction of ADUs that are not connected to or served by public sewers;
• a prohibition or restriction on the construction of ADUs in residential zones with a density of one dwelling unit per acre or less within areas designated as wetlands, fish and wildlife habitats, flood plains, or geologically hazardous areas; and
• restrictions on the use of ADUs for short-term rentals.

A city or county may not authorize the construction of an ADU in a location where development is restricted under other laws, rules, or ordinances as a result of physical proximity to on-site sewage system infrastructure, critical areas, or other unsuitable physical characteristics of a property.

An ADU is a dwelling unit located on the same lot as a single-family housing unit, duplex, triplex, townhome, or other housing unit. A detached ADU is a dwelling unit that is separate and detached from another housing unit and is located on the same property. 

Counties must 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 attached or 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 located within the same acre as the primary dwelling unit and utilize the same driveway as the primary dwelling; and
• the detached ADU must be sited to prevent loss of land designated as agricultural land or forestland under this chapter.

A parcel may not be subdivided for the purposes of avoiding the limits on development regulations established by this act.

Counties may allow detached ADUs by adopting development regulations substantially similar to those in effect as of January 1, 2024, in a county with a population exceeding 2 million.

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.
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