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.  Each city in a county must be included in a UGA.  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.  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 mandatory elements, including a housing element.  The housing element must ensure the vitality and character of established residential neighborhoods.

By July 1, 2024, cities planning under the Growth Management Act may not prohibit, within a residential zone that allows for the development of detached single-family residences, the splitting of a single residential lot into two residential lots if:

• the resulting lots are at least 1,500 square feet;
• the resulting lots are at least 40 percent of the size of the original lot;
• the original lot was not created by splitting a single residential lot authorized by this Act;
• the lot split would not require demolition or alteration of any housing that is rent restricted, rent subsidized, or that has been occupied by a tenant paying market-rate rent within the preceding 12 months; and
• any construction on the resulting lots is subject to all existing state and local laws regarding stormwater runoff, critical areas, shorelines, and conservation areas.

A fully planning city also may not impose regulations on a residential lot that is the result of splitting a single lot that:

• requires more than one off-street parking space per lot;
• requires more than 20 feet of frontage width per lot;
• requires easement widths of more than four feet for access to rear lots;
• imposes permitting requirements, design standards, or impact fees on construction on a lot resulting from a lot split that are greater than those imposed on new residential construction generally within the same zone; or
• imposes requirements for dedications of rights-of-way or for the construction of off-site improvements.

Any conflicting provisions in local development regulations after July 1, 2024, are superseded, preempted, and invalidated.