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 are required to designate urban growth areas (UGAs) within their boundaries sufficient to accommodate a planned 20-year population projection range provided by the Office of Financial Management (OFM).  Each city located within a planning county must be included within a UGA.
 
The GMA directs fully planning jurisdictions to adopt internally consistent comprehensive land use plans.  Comprehensive plans must contain certain required elements including a housing element that ensures the vitality and character of established residential neighborhoods.  Comprehensive plans are implemented through locally adopted development regulations, and both must be reviewed and revised every ten years.
 
Density Requirements.  No later than six months after its next required comprehensive plan update, fully planning cities meeting population requirements must allow for the development of a minimum number of units on all residential lots, unless zoning permitting higher densities or intensities applies, as follows:

• cities with a population of at least 75,000 must include authorization for at least: four units per lot; six units per lot if at least two of the units are affordable housing; and six units per lot within one-quarter mile walking distance of a major transit stop;
• cities with a population of at least 25,000 but less than 75,000 must include authorization for at least: two units per lot; four units per lot if at least one unit is affordable housing; and four units per lot within one-quarter mile walking distance of a major transit stop; and
• cities with a population less than 25,000, within a contiguous UGA with the largest city in a county with a population of more than 275,000, must include authorization for the development of at least two units per lot.

A major transit stop is defined as a stop on a high-capacity transportation system, commuter rail stops, stops on rail or fixed guideway systems, and stops on bus rapid transit routes.
 
The density requirements do not apply to:

• lots designated with critical areas or their buffers;
• a watershed serving a reservoir for potable water if that watershed is listed, as of July 23, 2023, as impaired or threatened under the federal Clean Water Act; or
• lots that have been designated urban separators by countywide planning policies as of July 23, 2023. 

Middle Housing Requirements.  A city must allow at least six of the nine of middle housing types and may allow accessory dwelling units to achieve the unit density requirements. Middle housing is defined as buildings that are compatible in scale, form, and character with single-family homes and contain two or more attached, stacked, or clustered homes including duplexes, triplexes, fourplexes, fiveplexes, sixplexes, townhouses, stacked flats, courtyard apartments, and cottage housing.  The GMA defines courtyard apartments as up to four attached dwelling units on two or three sides of a court.
 
Any city subject to the middle housing requirements may not require:

• standards for middle housing that are more restrictive than those required for detached single-family residences, but may apply objective development regulations that are required for detached single-family residences, including but not limited to set-back, lot coverage, stormwater, clearing, and tree canopy and retention requirements to ensure compliance with existing ordinances intended to protect critical areas and public health and safety;
• off-street parking as a condition of permitting development of middle housing within one-half mile walking distance of a major transit stop;
• more than one off-street parking space per unit as a condition of permitting development of middle housing on lots smaller than 6000 square feet before any zero lot line subdivisions or lot splits;  and
• more than two off-street parking spaces per unit as a condition of permitting development of middle housing on lots greater than 6000 square feet before any zero lot line subdivisions or lot splits. 

The parking provisions do not apply:

• if the local government submits to the Department of Commerce (Commerce) an empirical study prepared by credentialed transportation or land use planning expert that clearly demonstrates, and Commerce finds and certifies, the application of parking provisions in a defined area would be significantly less safe for vehicle drivers or passengers, pedestrians, or bicyclists than if the jurisdiction's parking requirements were applied in the same location for the same number of detached houses; or
• to portions of cities within a one-mile radius of a commercial airport with at least 9 million annual enplanements.

Density Requirements.  The minimum density requirements for residential lots within one-quarter miles walking distance of a major transit stop includes any bus rapid transit route stops under construction.

The exemption from minimum density requirements for lots with critical areas or their buffers is limited to that portion of a lot, parcel, or tract with a critical area or buffer except for critical aquifer recharge areas where a single-family detached house is an allowed use provided that any requirements to maintain aquifer recharge are met.  Until June 30, 2026, any additional residential capacity required by lots, parcels, or tracts with critical areas or critical area buffers may not be considered an inconsistency with countywide planning policies, multicounty planning policies, or growth targets.
 
Middle Housing Requirements.  A city with at least 25,000 population that is subject to minimum residential density requirements must allow at least six of the nine types of middle housing.  A city with less than 25,000 population may choose the number of types of middle housing to allow to meet minimum residential density requirements.  The four-unit limit in the definition of courtyard apartments is removed. 
 
In applying objective development regulations to middle housing, fully planning cities may apply regulations related to set-back, lot coverage, stormwater, clearing, and tree canopy and retention requirements, and language related to compliance with existing ordinances intended to protect critical areas and public health and safety is removed.
 
A city may not require more than one off-street parking space per unit for middle housing constructed on lots no greater than 6000 square feet before any zero lot line subdivisions or lot splits.
 
Lots created through the splitting of a single residential lot are exempt from the middle housing and minimum density requirements.