(1)(a) Cities and counties planning under this chapter must adopt or amend by ordinance, and incorporate into their development regulations, zoning regulations, and other official controls the requirements of this section and of RCW 36.70A.681, to take effect six months after the jurisdiction's next periodic comprehensive plan update required under RCW 36.70A.130.

(b) In any city or county that has not adopted or amended ordinances, regulations, or other official controls as required under this section, the requirements of this section and RCW 36.70A.681 supersede, preempt, and invalidate any conflicting local development regulations.

(2) Ordinances, development regulations, and other official controls adopted or amended pursuant to this section and RCW 36.70A.681 must only apply in the portions of towns, cities, and counties that are within urban growth areas designated under this chapter.

(3) Any action taken by a city or county to comply with the requirements of this section or RCW 36.70A.681 is not subject to legal challenge under this chapter or chapter 43.21C RCW.

(4) Nothing in this section or RCW 36.70A.681 requires or authorizes a city or county to authorize the construction of an accessory dwelling unit 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.

(5) Nothing in this section or in RCW 36.70A.681 prohibits a city or county from:

(a) Restricting the use of accessory dwelling units for short-term rentals;

(b) Applying public health, safety, building code, and environmental permitting requirements to an accessory dwelling unit that would be applicable to the principal unit, including regulations to protect ground and surface waters from on-site wastewater;

(c) Applying generally applicable development regulations to the construction of an accessory unit, except when the application of such regulations would be contrary to this section or to RCW 36.70A.681;

(d) Prohibiting the construction of accessory dwelling units on lots that are not connected to or served by public sewers; or

(e) Prohibiting or restricting the construction of accessory dwelling units in residential zones with a density of one dwelling unit per acre or less that are within areas designated as wetlands, fish and wildlife habitats, flood plains, or geologically hazardous areas.

Findings—Intent—2023 c 334: "(1) The legislature makes the following findings:

(a) Washington state is experiencing a housing affordability crisis. Many communities across the state are in need of more housing for renters across the income spectrum.

(b) Many cities dedicate the majority of residentially zoned land to single detached houses that are increasingly financially out of reach for many households. Due to their smaller size, accessory dwelling units can provide a more affordable housing option in those single-family zones.

(c) Localities can start to correct for historic economic and racial exclusion in single-family zones by opening up these neighborhoods to more diverse housing types, including accessory dwelling units, that provide lower cost homes. Increasing housing options in expensive, high-opportunity neighborhoods will give more families access to schools, parks, and other public amenities otherwise accessible to only the wealthy.

(d) Accessory dwelling units are frequently rented below market rate, providing additional affordable housing options for renters.

(e) Accessory dwelling units can also help to provide housing for very low-income households. More than 10 percent of accessory dwelling units in some areas are occupied by tenants who pay no rent at all; among these tenants are grandparents, adult children, family members with disabilities, friends going through life transitions, and community members in need. Accessory dwelling units meet the needs of these people who might otherwise require subsidized housing space and resources.

(f) Accessory dwelling units can meet the needs of Washington's growing senior population, making it possible for this population to age in their communities by offering senior-friendly housing, which prioritizes physical accessibility, in walkable communities near amenities essential to successful aging in place, including transit and grocery stores, without requiring costly renovations of existing housing stock.

(g) Homeowners who add an accessory dwelling unit may benefit from added income and an increased sense of security.

(h) Accessory dwelling units provide environmental benefits. On average they are more energy efficient than single detached houses, and they incentivize adaptive reuse of existing homes and materials.

(i) Siting accessory dwelling units near transit hubs, employment centers, and public amenities can help to reduce greenhouse gas emissions by increasing walkability, shortening household commutes, and curtailing sprawl.

(2) The legislature intends to promote and encourage the creation of accessory dwelling units as a means to address the need for additional affordable housing options." [ 2023 c 334 § 1.]