The bill authorizes counties in Washington that are required or choose to plan under RCW 36.70A.040 to incorporate middle housing into their development regulations and zoning ordinances. Specifically, it mandates that at least one middle housing unit must be allowed on each parcel that permits single-family residences within designated urban growth areas and certain limited areas of more intensive rural development. The bill stipulates that counties may not authorize more than four residential units per lot in these limited areas and outlines that the standards for middle housing cannot be more restrictive than those for detached single-family residences. Additionally, it requires that middle housing in these areas be served by sewer service and that the same development permit and environmental review processes applicable to single-family residences also apply to middle housing.

Furthermore, the bill amends existing laws to clarify that the adoption of ordinances and development regulations to implement these middle housing provisions will not be subject to administrative or judicial appeals. It also reenacts and amends sections of RCW 43.21C.495 and RCW 36.70A.280, ensuring that actions taken by cities or counties to implement the new middle housing regulations are exempt from such appeals. This legislative change aims to facilitate the development of middle housing options, thereby addressing housing shortages and promoting more diverse residential communities.