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. Specifically, it mandates that at least one middle housing unit must be allowed on each parcel that permits single-family residences in designated urban growth areas and limited areas of more intensive rural development. The bill stipulates that if a county opts to allow middle housing, it cannot authorize more than four residential units per lot in these areas. Additionally, the development regulations must not impose more restrictive standards on middle housing than those for detached single-family residences and must ensure that middle housing is served by sewer service.

The bill also includes provisions that exempt the adoption of ordinances and development regulations related to middle housing from administrative or judicial appeals. It amends existing laws to clarify that actions taken by a county to implement the new middle housing regulations are not subject to appeals, thereby streamlining the process for counties to adopt these regulations. Furthermore, the bill updates the growth management hearings board's authority to include noncompliance petitions related to the new middle housing provisions, ensuring that these regulations are integrated into the broader framework of growth management in Washington.