The bill adds a new section to chapter 19.27 RCW directing the State Building Code Council to adopt by rule a mandatory appendix to the International Building Code that establishes a performance-based code compliance pathway for "middle housing buildings" (defined as residential buildings with one to 24 dwelling units, and that are one to six stories above grade plane and not classified as high-rise under IBC section 202). This is a procedural and regulatory change creating an alternative compliance framework (a performance-based code) rather than a change to criminal penalties. The council must convene a technical advisory group—including the Departments of Labor and Industries and Commerce, the University of Washington, Washington State University, and other subject matter experts—to develop recommended performance criteria categories and targets for building systems, subsystems, components, products, and materials. The advisory group must consider existing and emerging nationally recognized standards and certification frameworks (for example, work by the National Institute of Building Sciences). The council must adopt rules to recognize housing systems and plan certifications from independent, nationally recognized third-party evaluation or certification organizations; certifications so recognized are deemed to demonstrate compliance with applicable state building code provisions and must be accepted by local permitting authorities for review and inspection within the scope of the certification. The council must also seek to ensure the pathway supports integrated compliance across the IBC, Washington state energy codes, and rules addressing building-related greenhouse gas emissions, to the extent practicable and consistent with existing statutory authority. Required recommendations from the advisory group must be provided in time for the council to adopt or amend rules and codes as necessary for implementation in the first substantive update to the state building code occurring after January 1, 2027 (per RCW 19.27.031 and 19.27.032). The text does not include the specific performance criteria or technical requirements for the appendix, does not define the criteria for recognizing third-party organizations beyond requiring council rulemaking, and does not provide the detailed timing or procedural steps tied to the referenced RCWs.

If enacted, the State Building Code Council would add a required performance-based compliance pathway for middle housing (residential buildings of one to six stories that are not high-rises). That pathway would let builders and designers meet measurable performance targets instead of following only prescriptive code details, and the council must set rules to recognize qualifications from independent, nationally recognized third-party certification organizations that local permitting authorities would accept for permit review and inspection within the scope of those certifications. A technical advisory group including state agencies, the University of Washington, Washington State University, and other experts must develop the performance criteria and recommend how the pathway aligns with Washington’s energy and greenhouse gas rules, with rules adopted in time for the council’s first substantive code update after January 1, 2027. Directly affected parties include the State Building Code Council (new rulemaking and oversight duties), local permitting authorities (required to accept recognized third‑party certifications within scope), developers, designers, and builders of middle housing (new compliance options but likely extra up-front costs for third‑party certification), and certification organizations (opportunity to provide recognized compliance services). The law could increase design flexibility and shift some review work to accredited certifiers, but it also creates uncertainty because the specific performance targets, certification recognition criteria, and the practical alignment with energy and greenhouse gas rules are not defined in the text provided.