Marin County adopts 2025 California Building Standards Code with stricter WUI exterior-material rules, effective 1 January 2026

The Marin County Board of Supervisors, acting at the county level for unincorporated areas, adopted the 2025 California Building Standards Code (Title 24) on 9 December 2025 via Ordinance No. 3838, with local amendments to the new Wildland-Urban Interface (WUI) provisions. The ordinance took effect 1 January 2026, the same date the statewide Title 24 update became mandatory across California. The amendments repeal and replace portions of Marin County Code Title 19 (Marin County Building Code) and apply to new permit applications submitted in unincorporated county territory.

What the state code introduced

The 2025 California Building Standards Code creates a standalone Wildland-Urban Interface Code, codified as Title 24, Part 7 - a significant structural change from prior editions, which scattered WUI requirements across multiple code books. The state code allows fire-retardant-treated wood (FRTW) as a compliant exterior wall material under Sections 504.5 and 504.5.2 of the California WUI Code. The 2025 California Building Standards Code is based on the International Building Code 2024 (IBC 2024) with California-specific amendments, and applies to all permit applications submitted on or after 1 January 2026.

Marin's local amendments

County building officials told the Board that local amendments were proposed to reduce fire risk by tightening allowable exterior materials and clarifying limits on repairs and replacements. Staff recommended removing the option to meet WUI requirements using fire-retardant-treated wood, citing maintenance and durability concerns - specifically, that FRTW "often requires ongoing maintenance that cannot be regulated through the initial building permit process."

The Board adopted a narrower version of the staff proposal. Ordinance No. 3838 prohibits FRTW as an exterior wall material for new construction in designated WUI and Fire Hazard Severity Zones, but does not extend the prohibition to remodels or additions, where FRTW remains a permitted compliance pathway. Several supervisors raised concerns about cost and aesthetic mismatches for existing wood-sided homes, which drove the decision to limit the ban to new construction.

The amendment applies under Title 24, Part 7, Chapter 5 (Special Building Construction Regulations), Sections 504.5 and 504.5.2. It affects general contractors, framers, and siding installers working on new residential and commercial construction in unincorporated Marin County WUI and Fire Hazard Severity Zones.

Permit applications already in progress

The code in effect at the time a complete permit application is submitted determines the standards that govern that project for its full duration. Applications accepted as complete before 1 January 2026 are reviewed under the 2022 California Building Standards Code. Applications submitted on or after that date must comply with the 2025 code, including Marin's local WUI amendments.

What to watch

At its 27 January 2026 meeting, the Board of Supervisors received a staff recommendation to amend Title 19 further - this time to remove the local FRTW prohibition and realign Marin County's exterior wall material requirements with the state WUI Code provisions in Sections 504.5 and 504.5.2, which allow FRTW as a compliance pathway. Whether that rollback was adopted, and on what timeline, is a determination for the authority having jurisdiction; practitioners should verify the current status of Marin County Code Section 19.04.064 directly with the county's Community Development Agency before specifying exterior wall materials for new WUI-zone projects.