Building an ADU in a Fire Zone

Are ADUs required to comply with Wildfire codes?

Yes. If your property is located in a designated Wildland-Urban Interface (WUI) or a High Fire Hazard Severity Zone (HFHSZ), any new accessory structure must comply with the 2025 California Wildland-Urban Interface Code (Title 24, Part 7). This ensures the structure resists ignition from flying embers and direct flame contact.

What are the exterior wall requirements for an ADU in a fire zone?

Under Section 504.5 of the code, the exterior walls of your ADU must be built with noncombustible materials, heavy timber, or feature a 1-hour fire-resistance-rated assembly on the exterior side. Certain tested and approved ignition-resistant wall assemblies are also permitted.

Can I use standard windows on my ADU?

No. Standard glass shatters quickly under intense radiant heat. Section 504.8 requires all exterior windows and glazed doors to be constructed with multilayered glass (where at least one pane is tempered glass), glass blocks, or glazing that carries a minimum 20-minute fire-protection rating.

What type of roof and vents are required?

Your ADU must have a Class A fire-rated roof assembly (Section 504.2). Additionally, any ventilation openings for attics or crawl spaces must be fully covered with Wildfire Flame and Ember Resistant (WUI) vents approved by the State Fire Marshal (Section 504.10). Roof vents must use noncombustible, corrosion-resistant mesh with openings no larger than 1/8-inch to prevent burning embers from entering the building.

Am I allowed to build a wood deck attached to my ADU?

Yes, but the materials are strictly regulated to prevent the deck from catching fire and spreading it to the house. Section 504.7.3 requires the walking surfaces of decks, porches, and balconies to be constructed with noncombustible materials, approved ignition-resistant materials, exterior fire-retardant-treated wood, or heavy timber. Additionally, the underfloor area of the deck may need to be fully enclosed.