Building an ADU in a fire hazard zone in California

Fire hazard severity zones

The State Fire Marshal maps fire hazard in three classes: moderate, high and very high. According to the State Fire Marshal's 2025 rulemaking, the maps are based on fuel, slope, fire weather and wind. The old 2007 maps marked only very high zones in cities. New maps covering all three classes in Local Responsibility Areas were sent to cities and counties in March 2025.

Cities then had to act. Government Code section 51179 requires a city to adopt moderate, high and very high zones by ordinance within 120 days of receiving the state's recommendations. A city can't lower a zone the state drew. It can only raise one or add areas, with a finding supported by substantial evidence for new very high areas.

The table follows the State Fire Marshal's emergency code text, readopted in April 2026. Cities can add areas, so confirm with yours. To find your zone, use your city's adopted map, not an older state viewer.

Can a city ban an ADU in a fire hazard zone?

Not on the map alone. Government Code section 66314(a) lets a city designate where ADUs are allowed in residential zones, but only based on water and sewer capacity and the impact on traffic flow and public safety.

HCD's ADU handbook addresses fire maps directly. It says some agencies have used high and very high fire hazard severity zone maps to restrict or prohibit ADUs, but that, according to CAL FIRE, the maps are meant to govern building materials and defensible space, not to restrict housing. A high fire zone “is not a conclusive rationale” for restricting ADUs, and any restriction should be backed by findings about the ADUs' impact on public safety.

The handbook adds two limits on local restrictions:

• State-guaranteed ADUs stay allowed. Designated areas can't preclude the units in section 66323: a conversion ADU, an 800 square foot detached ADU with 4-foot setbacks, and a junior ADU on a single-family lot, plus the multifamily options.
• A full stop needs HCD. To limit all ADUs, including 66323 units, a city must adopt a moratorium under Government Code section 66300, with HCD's approval, and only to protect against an imminent threat to health and safety.

What a city can do is apply objective standards, such as setbacks and fire access, and enforce the building and fire codes. That's where most fire-zone requirements come from. For the full statute, see California ADU laws and our summary of the HCD ADU handbook.

Chapter 7A and the WUI code

For years, wildfire construction rules lived in Chapter 7A of the California Building Code and Section R337 of the Residential Code. That changed on January 1, 2026. The 2025 Building Standards Code added Part 7, the California Wildland-Urban Interface Code, and the 2025 Residential Code's R337 is now just a note pointing there. If someone asks about “Chapter 7A” for your ADU, they mean these rules.

Does the WUI code apply to an ADU?

Yes, if your lot is in a covered area. Under the State Fire Marshal's emergency text, the code applies to new buildings with residential and similar uses, and to new structures accessory to them. An ADU is a dwelling, so it's covered. The exemption for Group U accessory buildings at least 50 feet from the house covers sheds and garages, not living space.

What it regulates

State law directs these standards at the parts of a building embers get into. The State Fire Marshal's rulemaking lists roofs, exterior walls, projections such as porches, decks, balconies and eaves, and openings such as attic and eave vents and windows. Eaves show the level of detail: the emergency text lists what may form the underside of open eaves, such as 2-inch nominal lumber, a layer of 5/8-inch Type X gypsum sheathing or a 1-hour rated exterior assembly. Your designer specifies the exact products against the code, and they cost more than standard materials.

Defensible space and Zone 0

Building to code is half of it. The other half is the land around the ADU. Government Code section 51182 requires owners of occupied buildings in very high fire hazard severity zones to maintain 100 feet of defensible space, with an ember-resistant zone within 5 feet of the structure. New and existing structures must meet the same ember-resistant standard.

The statewide details for that 5-foot zone, known as Zone 0, aren't in force yet. Wildfire Los Angeles reports that the Board of Forestry approved Zone 0 rules on August 19, 2026, then withdrew the package from state legal review on September 8, 2026, so it hasn't taken effect. Check the Board's status before you plan landscaping, and check your city: some, like San Diego, have adopted local Zone 0 rules already.

The statute measures defensible space from each side of the structure. A detached ADU is an occupied structure too, so plan the ground around it, not just around the house. On a tight lot, that shapes where the ADU goes and what can be planted or stored next to it.

ADU fire sprinklers

A fire zone doesn't change the sprinkler test under state ADU law. Section 66314 and section 66323 both say an ADU can't be required to have sprinklers if they aren't required for the main house, and building the ADU can't trigger sprinklers in the house.

That isn't the whole fire department review. Fire officials still check access and water supply, and some local programs require sprinklers on their own terms; San Diego's bonus ADUs are one, covered below. Sprinklers also shrink the building code's fire separation distances. Our ADU building code guide has the numbers.

Fire zone ADUs in San Diego

The City of San Diego updated its fire map in 2025 and its wildfire code in 2026.

• The 2025 map. The city's public notice says it adopted an updated Fire Hazard Severity Zone map by Ordinance O-21992, final passage July 31, 2025, including extra areas the State Fire Marshal hadn't mapped as very high. Building permit applications deemed complete after August 30, 2025, are subject to it. San Diego Fire-Rescue's map page links an interactive version.
• A 4-foot setback. Outside fire zones, a city ADU 16 feet or lower needs no interior side or rear setback. SDMC § 141.0302 requires 4 feet in high and very high zones, at any height, to keep defensible space between structures and native vegetation. The Fire Code Official can require more. See ADU setbacks.
• Bonus ADUs. The same section limits the city's ADU Home Density Bonus in high and very high zones: the lot must front an improved public street with at least two evacuation routes, and can't front a cul-de-sac or have only one way in and out. Bonus and affordable ADUs also need sprinklers.
• The city's WUI code. Ordinance O-22042, final passage January 29, 2026, adopted the state WUI code with local changes. It sets fuel modification zones of 0–5, 5–30 and 30–100 feet, requires them to extend at least 100 feet from a structure unless alternative compliance is approved, and adds local Zone 0 rules: no ember-prone plants, mulch, firewood, combustible boards or trellises within 5 feet. Those apply to new structures from the ordinance's effective date and to existing ones a year later.
• Brush management. IB-400 warns that more setback may be needed if a site sits in the Brush Management Zone.

According to the city's 2025 code fact sheet, the local WUI rules carry over the ignition-resistant construction from the old Chapter 7A and R337, with San Diego's earlier amendments. Builders who work in the city's canyon and hillside neighborhoods deal with these rules often; ask about fire-zone experience when you compare verified ADU builders in San Diego.

Outside the city, the County and each city adopt their own maps and WUI amendments. Check with your local building and fire departments.

SB 9 in fire zones

SB 9 is stricter than ADU law about fire. Government Code section 65852.21 requires an SB 9 two-unit project to meet the site rules in section 65913.4, which exclude very high fire hazard severity zones unless the site has adopted fire hazard mitigation measures under existing building standards, including the defensible space and Chapter 7A rules. A city also doesn't have to allow ADUs on a lot that uses both the SB 9 duplex and lot-split laws. Our SB 9 vs ADU guide compares the two paths.

Before you design

• Look up your zone on your city's adopted map, and note whether it's state or local responsibility land.
• Ask whether the main house has, or must have, sprinklers. That sets the ADU's requirement.
• Site the ADU for defensible space. Keep 5 feet clear of plants and combustibles, and plan for a 4-foot setback in San Diego's high and very high zones.
• Budget for ember-resistant materials and possibly brush clearing. See what an ADU costs in San Diego.
• Hire someone who has built in a fire zone. Read how to hire an ADU contractor and check a contractor's license.

Common questions