ADUs in Fire Hazard Severity Zones (LA).

Large parts of Los Angeles County are mapped as Fire Hazard Severity Zones, or FHSZs. Many homeowners in the hills and canyons wonder whether they can build an ADU at all. The short answer: state law limits how far a city can go in banning them, but fire safety rules still add requirements.

LA ADU Guide is an information site. This is not fire engineering or legal advice. Rules in this area are changing, so confirm with your fire and building departments.

Can a city ban ADUs in a fire zone?

Not easily. Gov. Code section 66314(a) lets a local agency designate areas where ADUs may be permitted only based on "the impact of accessory dwelling units on traffic flow and public safety." HCD's technical guidance states that fire hazard zones should not by themselves restrict ADUs.

This came to a head in Los Angeles. The City ordinance said "No ADU is permitted on any lot that is located in both a Very High Fire Hazard Severity Zone ... and a Hillside Area." In November 2024, HCD found this out of line with state law. It said the City "has not adequately demonstrated that ADUs will impact public safety in the VHFHSZ" and must either remove the ban or adopt, by resolution, "specific evidence of areas within the VHFHSZ that would impact public safety."

We did not find a confirmed record of how the City resolved this. A 2024 draft ordinance in City files kept a hillside-plus-VHFHSZ limit with a sprinkler-based path, but we could not confirm what was adopted. Ask City Planning what rule applies to your lot today.

Conversions and small ADUs

Section 66323 requires ministerial approval of certain ADUs, including conversions of existing space and one detached ADU with four-foot setbacks. The HCD letter to Los Angeles notes that front setbacks cannot be imposed on ADUs and JADUs under section 66323(a). These state allowances apply to lots in fire zones too, unless the city has made specific findings under section 66314(a).

Sprinklers

The State Fire Marshal's March 2025 bulletin gives the baseline:

• "All ADUs, attached or detached, require fire sprinklers if the primary dwelling is equipped with a fire sprinkler system."
• Adding an ADU "shall not trigger fire sprinklers to be installed in the primary dwelling."
• "Automatic residential sprinkler systems are required in all newly constructed dwellings including those built with an ADU."

So a new detached ADU on a lot where the main house has no sprinklers may or may not need them, depending on how the local agency applies the rules. A conversion of existing space is treated differently from new construction. Confirm with the fire department that reviews your project.

SB 897 (2022) also says building an ADU does not change a home's residential occupancy classification unless the local agency finds an adverse effect on public health and safety.

Access and water supply

The same bulletin says newly constructed ADUs "are required to comply with the standards for fire protection such as water supply and fire department access."

For unincorporated LA County, the Fire Department's guideline EG-07 (revised November 26, 2024) says:

• Buildings in a Fire Hazard Severity Zone shall have fire flow of not less than 1,250 gallons per minute for a 1-hour duration at 20 psi.
• All portions of a new ADU shall be within 150 feet of an approved vehicular access roadway.

These figures come from the County document and apply to areas the County Fire Department serves. They are not the City of Los Angeles rules. The City is served by the Los Angeles Fire Department, which has its own requirements. We did not find a City source we could cite, so ask LAFD or LADBS.

Building materials in a fire zone

Homes in mapped zones, and in Wildland-Urban Interface areas, must meet California wildfire-resistant construction standards for new buildings. Typical items include Class A roofing, ember-resistant vents and ignition-resistant exterior materials. LADBS and LA County Public Works publish current construction notes for fire zones. Ask for the version in force on the day you apply, since new code cycles take effect and local amendments change.

SB 9 and fire zones

SB 9 is treated differently from ADU law. LA County's summary says an SB 9 project is not permitted in a Very High FHSZ, or High or Very High zone, "unless all new and existing structures on the project site will be in compliance with fire hazard mitigation measures." Properties in the Palisades and Eaton fire boundaries are also excluded under Executive Order N-32-25. See SB 9 lot splits.

Fire rules at a glance

How to check your lot

1. Look up your address on CAL FIRE's Fire Hazard Severity Zone map, and on the City or County mapping tools. Note that the City of Los Angeles designates its own VHFHSZ under section 51178.
2. Find out which agency permits and which fire department reviews you.
3. Ask whether your lot is also in a Hillside Area, which can add grading and access rules.
4. Ask the permit counter for the written code sections that apply to ADUs in your zone.
5. Check whether your insurer requires anything additional. We have no source on this, so ask your insurer.

Checklist

• Confirm the zone and your permitting agency.
• Ask whether sprinklers are required, citing the State Fire Marshal bulletin.
• Check road access distance and fire flow.
• Ask what roofing and vent standards apply.
• Get the answers in writing before design.
• Vet any contractor through the state board, as in hiring an ADU builder.

For the wider framework, see California ADU rules and permit process in Los Angeles. More in the ADU rules hub. This page is general information only.