What changes for an ADU in a Very High Fire Hazard Severity Zone?

A fire hazard zone does not by itself rule out an ADU, but the ADU must meet California's wildfire construction standards, which are now in the 2025 California Wildland-Urban Interface Code (Title 24, Part 7), effective January 1, 2026. That new standalone code gathers the wildfire rules that used to sit in the Building and Residential Codes, and it affects the roof, walls, windows, doors, and every vent, which in turn shapes where mechanical and electrical equipment goes.

Can a city ban ADUs in fire zones?

Not just because of the map. HCD's ADU Handbook says the fire hazard maps are meant to govern building materials and defensible space, not to restrict housing, and that a fire hazard zone alone is not a conclusive reason to restrict ADUs. A city may limit where ADUs go based on public safety, but it needs findings, and those limits cannot preclude the state-protected 66323 units, such as conversions of existing space.

Wildfire construction items that touch your project

• Vents (Section 504.10): ventilation openings for attics, eaves, gable ends, underfloor areas, crawl spaces, and other openings intended for ventilation must use vents listed by the State Fire Marshal as flame- and ember-resistant or tested to ASTM E2886. Sloped-roof vents such as dormer vents need noncombustible, corrosion-resistant mesh from 1/16 to 1/8 inch, and gable-end and dormer vents must be at least 10 feet from lot lines.
• Roof: a Class A roof assembly.
• Windows and doors: multilayer glazing with at least one tempered pane or another listed option, and ember-resistant exterior doors.
• Garage doors: gaps limited to 1/8 inch, and automatic openers on vehicle doors serving a residence need battery backup.
• Defensible space: vegetation management around structures, which local plan check may want shown on the site plan.

Existing buildings and conversions

Part 7 covers new buildings and, through its provisions for additions and alterations, work on existing buildings in these zones. The State Fire Marshal has continued to revise those provisions and some cities adopt local amendments, so ask your building department how much of an existing garage or house must be upgraded for your project.

Sprinklers and Title 24

• Under state law, fire sprinklers are required in an ADU only if the primary residence is required to have them, and building the ADU cannot trigger sprinklers in your house.
• The 2025 Energy Code exempts windows in buildings that meet Part 7 in Fire Hazard Severity Zones from the mandatory maximum window U-factor (Exception 3 to Section 150.0(q)). The Title 24 report must still show overall compliance.

What it means for your MEP plans

• Mechanical: specify listed ember-resistant vents wherever ventilation openings are needed, including new attic or underfloor vents created when a garage is insulated or given a raised floor.
• Penetrations: coordinate exhaust fan, dryer, and range hood terminations and mini-split line-set penetrations with the wildfire-rated wall and roof assemblies, and ask plan check which terminations it treats as ventilation openings.
• Electrical: include battery backup for any automatic garage door opener.

More context: the California ADU requirements guide, fire sprinklers for ADUs, and the ADU mechanical guide.

Sources: hcd.ca.gov, dbs.lacity.gov, paradiseca.gov, energy.ca.gov.