Can an ADU Be a Wildfire Refuge? What a Fire-Resistant Backyard Unit Can and Cannot Do

# Can an ADU Be a Wildfire Refuge?

Direct answer. A backyard unit built to ignition-resistant standards is very likely to still be standing after a wildfire passes, and that is reason enough to build it that way: it becomes the family's home during the rebuild and one less burning structure on the street. What it is not is a place to plan on riding out a fire. Fire agencies across the West are unanimous that early evacuation is the only reliable survival strategy, and no U.S. standard defines a residential wildfire refuge the way ICC 500 defines a storm shelter. Build the ADU so it survives without you in it. If a fast-moving fire ever cuts off the road, a hardened unit with a non-combustible perimeter is a better last resort than a car on a blocked road — and that is the whole claim BuiltStrong makes.

First, the thing to say plainly

[code] California's statewide guidance tells residents to consider leaving even when a fire seems a safe distance away, to go immediately when ordered, and to call 911 if trapped; CAL FIRE's Ready for Wildfire program is built on leaving early. [tested] NIST's case study of the 2018 Camp Fire — 157 first-responder accounts, radio logs and field data — identified 23 life-threatening entrapment and burnover events in the first 24 hours; 17 directly affected evacuating civilians and 12 happened on major evacuation roadways. Eighty-five people died, most in the first 12 hours. The authors' conclusion is that fast-moving fires may force last-minute, large-scale evacuations. That is the case for leaving before the order, not for staying.

[tested] A 2025 study of U.S. wildfire deaths from 2008 to 2024 found them concentrated in communities with few exit routes and recommended pre-planned refuge areas — and UC Berkeley fire scientist Michael Gollner called that one of the most controversial questions in wildfire response, because a designated refuge could deter early evacuation and put residents and firefighters at greater risk. Cal Fire's local units know where refuges are but are reluctant to publicize them for the same reason. [tested] Shelter-in-place is not common policy in California; Rancho Santa Fe is the well-known exception, and the Forest Service study of it found resident understanding of the policy was lacking. [tested] The Yuba Fire Safe Council notes that Australia's experience with shelter-in-place shows the policy risks lives when protocol isn't followed.

There is no wildfire equivalent of a tornado safe room. [code] ICC 500 and FEMA P-361 are wind standards. No U.S. building code, ICC standard or FEMA publication defines, tests or certifies a residential wildfire refuge, and no state or federal fire agency recommends building one in place of evacuation. BuiltStrong will not call any unit a "wildfire safe room," and you should be wary of anyone who does with reviewer that no NFPA document (1140, 1141) or state program creates such a designation.

So this page is not a how-to for staying. It is a how-to for building a unit that survives, with an honest note on what it offers if you ever have no choice.

What survival requires: the details, not the wall

[judgment] Structures ignite in wildfires three ways: embers lodging in something that burns, radiant heat cracking windows or igniting adjacent fuel, and direct flame from vegetation or an attached deck. A concrete box with the wrong vents and a wood deck fails the first and third tests. The package that works is the same in every state:

• Roof. [code] Class A roof covering (metal, tile, or asphalt shingles in a Class A assembly); no debris in valleys or gutters; non-combustible or enclosed eaves. Chapter 7A requires Class A in very high hazard zones.
• Vents. [code] Ember-resistant vents listed under Chapter 7A, or no attic at all. A flat or vaulted roof with no vented attic removes the single most common ember entry point [judgment].
• Walls. [code] Non-combustible or ignition-resistant cladding — stucco, fiber-cement, metal, masonry — over sheathing that carries an ASTM E136 noncombustible report if the assembly needs it (see /library/mgo-board). [judgment] ICF, SCIP, concrete block and steel-frame units get this for free; wood frame gets it from the cladding and sheathing choice.
• Glazing. [code] Multi-pane with at least one tempered pane under Chapter 7A; keep windows small and away from the fuel side [judgment]. See /library/impact-glazing for the overlap with hurricane glazing.
• Deck and the first five feet. [code] AB 1455 writes a five-foot ember-resistant Zone 0 into California law for VHFHSZ and SRA once regulations are final; the IBHS Wildfire Prepared Home program asks for the same nationally current IBHS checklist wording. Non-combustible decking, no mulch, no fence boards touching the wall.
• Doors and garage. Weatherstripped, solid-core or metal; no gap under the garage door [judgment].

If you are ever trapped: what a hardened ADU offers

This section exists because people will search for it, and a wrong answer is dangerous. [tested] NIST defines a safety zone as an area with ember and smoke exposure but no hazardous fire exposure, sized by the expected flame height of surrounding fuels; a temporary refuge area is a smaller fallback offering short-term relief. A hardened ADU in the middle of a wide, non-combustible yard can function as a temporary refuge: it keeps embers and radiant heat off people for the minutes the fire front takes to pass [judgment]. It does not supply breathable air, it can fill with smoke through any gap, and it is only as good as the five feet around it [judgment]. The Camp Fire record shows where people actually died — on the roads, trying to leave late — which is the strongest argument for leaving early, and the only sense in which a refuge is "safer than the alternative" [judgment].

If you design for this possibility, the honest list is short: a non-combustible exterior and roof; no vented attic; tight doors and windows that can be closed; a room with the fewest windows for people to gather in; a wide non-combustible perimeter; water and an N95 or better mask for each person; and a plan that treats all of this as what happens after evacuation failed, never instead of it. this paragraph with the fire-service reviewer before publishing; it is the one place on the site where being wrong costs lives.

Permitting in fire zones

[code] California cannot ban ADUs in VHFHSZ outright; the unit must meet Chapter 7A and, going forward, Zone 0, and new permits in those zones need the building official's wildfire-standards certification under AB 1455. Oregon, Colorado and other states have WUI codes or local overlays with similar packages per state. See /adu for the general ADU law picture and /fire for the hazard hub.

Why build it strong if you're leaving anyway

Three reasons, in order of weight: the unit that survives is where you live during a 12- to 24-month rebuild (see /rebuild/after-a-wildfire); a surviving structure is one less source of flames and embers for the neighbors; and insurers in wildfire markets look at the hardened-home checklist, so a Chapter 7A-compliant unit is easier to insure and may earn credits specific programs, e.g., California's Safer from Wildfires framework.

Next step

If you are rebuilding after a fire, or adding a unit in a mapped hazard zone, bring the parcel and the zone designation to a BuiltStrong advisory session. We'll check the Chapter 7A or local WUI package against your design, choose among the non-combustible systems that fit, and connect you with builders in the network who build in the WUI. Start at /start.