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Building Code Reference

Got a CRC R311.3 correction?

CO alarm fuel appliances

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CRC R311.3 sets where carbon monoxide alarms go in any dwelling unit that R311.2 covers: one with a fuel-burning appliance, a fireplace, or an attached garage with an opening into the dwelling. Alarms must be installed outside each separate sleeping area, on every occupiable level of the home including basements, and inside any bedroom where a fuel-burning appliance is located in the bedroom or its attached bathroom. Most single-family and ADU permits hit this correction when CO alarm locations are simply absent from the plans.

How to fix it

  1. Mark CO alarm locations on every floor of the plan

    Add CO alarm symbols to the floor plan on every occupiable level — main floor, second floor, basement. Use a distinct symbol from your smoke alarm symbols, or label them clearly. 'CO/SMOKE COMBINATION ALARM — LISTED UL 2034/UL 217' is a clean way to call them out if you're using combination units. The plan-checker needs to see a specific location on the drawing, not just a general note that says 'per code.'

  2. Place one alarm outside each separate sleeping area

    Every cluster of bedrooms needs a CO alarm in the hallway or common area immediately outside. If bedrooms are separated by a common area or split across different floors, each cluster gets its own alarm. A 3-bedroom house with all bedrooms on one hall typically needs one alarm in that hallway. A split-level where two bedrooms are upstairs and one is downstairs needs coverage on both levels. Show each location on the floor plan and tie it to the bedroom cluster it serves.

  3. Add a separate alarm inside any bedroom with a fuel-burning appliance

    If a bedroom or its attached bathroom has a gas appliance — a gas fireplace insert in the master suite, a wall heater in a converted room — that bedroom needs its own CO alarm inside the room, in addition to the hallway unit. Mark it on the floor plan and add a note referencing the appliance that triggers the requirement. This inside-the-room placement is a separate code requirement from the outside-the-sleeping-area placement, and plan-checkers track them as distinct items.

  4. Identify trigger appliances on a schedule or keynote

    Add a note or keynote schedule listing which fuel-burning appliances or conditions trigger CO alarms under R311.2 in this project. Something like: 'FUEL-BURNING APPLIANCES: GAS FURNACE (GARAGE), GAS WATER HEATER (GARAGE), GAS RANGE (KITCHEN) — CO ALARMS REQUIRED PER CRC R311.2, LOCATED PER R311.3.' This connects the alarm locations to a specific code trigger, which closes the loop for the plan-checker and cuts the chance of a follow-up correction asking you to justify the placement.

  5. Cover the attached garage trigger even in all-electric homes

    If the home has an attached garage with an opening that communicates with the dwelling, you need CO alarms in the unit regardless of whether the home uses gas appliances. The garage-to-house door creates the CO pathway the code targets. Mark the CO alarm location on the floor plan for the level that has direct access from the garage. Don't leave it off because you went all-electric — the plan-checker will flag it.

  6. Confirm UL 2034 listing in specs or general notes

    CO alarms must be listed in accordance with UL 2034 and approved and listed by the State Fire Marshal. Include this in your general notes or electrical schedule. If you're using combination smoke/CO alarms, confirm they carry both UL 217 (smoke) and UL 2034 (CO) listings. A smoke alarm that isn't also listed as a CO alarm does not satisfy R311.3, and an inspector will catch it at rough. Calling out the listing in the specs gives the plan-checker a clear path to approval without a call back.

  7. Confirm basement and lower-level coverage is shown

    Basements are occupiable levels. If the project has a basement, it needs a CO alarm if there are fuel-burning appliances anywhere in the structure. Show it on the basement floor plan as a separate symbol. Don't assume the main-floor alarm covers the basement; the code requires each occupiable level to have coverage, and a plan-checker will read it strictly. A two-level basement counts as two levels.

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Why you got flagged

Plan-checkers flag R311.3 most often when CO alarm locations don't appear anywhere on the submitted plans. The correction doesn't mean your home lacks CO alarms — it means the plans don't show them. If you submitted a floor plan without marking where the CO alarms go, every plan-checker in Orange County will write it up.

A second common flag: the plans show smoke alarm locations but not CO alarm locations. These are separate requirements. CRC R310.3 covers smoke alarms; R311.3 covers CO alarms. Labeling your smoke alarm symbols as 'combination smoke/CO alarms' helps, but only if the fixture spec confirms they're listed as CO alarms, not just smoke detectors.

The trigger appliances catch people off guard. Gas range, gas water heater, gas furnace, wood-burning fireplace, gas fireplace insert — any one of these requires CO alarms in the unit. Some contractors assume CO alarms are only required when there's a full combustion HVAC system. That's not the standard. A single gas water heater in the garage counts.

Attached garages with an opening into the dwelling are their own trigger under R311.2. Even if the entire unit is all-electric, that opening creates a CO pathway. Plan-checkers know this and will flag any plan with an attached garage that doesn't mark a CO alarm on the level with garage access.

The official code

California Residential Code R311.3 — Carbon Monoxide Alarm Location, 2025 Edition

Read the source →

Common questions

Does CRC R311.3 apply to ADUs with only electric appliances?
It depends on the garage. If the ADU is attached to a garage, or the main house it shares a lot with has an attached garage that creates a CO pathway into the ADU, alarms may still be required. The attached-garage trigger doesn't care whether the living unit runs on electric. The garage creates a potential CO entry point regardless of what burns inside the home. If the ADU is a fully detached structure with no gas appliances and no garage connection, R311.3 generally doesn't apply. Confirm with your plan-checker because local interpretations of 'attached' vary.
Can combination smoke/CO alarms satisfy both R310.3 and R311.3?
Yes, a combination alarm satisfies both requirements in one device, as long as the unit is listed under both UL 217 for smoke and UL 2034 for CO. Not all combination alarms on the shelf carry both listings — check the spec sheet. In your plans, label them as 'COMBINATION SMOKE/CO ALARM, UL 217 / UL 2034 LISTED' so the plan-checker sees the compliance path without having to guess. Generic symbols labeled just 'smoke alarm' won't clear the R311.3 correction even if the fixture you plan to install is a combination unit.
How close to a bedroom door does the CO alarm need to be?
The code requires placement outside each separate sleeping area but doesn't set a maximum distance in feet. Plan-checkers in practice want the alarm within the immediate hallway or landing that directly serves the bedrooms, not at the far end of a long corridor. A useful test: if someone sleeping with their door closed could plausibly not hear the alarm, it's too far. Show the alarm on the floor plan within the hallway section that directly serves the bedrooms, and you won't get a question about it.
My furnace is in the garage, not inside the living space. Do I still need CO alarms inside the house?
Yes. A gas furnace in an attached garage is a fuel-burning appliance connected to the dwelling. CO from the garage can migrate through the garage-to-house door, ductwork penetrations, and wall assemblies. R311.3 doesn't require the appliance to be inside the living envelope to trigger the requirement. If the appliance is in an attached garage, it counts. Show CO alarm locations in the living space on every occupiable level and outside each sleeping area.
Are CO alarms required on every floor or just near bedrooms?
Both. R311.3 has two placement rules that work together: one alarm outside each separate sleeping area, and one alarm on every occupiable level. A two-story house with all bedrooms upstairs needs a hallway alarm near the bedrooms and a separate alarm on the main floor. A basement is an occupiable level and needs its own alarm. Show one per floor plus one per bedroom cluster on the floor plans, and you'll cover both prongs of the requirement.
The correction letter says CO alarm locations are missing. Can I just add a note saying 'CO alarms per code'?
No. A generic note won't clear the correction. Plan-checkers need to see actual symbols on the floor plan showing where each alarm is located, plus a note or legend identifying them as CO alarms and citing the UL listing. A keynote like 'CO ALARM, UL 2034 LISTED, OUTSIDE EACH SLEEPING AREA AND ON EACH OCCUPIABLE LEVEL PER CRC R311.3' placed at each alarm location is the standard format. You also want a brief appliance schedule or note identifying which appliances trigger the requirement. That gives the plan-checker everything in one place.

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