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Building Code Reference

Got a CRC R311.2 correction?

CO alarm locations

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 R311.2 requires carbon monoxide alarms in every dwelling unit that has a fuel-burning appliance, a fireplace, or an attached garage with an opening into the living space. Alarms must be placed outside each sleeping area, on every occupiable level (including basements), and inside any bedroom that contains a fuel-burning appliance or has an attached bathroom with a fuel-burning appliance. They must be hardwired with battery backup and interconnected so all alarms sound together.

How to fix it

  1. Confirm which trigger applies to your project

    R311.2 applies if any of these are true: (1) the unit has a fuel-burning appliance (gas range, furnace, water heater, dryer), (2) there is a fireplace (gas or wood-burning), or (3) there is an attached garage with any opening into the dwelling. An alarm is also required inside any bedroom that contains a fuel-burning appliance or has an attached bathroom with a fuel-burning appliance. Check your project against all triggers. An ADU with a gas range and no garage still needs CO alarms. A new detached unit with all-electric appliances and no attached garage does not.

  2. Mark CO alarm locations on the floor plan

    Add a dedicated CO alarm symbol to every required location on your floor plan sheet. Required locations are: (a) outside each separate sleeping area, (b) on every occupiable level including basements, and (c) inside each bedroom that contains a fuel-burning appliance or has an attached bathroom with a fuel-burning appliance. Use a distinct symbol from your smoke alarm legend, or use a combination CO/smoke symbol and label it clearly. If the plan-checker can't tell which symbol is CO vs. smoke, expect a correction.

  3. Add CO alarms to every occupiable level

    Count your levels. Finished basement or bonus room? That's an occupiable level and needs coverage. Unfinished crawl space? Not occupiable, no alarm needed. If you have a split-level layout, plan-checkers typically interpret each distinct occupiable floor as its own level. When in doubt, add the alarm. Missing one level is a common reason for a second round of corrections on this section.

  4. Show hardwired power with battery backup on the electrical plan

    Battery-only CO alarms fail R311.6 (power source) in new construction. Your electrical plan must show CO alarms connected to a circuit, with a note that battery backup is provided. Combination CO/smoke alarms wired to the existing smoke alarm circuit are acceptable if labeled as combination units. Add a note on the electrical schedule: "CO alarms: 120V hardwired, battery backup, UL 2034 listed." Missing this note on the electrical sheets is a top-five reason this correction comes back on resubmit.

  5. Note interconnection on the plans

    All CO alarms in the unit must be interconnected so that when one triggers, all alarms sound. Show the interconnect wiring on the electrical plan or add a general note: "CO alarms shall be interconnected per CRC R311.5." If you are using a combination alarm that shares the smoke alarm interconnect, confirm the product listing covers CO (UL 2034) in addition to smoke (UL 217). Some alarms marketed as "combination" only include a CO sensor on certain SKUs, so check the product data sheet before specifying.

  6. Address attached garage openings explicitly

    If the project has an attached garage with any door or pass-through into the living space, add a CO alarm note near that area on the floor plan. The plan-checker is specifically checking whether the garage connection triggers coverage. A sentence in the notes section of the floor plan works: "CO alarms required per CRC R311.2, attached garage with dwelling opening confirmed." This signals that you saw the trigger condition and addressed it.

  7. Cross-reference your smoke alarm sheet

    Smoke alarms (CRC R310) and CO alarms are often reviewed together. If you're revising for a CO alarm correction, take 10 minutes to confirm your smoke alarm locations are current. A second correction for smoke alarms after you resubmit costs another 60-day review cycle. Check that every bedroom and hallway outside sleeping areas has a smoke alarm shown, and that they are hardwired with battery backup.

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Why you got flagged

Most CO alarm corrections come down to one thing: the plans don't show them at all. Plan-checkers scan for CO alarm symbols on the floor plan the same way they look for smoke alarms, and if they're not drawn and labeled, the sheet gets flagged. The correction note on this section says it plainly: alarms are either omitted entirely or the wiring isn't called out.

A second common trigger is misunderstanding which levels need coverage. The code says every occupiable level, including basements. A plan that shows CO alarms on the first and second floors but skips a finished basement will get a correction. Basements count even if they're primarily used for storage, as long as they're occupiable space.

Attached garages trip contractors up more than almost anything else in this section. If the garage has any door or opening into the dwelling, the unit needs CO coverage per R311.2, even if there's no fuel-burning appliance inside. Plan-checkers look at the garage-to-dwelling connection on the floor plan. If an ADU or converted garage has that opening, expect this correction unless CO alarms are explicitly shown and the interconnect is noted.

Hardwiring is the last piece that catches people. Battery-only CO alarms don't satisfy R311.6 in new construction. The plans need to show the alarms tied to a circuit, with battery backup noted. If the electrical plan just shows a smoke alarm symbol and the plan-checker can't confirm it's a combination or dedicated CO alarm, they'll flag it and ask for clarification.

The official code

California Residential Code (2025 edition) — Title 24 Part 2.5, Section R311.2 Carbon Monoxide Alarms, Where Required

Read the source →

Common questions

Does CRC R311.2 apply to ADUs?
Yes, if the ADU has a fuel-burning appliance (gas range, water heater, furnace), a fireplace, or an attached garage with an opening into the unit. Most California ADUs have at least a gas water heater or range, so the answer for most ADU projects is yes. A detached ADU with all-electric appliances and no garage connection does not need CO alarms under R311.2, but check your local jurisdiction since some cities have amended the trigger conditions.
Can I use battery-only CO alarms in a new single-family home?
No. CRC R311.6 requires hardwired alarms with battery backup in new construction. Battery-only units do not meet the code requirement and will be flagged at plan check. In existing buildings being renovated, the code may allow battery-only alarms where hardwiring is not practical, but for new builds you need a 120V hardwired unit. Combination CO/smoke alarms wired to the smoke alarm circuit are the most common approach on new projects.
Where exactly does the CO alarm go outside the sleeping area?
The code specifies "outside each separate sleeping area" but does not set a maximum distance in feet. Place the alarm in the hallway or space immediately adjacent to the bedroom entries. If bedrooms are on both ends of a long hallway, you may need two alarms to cover both clusters. A general note on the plans stating the alarm location satisfies the requirement.
My project has a gas fireplace but no garage. Do I still need CO alarms?
Yes. A gas fireplace is a fuel-burning appliance and is an independent trigger under R311.2. The full requirement applies: outside sleeping areas, on every occupiable level, and inside any bedroom with a fuel-burning appliance. Fireplace addition permits that don't think through the CO alarm requirement are a common source of corrections. If you're adding or replacing a gas fireplace, confirm CO alarms are shown on the plans before you submit.
Do CO alarms need to be interconnected in a single-family home?
Yes. When more than one CO alarm is required, they must be interconnected so that triggering one sets off all of them. The electrical plan should show the interconnect wiring or include a general note calling out interconnection per CRC R311.5. Most hardwired combination CO/smoke alarms handle interconnection through the same three-wire system used for smoke alarms, so if you're already running interconnected smoke alarms, adding CO coverage to the same circuit is usually straightforward.
Does a finished basement need a CO alarm?
Yes, if the basement is occupiable space. The code requires coverage on every occupiable level, and a finished basement counts. An unfinished, non-habitable crawl space does not. If your project has a finished basement with a bedroom, home office, or family room, add a CO alarm to the basement floor plan. Missing the basement is one of the most common reasons a CO alarm correction comes back on resubmit.
What listing standard does the CO alarm need to meet?
UL 2034 is the listing standard for carbon monoxide alarms. Combination CO/smoke alarms must be listed to both UL 2034 (CO) and UL 217 (smoke). Include the listing standard in your electrical plan notes or device schedule. Some plan-checkers will ask for this explicitly if your plans just say "CO alarm" without specifying the listing. Noting "UL 2034 listed" in the device schedule prevents a back-and-forth.

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