Smoke Detector & Carbon Monoxide Alarm Requirements
Smoke and CO alarm requirements are among the most consistently enforced life-safety items on a residential inspection. Get the location wrong and you fail — no exceptions. This guide covers exactly where alarms are required, when hardwiring is mandatory, and the interconnection rules that most DIYers miss.
Quick Reference — IRC R314 & R315
Smoke alarms required in: Each sleeping room • Outside each sleeping area • Each story including basement
CO alarms required: Outside each sleeping area when fuel-burning appliances or attached garage present
New construction: Hardwired with battery backup • Must be interconnected
IRC sections: Smoke → R314 • CO → R315
Smoke Alarm Requirements — IRC R314
IRC 2021 Section R314 establishes where smoke alarms are required in one- and two-family dwellings. The three-location rule covers the scenarios where people are most at risk: asleep, in transit between sleeping rooms, and anywhere below bedrooms where a fire can originate undetected.
Required Locations (IRC R314.3)
- Inside each bedroom (sleeping room) — A closed bedroom door significantly delays warning from a hallway alarm. The inside-the-room alarm is the one that actually wakes the occupant.
- Outside each sleeping area in the immediate vicinity — The hallway serving the bedrooms, typically within 10 feet of any bedroom door. The living room or kitchen does not satisfy this requirement.
- On each story including the basement — Every floor level requires a smoke alarm. An unfinished basement with a furnace and laundry still requires one. Attics used only for storage are exempt.
| Location | Required? | Notes | IRC Section |
|---|---|---|---|
| Inside each bedroom | ✓ Required | Both bedroom AND hallway are independently required | R314.3.1 |
| Hallway outside sleeping rooms | ✓ Required | Immediate vicinity of bedroom doors | R314.3.2 |
| Each story, including basement | ✓ Required | Unfinished basements included | R314.3.3 |
| Attached garage interior | Not required | CO alarm required instead (R315) | — |
| Kitchen | Not required | Cooking fumes cause nuisance alarms; hallway is correct location | — |
| Attic (storage only) | Not required | Habitable attic spaces are required | — |
Hardwired vs. Battery — IRC R314.4
- New construction: Hardwired to building electrical with battery backup. Battery backup ensures function during power outage.
- Existing construction — replacements: Battery alarms are permitted when the existing construction was not originally wired for hardwired alarms.
- 10-year sealed battery alarms: IRC 2021 accepts sealed lithium battery alarms (listed to UL 217) in existing construction where hardwiring is impractical. These prevent the documented failure mode of occupants removing batteries and never replacing them.
Most Common Inspection Failure
Missing the bedroom interior alarm. The outside-hallway alarm is not a substitute for the inside-bedroom alarm — both are independently required. If you are finishing a basement bedroom, you need an alarm inside that room regardless of what exists upstairs.
Interconnection — IRC R314.5
In new construction, all smoke alarms must be interconnected — when one activates, all sound simultaneously. A basement fire triggering only the basement alarm will not wake someone sleeping on the second floor with a closed door. Interconnection can be hardwired (third interconnect wire) or wireless RF listed for interconnect use. Existing construction is generally exempt when replacing alarms, but adding new bedrooms or finishing a basement triggers interconnection for the new alarms.
Carbon Monoxide Alarm Requirements — IRC R315
CO is odorless and colorless. Without a detector you cannot know it is present until symptoms appear, at which point evacuation may be difficult. The IRC requires CO alarms in dwellings with fuel-burning appliances or attached garages.
When CO Alarms Are Required (IRC R315.1)
CO alarms are required when either condition exists:
- Fuel-burning appliances: Gas furnace, gas water heater, gas range, oil boiler, wood stove, fireplace, gas dryer — any combustion appliance triggers the requirement.
- Attached garage: A garage sharing a wall or ceiling with the living space requires CO coverage regardless of what appliances are in the garage. Cars, generators, and small engines all produce lethal CO.
Required CO Alarm Locations (IRC R315.3)
CO alarms must be installed outside each separate sleeping area in the immediate vicinity of the bedrooms. Unlike smoke alarms, the IRC does not require CO alarms inside each individual bedroom — a hallway alarm satisfies coverage for adjacent bedrooms. Additional alarms are required on each story containing sleeping rooms and, where a fuel-burning appliance is on a floor directly below sleeping areas, on the sleeping floor above.
CO Alarms vs. Smoke Alarm Placement
CO disperses relatively evenly in room air. CO alarms can be ceiling or wall-mounted — follow manufacturer instructions. Do not install within 5 feet of a cooking appliance or in high-humidity spaces. Combination smoke/CO units (listed to both UL 217 and UL 2034) are code-compliant and satisfy both requirements at a single location, but note that the smoke and CO location requirements are still independently applied.
When These Requirements Are Triggered
- New construction & additions: Full compliance — hardwired, interconnected, all required locations.
- Basement finishing with a sleeping room: Smoke alarm inside the new bedroom, smoke alarm in any new hallway, and CO alarm if fuel-burning appliances exist.
- Permit work in sleeping areas: Most AHJs require alarms in the affected area to be brought into compliance when a permit is pulled for bedroom work.
- Real estate sale: Many states independently require working smoke detectors as a condition of sale, separate from the building code.
Photoelectric vs. Ionization Alarms
The IRC does not specify detection technology — both are code-compliant. Ionization alarms respond faster to fast-flaming fires and are more prone to kitchen nuisance alarms. Photoelectric alarms respond faster to smoldering fires (the most common residential fire type) and produce fewer false alarms from cooking. Dual-sensor alarms combine both. Several jurisdictions now require photoelectric alarms by local amendment — confirm with your AHJ.
Common Inspection Failures
- Missing bedroom interior alarm — hallway alarm is present but inside-the-bedroom alarm was skipped
- Alarm installed in kitchen or within 3 feet of cooking appliances
- Battery-only standalone alarms in new construction instead of hardwired interconnected units
- No CO alarm despite attached garage
- No basement alarm — treating unfinished basements as exempt from smoke alarm requirements
Frequently Asked Questions
Where are smoke detectors required under IRC 2021?
IRC R314.3 requires smoke alarms inside each sleeping room, outside each sleeping area in the immediate vicinity of bedroom doors, and on each story of the dwelling including basements. All three location types are independently required.
Does a smoke detector have to be hardwired?
In new construction, yes — hardwired with battery backup per IRC R314.4. In existing construction where hardwiring is impractical, battery alarms (including 10-year sealed lithium units) are accepted as replacements.
Do smoke detectors have to be interconnected?
In new construction, yes. IRC R314.5 requires all smoke alarms to be interconnected so that activating one sounds all of them. Wireless interconnect is an accepted alternative to hardwired interconnect.
Where are carbon monoxide alarms required under IRC 2021?
CO alarms are required outside each separate sleeping area where fuel-burning appliances are present or an attached garage shares a wall with the dwelling. Per IRC R315.3, one hallway alarm can cover adjacent bedrooms — inside-bedroom CO alarms are not individually required.
Do I need a CO alarm in an all-electric home?
Only if you have an attached garage. An all-electric home with no attached garage and no combustion appliances is not required to have CO alarms under the IRC. Check local amendments, as some jurisdictions are stricter.
How high should a smoke detector be mounted?
Follow the manufacturer's instructions. NFPA 72 specifies ceiling or high-wall mounting. Avoid placement within 3 feet of cooking appliances, within 3 feet of a bathroom door, or near supply air registers that would dilute smoke before reaching the sensor.