Every time someone in the house sears a steak, the hallway alarm goes off, and every time the fix is the same: wave a dish towel at the ceiling until it stops. Eventually somebody pulls the battery, and it stays out. That is how a working alarm becomes a decoration — and in most houses the root cause is not a defective alarm. It is an alarm installed too close to the stove.
The short answer: California requires smoke alarms to be installed at least 20 feet from a permanently installed cooking appliance, with reduced distances of 10 feet or 6 feet for certain alarm types, at least 3 feet from a bathroom door where there is a tub or shower, and not within 36 inches of a supply register or the tip of a ceiling fan blade (§ R310.3). Carbon monoxide alarms have their own required locations (§ R311.3).
Key numbers
| Rule | Distance | Where it comes from |
|---|---|---|
| Smoke alarm from a permanently installed cooking appliance | 20 feet minimum, horizontal | § R310.3 |
| Same, for ionization alarms with a hush switch, or photoelectric alarms | 10 feet or greater | § R310.3 |
| Same, photoelectric in an open kitchen where 10 feet is impossible | Greater than 6 feet | § R310.3 |
| Smoke alarm from a bathroom door with a tub or shower | 3 feet minimum, horizontal | § R310.3 |
| Smoke alarm from a forced-air supply register | 36 inches, and out of the direct airflow | § R310.3 |
| Smoke alarm from a ceiling fan blade tip | 36 inches | § R310.3 |
| Temperature range where alarms may be installed | Not where temperatures fall below 40°F or exceed 100°F | § R310.3 |
| CO alarms | Outside each sleeping area, and on every occupiable level including basements | § R311.3 |
The kitchen distances, and why there are three of them
The base rule is 20 feet of horizontal distance between a smoke alarm and a permanently installed cooking appliance. That is a long way in a modern floor plan, and the code knows it — so the distance shrinks depending on how the alarm senses smoke (§ R310.3):
- Ionization alarms with an alarm-silencing switch, and photoelectric alarms, may be installed 10 feet or greater from the appliance.
- A photoelectric alarm may be installed more than 6 feet away where the kitchen or cooking area and the adjacent spaces have no clear interior partitions and holding the 10-foot distance would make it impossible to place an alarm the code requires elsewhere.
- Alarms specifically listed for use in close proximity to a cooking appliance are also recognized.
The practical takeaway for an open-plan house: buy photoelectric. Ionization sensors respond to the fine particles that cooking throws off, which is why they nuisance-trip; photoelectric sensors are less bothered by them, and the code rewards that with a shorter allowed distance.
The other four clearances people miss
- Bathroom doors. Smoke alarms must be at least 3 feet horizontally from the door or opening of a bathroom containing a bathtub or shower, unless that would prevent placing an alarm required elsewhere. Steam behaves like smoke to a sensor.
- Supply registers. No alarm within a 36-inch horizontal path of a forced-air heating or cooling supply register, and it must be installed outside the direct airflow. Moving air can push smoke past the sensor.
- Ceiling fans. No alarm within 36 inches of the tip of a paddle fan blade, for the same reason.
- Temperature and mounting surface. Alarms may not go in unfinished attics or garages, or anywhere temperatures can fall below 40°F or exceed 100°F. Where the mounting surface could get much hotter or colder than the room — a poorly insulated ceiling under an unfinished attic, or an exterior wall — the alarm goes on an inside wall instead.
There is also a stairway rule worth knowing: where stairs lead to other occupied levels, an alarm must be located so that smoke rising in the stairway cannot be blocked from reaching it by an intervening door or obstruction.
Carbon monoxide alarms: a different list
Carbon monoxide is odorless, so its alarms are placed around sleeping people rather than around smoke paths. CO alarms in dwelling units must be installed and maintained per the manufacturer's published instructions, in all of the following locations:
On every occupiable level of a dwelling unit, including basements.
Plus, from the same section (§ R311.3):
- Outside each separate sleeping area, in the immediate vicinity of the bedrooms.
- Inside the bedroom, where a fuel-burning appliance is located within that bedroom or its attached bathroom.
That third one catches houses with a wall furnace or a gas fireplace in a master suite. If the appliance is in the room where someone sleeps, the alarm goes in that room too.
Power, interconnection, and what has to be on the box
Carbon monoxide alarms must take their primary power from the building wiring where the building is served by commercial power, and switch to a battery when that power is interrupted; the wiring must be permanent, with no disconnecting switch other than overcurrent protection (§ 915.4.1). Battery-powered alarms are acceptable in buildings without commercial power.
Existing homes get real relief here. In buildings built before January 1, 2011, CO alarms may be battery-powered or plug-in with battery backup where no construction is taking place; where repairs or alterations do not remove interior wall and ceiling finishes in the areas needing alarms; where work is limited to exterior surfaces such as roofing, siding, windows, doors, a porch or a deck; or where the work is plumbing, mechanical or electrical that does not open up those finishes.
Where more than one CO alarm is required within a dwelling unit, they must be interconnected so that one alarm sounding activates all of them (§ 915.4.4) — with a matching set of exceptions for pre-2011 buildings, including one that lets listed wireless alarms satisfy the requirement without physical wiring.
On listings: CO alarms must be listed and labeled to UL 2034, and combination CO and smoke alarms to both UL 217 and UL 2034 (§ R311.1.1). California adds a step no other state does: no person may install, market, distribute, offer for sale or sell any carbon monoxide device in California unless the device and its instructions have been approved and listed by the Office of the State Fire Marshal.
Combination units are allowed in both directions — a combination smoke and CO alarm may be used in place of a smoke alarm (§ R310.5), and a combination CO/smoke alarm is an acceptable alternative to a CO alarm (§ 915.4.3). Both must carry the State Fire Marshal listing.
What this article does not cover
The clearances above are the placement rules. The room-by-room list of where smoke alarms are required in a dwelling is set out separately in § R310.3 — read that section for the full list before you plan a layout, because the clearances only tell you where an alarm may not go. Requirements for licensed residential care facilities are different again and stricter (§ R335.5).
These are 2025 California code requirements, in effect statewide since January 1, 2026, and they are minimums — your city adopts the state code and may amend it to be stricter, and separate state law governs when alarms must be present in existing homes and rentals.
Sources
- § R310.3 — Smoke alarm location (California Residential Code)
- § R310.5 — Combination alarms (California Residential Code)
- § R311.1 — Carbon monoxide alarms, general and listings (California Residential Code)
- § R311.3 — Carbon monoxide alarm location (California Residential Code)
- § 915.4 — Carbon monoxide alarms (California Fire Code)
- § R335.5 — Fire protection provisions, licensed care facilities (California Residential Code)
