The alarm in the hallway outside your kitchen goes off every time someone sears a steak, so at some point a previous owner moved it — or took the battery out. Then a remodel brings an inspector through, and the conversation turns to where alarms are allowed to be, how they are wired, and whether they have to talk to each other. Most of that is settled by a few short provisions.
In a California home, smoke alarms must sit at least 20 feet from a permanently installed cooking appliance (with narrower distances allowed for certain alarm types), at least 3 feet from a bathroom door, and at least 36 inches from a supply register or a ceiling fan blade tip (§ R310.3). Where more than one is required, they must be interconnected so that one sounding sets off all of them (§ R310.4).
Key numbers
| Requirement | Distance or rule | Source |
|---|---|---|
| From a permanently installed cooking appliance | 20 feet minimum | § R310.3 |
| Ionization alarm with a silencing switch, or a photoelectric alarm | 10 feet or greater allowed | § R310.3 |
| Photoelectric alarm, open kitchen with no interior partitions | More than 6 feet allowed | § R310.3 |
| From the door of a bathroom with a tub or shower | 3 feet minimum | § R310.3 |
| From a forced-air supply register | 36 inches minimum, and out of the direct airflow | § R310.3 |
| From the blade tip of a ceiling fan | 36 inches minimum | § R310.3 |
| Ambient temperature range of the space | Not below 40°F or above 100°F | § R310.3 |
| Power | Building wiring plus battery backup, where a commercial power source serves the building | § 1103.8.3 |
Why alarms end up in the wrong place
Every clearance above exists to stop an alarm crying wolf, because an alarm that nuisance-trips is an alarm somebody disables.
Cooking. Twenty feet is a long way in a small house, so the code gives two ways down. An ionization alarm fitted with an alarm-silencing switch, or a photoelectric alarm, may be installed 10 feet or more from the cooking appliance. And where a kitchen and the adjacent space have no clear interior partitions between them — the standard open-plan layout — a photoelectric alarm may go more than 6 feet away, on the reasoning that the 10-foot distance would otherwise leave nowhere legal to put an alarm that another part of the code requires (§ R310.3).
Steam. Keep alarms 3 feet from the door or opening of any bathroom containing a tub or shower, unless doing so would make a required alarm impossible to place.
Moving air. Draughts blow smoke past a sensor. Alarms may not sit within a 36-inch horizontal path of a forced-air heating or cooling supply register, and must be outside the direct airflow from it. The same 36 inches applies from the blade tip of a paddle fan.
Temperature. Alarms and detectors may not be installed in unfinished attics or garages, or anywhere else temperatures can fall below 40°F or exceed 100°F. And where a mounting surface could get much warmer or cooler than the room — a poorly insulated ceiling under an unfinished attic, or an exterior wall — the alarm goes on an inside wall instead.
Stairs. Where stairs lead to other occupied levels, an alarm must be placed so that smoke rising in the stairway cannot be blocked from reaching it by an intervening door or obstruction.
Interconnection: one sounds, all sound
Where more than one alarm is required in a dwelling or sleeping unit, the code is unambiguous:
the smoke alarms shall be interconnected in such a manner that the activation of one alarm will activate all of the alarms
The alarm must be clearly audible in every bedroom over background noise, with all intervening doors closed (§ R310.4).
Three exceptions matter to anyone in an older house. Interconnection is not required in buildings not undergoing alterations, repairs or construction of any kind. It is not required in existing areas where the work does not remove interior wall or ceiling finishes and expose the structure — unless an attic, crawl space or basement gives access to run the connection without touching those finishes. And it is not triggered by exterior-only work such as re-roofing, new siding, replacing windows or doors, or adding a porch or deck.
Power and wiring
For existing homes, alarms must draw primary power from the building wiring where that wiring is served from a commercial source, and must have a battery backup. They must signal when the batteries are low, and the wiring must be permanent, with no disconnecting switch other than the overcurrent protection (§ 1103.8.3).
Battery-only alarms remain acceptable in three situations: existing buildings where no construction is taking place, buildings not served from a commercial power source, and existing areas of buildings undergoing alterations that fall inside the same limits as the interconnection exceptions.
A combination smoke and carbon monoxide alarm may be used in place of a smoke alarm, provided the equipment is State Fire Marshal listed and approved (§ R310.5). Anything involving new wiring is permit-level electrical work — have it done by a licensed electrician and inspected.
When a remodel drags the rest of the house up to code
This is the part that surprises owners mid-project. Where an alteration, addition, change of occupancy or relocation is made to an existing residential building, the existing building must be provided with smoke alarms to the current standard — not just the new part. Work classified as a Level 1 alteration is excepted (§ 307.1).
Existing homes are otherwise not forced to re-do compliant work. Alarms installed to the code in effect when the house was built are accepted, as are alarms voluntarily installed in homes that never needed them, provided they met the rules in force at the time of installation (§ 1103.8.1).
One narrow case runs the other way: tiny houses and manufactured homes used for temporary emergency housing need a smoke alarm, but are not required to be hard wired (§ E107.3.3).
What this article does not cover
These provisions govern where alarms may sit, how they are powered and interconnected, and what work triggers an upgrade. The separate "where required" provision — how many alarms your home needs and in which rooms — is not reproduced in the sections cited here, so confirm that room-by-room list before you buy hardware. Carbon monoxide alarms are governed by their own section. Local ordinances and your fire authority can be stricter than the statewide code, and California's point-of-sale and rental smoke alarm duties sit in state statutes outside the building standards. These are the 2025 editions, in effect since January 1, 2026.
Sources
- 2025 California Residential Code § R310.3 — Smoke alarm location
- 2025 California Residential Code § R310.4 — Interconnection
- 2025 California Residential Code § R310.5 — Combination alarms
- 2025 California Fire Code § 1103.8 — Single- and multiple-station smoke alarms in existing buildings
- 2025 California Existing Building Code § 307.1 — Smoke alarms
- 2025 California Existing Building Code § E107.3 — Tiny houses and manufactured homes
