What this code requires
CRC R310.3 requires smoke alarms in every sleeping room, outside each sleeping area, and on every level of a home, including the basement. Companion sections also require each alarm to be hardwired to the building's electrical system with a battery backup (R310.6) and interconnected so that when one alarm trips, all alarms sound (R310.4). New construction and most substantial remodels trigger this requirement. ADUs, single-family residential additions, and whole-house renovations are the permit types most commonly flagged under R310.3.
How to fix it
Mark smoke alarm locations on every floor plan sheet
Add a smoke alarm symbol to each bedroom, to the hallway or area immediately outside each bedroom cluster, and to every level of the home including the basement. Use a standard SA or smoke alarm symbol from your legend. The plan-checker needs to count the alarms and verify they cover R310.3's required locations. If your floor plan has a sleeping loft, add an alarm in the room the loft is open to, near the loft, and give any bonus room used or intended as a sleeping room its own alarm. Don't leave it to a general note. The symbol on the plan is what gets reviewed.
Specify hardwired with battery backup in your notes
Add a general note on the electrical plan sheet or in the building notes block that reads: "Smoke alarms shall be hardwired to a dedicated circuit or branch circuit, with battery backup." Battery-only units are not permitted for new construction or additions under the current adopted California Residential Code. This note must appear on the plans — a verbal assurance at the counter will not pass plan check, and the inspector will verify at rough electrical.
Show the interconnection requirement on the electrical sheet
Add a note that all smoke alarms are interconnected — when one sounds, all sound. A note on the electrical sheet works: "All smoke alarms shall be interconnected per CRC R310.4." For multi-story projects, include a simple wiring diagram or riser showing the interconnect wire (typically the yellow traveler in a three-conductor cable) running from unit to unit. Wireless interconnect systems that meet UL 217 are also acceptable — note the method you're using on the plans so the plan-checker can verify compliance without guessing.
Count alarms against sleeping room inventory
Do a quick count: bedrooms + bonus rooms used as bedrooms = number of in-room alarms required, plus one in the room each sleeping loft opens to. Add one alarm per hallway or area immediately outside each sleeping cluster, and one alarm per additional level. A 3-bedroom single-story home with one central hallway needs a minimum of 4 alarms: 3 in bedrooms, 1 in the hallway. A two-story version adds at least one more for the lower level. Total your count and make sure the plan shows the same number of units. Mismatches between the room count and alarm count are an easy catch for plan-checkers.
For ADU permits — address the existing dwelling on the plans
When the permit is for an ADU or addition, the plan-checker may require the existing dwelling to comply as well. Show the existing alarm locations on the as-built or existing conditions sheet. If the existing alarms are battery-only, note whether they will be upgraded to hardwired with battery backup and interconnection; R310.6 lets battery-only alarms stay in existing areas where the work does not remove wall or ceiling finishes. Getting ahead of this question on the plans avoids a back-and-forth correction letter after you've already submitted the full package.
Confirm placement height and mounting location
Smoke alarms must be installed on the ceiling or on a wall with the top of the alarm between 4 inches and 12 inches below the ceiling. Keep alarms at least 3 feet from the door to a bathroom that contains a tub or shower — steam can trigger nuisance alarms and some plan-checkers flag locations that are too close. Note placement specs in your general notes if the correction letter called out mounting location as a specific issue. In rooms with peaked ceilings, mount the alarm within 3 feet horizontally of the peak but no closer than 4 inches from any wall or obstruction.
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Why you got flagged
Plan-checkers flag R310.3 when the permit drawings don't show smoke alarm locations at all. This is the most common trigger. A fire protection note that says "smoke alarms per code" without drawing specific locations on the floor plan won't pass — the plan-checker needs to see each alarm drawn in, with a symbol or callout identifying it. Relying on a general note is a shortcut that almost always generates a correction.
The second common flag is missing the hardwired plus battery-backup specification. Battery-only alarms are not compliant for new construction in California. Plans that show alarm locations but use a generic note like "smoke detector" instead of specifying hardwired with battery backup get marked up. The electrical plans or general notes must explicitly state the power type — plan-checkers will not assume hardwired from silence.
Interconnection is the third frequent miss. R310.4 requires that triggering one alarm triggers all of them. Plans that show hardwired alarms but don't include a note or wiring diagram showing the interconnect circuit will get flagged. Some contractors assume interconnection is implied by hardwiring; it isn't. The interconnect wire is a separate conductor that needs to be called out.
A fourth scenario hits ADU additions: when an ADU is added to a property, plan-checkers often review the entire dwelling for R310.3 compliance, not just the new unit. If the existing home has battery-only alarms or no alarms shown on the as-built drawings, the correction letter may require upgrading the main house alarms as part of the ADU permit.
The official code
California Residential Code — Smoke Alarms (CRC R310.3), Building Standards Commission
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