How much does it cost to install hardwired smoke and CO detectors?

The price is per detector, and the number you see depends on how the wiring gets there. The typical SoCal range is $150 to $450 for a small interconnected set as of October 2026. This is a desk estimate, not a published price list, so the set price is built from the cost of each unit plus the labor to run interconnect wire between them.

A quick way to estimate

Count the detectors. Required locations in a home usually include each bedroom, the area outside the sleeping rooms, and each level. Each added detector adds the unit cost plus the interconnect wire, and the minimum visit charge can push a one-detector job toward the top of the range. That is why the set price here is a range, not a formula.

Swap versus new location

Swap. Replacing an old hardwired detector with a new one on the same base is quick and cheap. Most of the cost is the device and the visit.

New location. Adding a detector where there is none means running cable from the nearest connection and, for an interconnected system, between the units. In a finished house, that means fishing cable through attics and walls, and patching. This is the case that pushes cost toward the top.

What the law says

• Smoke alarms: Health and Safety Code 13113.7 requires State Fire Marshal-listed smoke alarms in dwellings. When a building permit is issued for alterations, repairs or additions above $1,000, the permit issuer cannot sign off until the permittee shows the required alarms are listed devices. A battery alarm that met the standard when installed can still satisfy the law unless local rules say otherwise.
• CO devices: Section 17926 requires a carbon monoxide device in each existing dwelling unit that has a fossil-fuel appliance, a fireplace or an attached garage. Placement follows the current building standards for new construction, or the manufacturer’s instructions where feasible.
• Fines: a violation of either section is an infraction with a fine up to $200 per offense. A property owner gets a 30-day notice to correct under the CO section.
• Landlords: the owner of a rented home or apartment is responsible for testing and maintaining required alarms.

Local rules can be stricter, so ask your city. The smoke and CO detector page covers placement and detector types.

Permits and fees

A detector swap is often exempt from a permit. New detectors added during a remodel fall under the project’s permit. When the alterations on a dwelling exceed $1,000, the city’s final sign-off checks the alarms. Fees depend on the city. Look at your page, such as Los Angeles, Irvine or Riverside, and the permits guide.

How to compare quotes

1. Ask for a count and a location for each detector.
2. Ask if the units are hardwired with battery backup, and whether they are interconnected.
3. Ask whether smoke and CO are separate units or combined.
4. Ask if patching is included.
5. Ask about the manufacturer’s expiration date. If you are also getting an electrical inspection, bundle the detectors into it to avoid a second visit charge. The service call cost page explains minimum charges.

What is not included

• Detectors in outbuildings or detached garages, unless listed
• Drywall, ceiling repair or paint
• Fire alarm monitoring or a security system
• Fire sprinklers
• Permit fees, when the job needs a permit