Over the past few years, more and more states have passed laws or regulations that require home sellers to install smoke and carbon monoxide detectors before putting a house on the market. At last count, more than two-thirds of states had such requirements and required home inspections by fire officials to make sure the alarms are installed correctly and in working order.

This is an advantage for the buyer, but if you’re the seller, you may balk at the cost of buying and installing a number of detectors all at once. Guidelines set by the National Fire Prevention Association (NFPA) call for nine smoke detectors and four carbon monoxide detectors for a typical three-bedroom, two-bathroom house with an attic and basement (see our CO and smoke detector buying guide for tips on where to place them).

But when it comes to safety, the cost is a relative bargain. You can get a good standalone CO or smoke detector for $30, connected models cost about twice that. So even if you’re installing 10 alarms or more to meet the regulations, it will cost well under $500. The bonus? You’ll be safer while still living in the house.

Even if home buyers are looking for a house in a state that doesn't require alarm installations, it's wise to follow the NFPA's recommendations once they move in. Here's what else you should know.

Check the Alarm's Pedigree

In our smoke and carbon monoxide alarm ratings, every model that we score performs to the recommended safety standard. But the highest-scoring models alarm faster within a given time range. That’s crucial because in a span of only several minutes, a fire can reach its flashpoint—the point at which it becomes so hot it can set furnishings ablaze without even touching them. And carbon monoxide, a colorless, odorless gas, can kill you in minutes.

In addition to checking your alarms against our ratings, you can also pop them off the wall and look on the back for a UL symbol, for Underwriters Laboratories, which indicates it’s been tested to a widely accepted safety standard.  

Test the Batteries

This step is so obvious that it’s easy to skip, but according to the NFPA, nearly half of home fires that fail to trigger alarms occur in houses where batteries have expired or been removed from a smoke alarm. Even if the home you're considering received a sign-off from the fire department that the alarms are working, that inspection may have occurred months before the closing. In the meantime, the seller may have borrowed batteries from the alarm to power the remote control. If so, you'll want to know sooner rather than later.

Once you're settled in, check the batteries twice a year or as often as the instructions in the owner's manual recommend. A good reminder is to check the detectors when the time changes in the spring and the fall.

Do You Want Real-Time Monitoring?

Many security companies now offer smoke and carbon monoxide alarms that are monitored in real time. If an alarm is triggered, the fire department will be sent to your home. So when buying a house, make arrangements with the seller to transfer the monitoring contract to your name, or sign up for the service yourself. 

If you don’t want to pay a monthly monitoring fee, consider installing a network of interconnected alarms. These models are more expensive, but they trigger all the units in your home to sound when any alarm detects smoke or carbon monoxide, which is particularly valuable if you’re asleep upstairs and there’s a slow carbon monoxide leak from your furnace in the basement. Our top-rated model, the First Alert OneLInk SCO501CN, is a battery-powered combination photoelectric smoke and carbon monoxide detector that wirelessly connects to multiple units.

But note that because photoelectric smoke alarms are best at detecting smoldering fires, you'll still want ionization alarms as well. They're better at detecting fast, flaming fires. For more information, check our smoke and carbon monoxide alarm buying guide. No combination CO and smoke alarm currently on the market detects both types of fires as well as carbon monoxide. You need all three.