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Smoke and Carbon Monoxide Alarm Rules for Ontario Houses: Where, How Many, and What the Inspector Tests
Permits and code Life safety Plain English
Smoke and Carbon Monoxide Alarm Rules for Ontario Houses: Where, How Many, and What the Inspector Tests
The alarm rules are short, specific and the last thing the inspector tests before you get the keys. They are also the item most often wrong on a final because somebody bought the alarms at the hardware store the morning of. Here is the whole set.
Short version: New houses in Ontario need smoke alarms on every storey including the basement, in every bedroom, and in the hallway serving the bedrooms, all interconnected so they sound together, hard-wired with battery backup. Carbon monoxide alarms are required adjacent to every sleeping area when the house has a fuel-burning appliance, a fireplace or an attached garage. A secondary suite needs its own set, interconnected within the suite and with the main unit as the Code requires. Existing houses fall under the Fire Code, which requires working smoke alarms on every storey and outside sleeping areas and CO alarms near bedrooms where there is a fuel-burning appliance or attached garage.
Smoke alarms in a new house
- One on every storey, including the basement and any finished attic
- One in every bedroom (sleeping room)
- One in the hallway or corridor serving the bedrooms, within the distance the Code sets of each bedroom door
- All of them interconnected: when one sounds, they all sound
- Hard-wired to the house electrical with a battery backup, installed by the electrician under the ESA permit
- Ceiling or high-wall mounted per the manufacturer, away from the kitchen where cooking sets them off and away from bathroom doors where steam does
- In larger houses, additional alarms so that no point on a storey is farther from an alarm than the Code allows
The current edition of the Ontario Building Code carries the exact article; the OBC Code Navigator will give you the Part, Section and Article in ten seconds if you want to read the wording. A visual signal (strobe) is required in some sleeping rooms for occupants with hearing loss; ask if it applies.
Carbon monoxide alarms
Required when the house has any fuel-burning appliance (gas or propane furnace, boiler, water heater, range, fireplace, wood stove) or an attached garage, which is nearly every house in Ontario. They go adjacent to each sleeping area, so that someone asleep can hear them, and in any room with a fuel-burning appliance that is separated from the rest of the house in some configurations. Combination smoke and CO units are acceptable where both are required at the same spot, as long as the unit is listed for both. Mount per the manufacturer; CO mixes with air, so height matters less than location. Gas fireplaces and wood stoves both trigger the requirement.
Secondary suites and two-unit houses
A basement apartment or garden suite needs its own smoke alarms in the same pattern (every storey, every bedroom, the hallway), its own CO alarm adjacent to the sleeping area, and interconnection within the suite. The Code also requires interconnection between the units in some cases so that a fire in one wakes the other; the fire separation between the units is the other half of the rule. Legal basement apartment requirements covers the whole package.
What the inspector does at final
- Counts alarms against the plan: every storey, every bedroom, the hallway.
- Presses test on one and listens for all of them.
- Checks for the hard-wire and battery backup, and the ESA certificate.
- Checks the CO alarm location relative to the bedrooms.
- Fails the final if anything is missing. It is the easiest fail to fix and the most annoying one to get, because the re-inspection costs a day.
Occupancy permit requirements lists the rest of the final inspection.
Existing houses: the Fire Code
Once the house is occupied, the Ontario Fire Code takes over. It requires working smoke alarms on every storey and outside all sleeping areas in every home, and carbon monoxide alarms adjacent to sleeping areas where there is a fuel-burning appliance, fireplace or attached garage. Landlords are responsible in rental units. Replace smoke alarms every ten years and CO alarms per the date on the unit (often seven to ten years); test monthly; change backup batteries when the clocks change. The fire department enforces it, and after a fire the insurer asks.
Related guides on this site
Sources and further reading
Questions people ask
How many smoke alarms does a house need in Ontario?
One on every storey including the basement, one in every bedroom, and one in the hallway serving the bedrooms, all interconnected and hard-wired with battery backup in a new house. Larger storeys may need more to meet the maximum distance rule.
Where do carbon monoxide alarms go in a house?
Adjacent to every sleeping area, and in some configurations in the room with the fuel-burning appliance. Required whenever the house has a fuel-burning appliance, a fireplace or an attached garage.
Do smoke alarms have to be interconnected in Ontario?
In a new house, yes: when one sounds, all sound. They must be hard-wired with battery backup. In existing houses the Fire Code requires working alarms on every storey and outside sleeping areas.
Does a basement apartment need its own smoke alarms?
Yes: every storey, every bedroom and the hallway within the suite, interconnected, plus a CO alarm adjacent to the sleeping area, and interconnection with the main unit where the Code requires it.
Got a code question this page did not answer?
Ask it in plain English and the OBC Code Navigator answers with the Part, Section and Article from the Ontario Building Code. Two questions free. Your municipality and inspector still have the final word.
Reviewed and updated October 2026. Costs are Ontario ranges, not quotes; code references are to the Ontario Building Code in force at that date.
