What makes a basement apartment legal in Ontario?

Where these rules come from

The figures in this guide are taken from the Ontario government’s own guide, Add a second unit in your house, which summarises the Building Code rules for second units in existing houses. Three cautions:

• It applies to detached, semi-detached, and row houses more than five years old. Newer houses are treated as new buildings with different requirements.
• The province notes that the guide may not reflect every change made since the More Homes Built Faster Act, 2022.
• Your municipality’s building department decides how the rules apply to your house. Treat this guide as orientation, and confirm the details before you design.

1. Zoning and permission

Before anything else, the property must be allowed to have a second unit. Municipal zoning by-laws set rules on things like parking, entrances, servicing, and unit size. If the property cannot comply, a minor variance may be possible; without one, the unit cannot be added. Some municipalities also require licensing or registration. Mississauga, for example, requires second units to be registered.

2. Ceiling height

A basement unit needs a ceiling height of 1.95 m over the entire required floor area, including the route to the exit.

This is the requirement that most often decides whether a basement can be converted economically. If the basement is too low, the remedy is to lower the floor, which is major structural work.

3. Room sizes

The guide lists minimum floor areas, including:

Room	Minimum area
Living area	13.5 m²
Dining area, where separated by walls	7 m²
Kitchen	4.2 m²
Combined living, dining, and kitchen in a one-bedroom unit	11 m²
Main bedroom, without a built-in closet	9.8 m²
Other bedrooms, without a built-in closet	7 m²
Open-concept unit combining sleeping, living, dining, and kitchen	13.5 m²

The bathroom needs enough space for a sink, a toilet, and a shower or bath.

4. Windows

Where a window is not being used as an exit:

• living and dining rooms need window area equal to 5% of the floor area,
• bedrooms need 2.5%, and
• kitchens, bathrooms, and laundry rooms need no window.

5. Exits

Getting out in a fire is the centre of the whole set of rules.

• A separate exit for the unit is preferred.
• A shared exit is allowed if the exit area has a 30-minute fire separation and smoke alarms interconnected to both units.
• If one unit’s exit passes through the other unit, a second means of escape is required, typically a window.

For a basement or ground-floor escape window, the guide gives an unobstructed open area of at least 0.38 m², an openable portion at least 460 mm in each direction, and a sill no more than 900 mm above the floor or steps. It must open easily without tools.

6. Fire separation

• A 30-minute fire separation is required between the two units, and between each unit and any common area.
• This can be reduced to 15 minutes if the entire house has interconnected smoke alarms.
• Any existing floor or ceiling that is altered must be built to the 30-minute standard.

A fire separation can be a floor, a wall, a door with a self-closing device, or a combination. In practice it means fire-rated drywall on the basement ceiling and on walls around shared spaces, with insulation in the cavities and protected openings.

7. Smoke and carbon monoxide alarms

• Smoke alarms are required on every level, outside sleeping areas, in each bedroom of the second unit, and in shared areas such as entrances and laundry rooms. They must include a flashing light.
• Carbon monoxide alarms are required if the house has a fuel-burning furnace or appliance, or an attached garage. They go near sleeping areas of the second unit and in a separate furnace room where there is one.

8. Heating and ventilation

• A single furnace and shared duct system is permitted.
• In that case a duct-type smoke detector is required in the main supply or return duct, which shuts down the furnace when it detects smoke, so that smoke is not blown from one unit into the other.
• The bathroom and kitchen need ventilation, by an exhaust fan or an openable window.

A separate heating system for the unit goes beyond the minimum, and improves comfort and sound and odour separation.

9. Plumbing

The unit needs hot and cold water, a bathroom with a sink, toilet, and tub or shower, a kitchen sink, and access to laundry, either shared or in the unit. Each unit needs its own water shut-off valves. A backwater valve may be required in areas prone to sewer backup.

10. Electrical

Every room and space needs a light and a switch, with switches at the top and bottom of stairs. Electrical work requires a separate permit from the Electrical Safety Authority, which inspects it. This is in addition to the municipal building permit.

11. The permit

A second unit always requires a building permit. The application includes construction drawings and a fee, and inspections take place during construction. An existing unit built without a permit is legalised through the same process: drawings, a permit, upgrades to meet the rules, and inspections.

Is my basement a good candidate?

Check five things:

1. Height. Measure floor to underside of the ceiling, and note low ducts and beams.
2. Exit. Is there a side or rear door with its own stair, or room for one?
3. Windows. Are they big enough, or can they be enlarged?
4. Moisture. Is the basement dry all year?
5. Zoning. Does your municipality allow a second unit at your address?

We check all five at a free site visit and tell you plainly what your basement needs.

Next steps

See our legal basement apartment service and the basement renovation cost guide, which compares an apartment with an ordinary finished basement. When you are ready, book a free consultation.