Enlarging a basement window: what the building code says

2026-01-28 — Ottawa-Gatineau

Want to turn your basement into a bedroom, office or rental unit? In most cases, the building code requires an egress window — and the original small horizontal window almost never qualifies. Here are the rules to know.

Why an egress window?

In a fire, every bedroom needs a usable emergency exit. In a basement, that's usually the window — so it must be big enough for an adult to get through.

Minimum dimensions (Ontario — OBC)

• Minimum unobstructed opening: 0.35 m² (3.8 sq ft)
• No dimension under 380 mm (15")
• Must open from the inside, without tools
• If the window opens into a window well: at least 760 mm (30") of clearance in front

And in Quebec?

Requirements are very similar: at least 0.35 m² of clear opening, no dimension under 380 mm. Cities like Gatineau apply the Quebec Construction Code — always check with your city before cutting.

Do you need a permit?

• Ottawa: yes, enlarging an opening in a foundation wall generally requires a building permit (structural modification)
• Gatineau: a permit is also required to modify an opening in an exterior wall
• Good news: the process is simple for this type of work, and the contractor can guide you

Typical project sequence

• Verify required dimensions for your project
• Permit (if required)
• Excavation and cutting — see our service →
• Window well and window installation
• Inspection and compliance

Window well requirements

When the bottom of the window is below grade — almost always in a basement — a window well is required. The rules that matter: at least 760 mm (30") of clear space measured from the window outward, a well wide enough to allow the sash to open fully, and drainage at the bottom (a gravel bed connected to the weeping tile, or a drain) so the well does not fill with water. Wells deeper than about 44" should have a ladder or steps. A cover is allowed if it can be lifted from the inside without tools.

Bedroom vs other rooms

The egress requirement applies to bedrooms. A family room, office or gym in the basement does not need an egress window under the Ontario Building Code, but the basement as a whole still needs a second means of escape if it is a separate dwelling unit. In practice, most homeowners cut the egress window in the room they intend to use as a bedroom, and often a second one for natural light in the living area — cutting both on the same day costs less than two separate visits.

Secondary suites and rentals in Ottawa

If the basement becomes a rental unit (a "secondary dwelling unit" in Ottawa, or an "accessory dwelling" in Gatineau), the egress window is only one of several requirements: ceiling height, fire separation between units, smoke and CO alarms interconnected, and a separate entrance in many cases. Cities inspect these units; an unpermitted basement apartment without egress is a common reason for an order to vacate. Getting the window right first makes the rest of the approval simpler.

What it costs

Cutting the opening in a poured concrete wall typically runs $800 to $1,500; the full egress project with excavation, well, window and finishing usually lands between $3,500 and $7,000. See our detailed article: Egress window cost in 2026.

Frequently asked questions

Can I use a casement window instead of a slider?

Yes, and it is often the better choice: a casement opens fully and gives more clear opening for the same frame size, which makes it easier to reach the 0.35 m² minimum.

Does the existing small window count if I add a door?

A walk-out door to the exterior is itself a means of egress, so a bedroom with a proper exterior door does not need an egress window. Most basements do not have that option without a full walk-out excavation.

Who confirms the window is compliant?

The building inspector, at the final inspection under your permit. Keep the window manufacturer's clear-opening specification on hand — it is what the inspector will compare to the code minimum.

A code-compliant basement bedroom can add tens of thousands of dollars to your home's value — and let you rent legally.