You’re planning a bedroom in the basement, for a teenager, a parent or a tenant. The window down there is a small slider up near the ceiling. Is it big enough to count? If there were a fire, could someone actually get out through it in a winter coat? That’s what the egress rule is about.
legal basement apartment — plan the scope, drawings and next step.
In Ontario, an egress window must open from the inside without tools or special knowledge, give an unobstructed opening of at least 0.35 m² (3.77 sq ft) with no side smaller than 380 mm (15 in), and stay open on its own. If it opens into a window well, the well needs clear space in front of it. A new or enlarged egress window in a basement usually costs about $3,000–6,000 installed (GTA market range, 2026).
The part that trips people up is “unobstructed opening”. It’s not the size of the glass or the frame. It’s the hole you can climb through when the window is fully open. Here’s how to measure yours.
Prices are GTA market ranges in Canadian dollars, before HST, not a Capable Group price list. Scope and site conditions determine the quote.
The requirements, in plain words
On a small screen, swipe the table sideways to read all columns.
| Requirement | What it means at home |
|---|---|
| Opens from inside without keys, tools or special knowledge | No locked bars or hardware that needs keys, tools or special knowledge to release |
| Unobstructed opening at least 0.35 m² (3.77 sq ft) | Measure the clear open space, not the frame or the glass |
| No side of that opening under 380 mm (15 in) | A 380 × 380 mm opening is only 0.14 m², far too small. One side has to be much bigger |
| Stays open on its own | Nobody should have to hold it open while climbing out |
| Sill height | Above grade, the sill can be at most 1,000 mm off the floor. Basements are exempt from that limit, but someone still has to climb out: a fixed step or built-in bench under a high window is the usual answer |
| Window well | If the window opens into a well, leave at least 550 mm of clear space in front of it so a person can get out. An outward-opening sash must not obstruct escape. Any cover must open from inside without keys, tools or special knowledge |
Measure your window in 5 minutes
- Open the window as far as it goes.
- Measure the clear width and clear height of the open space, inside the frame and past any sash that stays in the way.
- Multiply them (in metres). You need 0.35 or more.
- Check that both numbers are 0.38 m or more.
- Check how it opens: it must stay open by itself and open without tools.
Two openings that pass: 400 × 875 mm, or 560 × 625 mm. Both give 0.35 m².
The window type decides most of this. A casement (it swings out like a door) gives almost its whole frame as clear opening, so it’s the easiest way to pass. A slider only opens half its width, so it has to be much bigger to pass. A small hopper near the ceiling may fail on clear-opening size; a high basement window also needs usable access.
Does every basement bedroom need one?
This is where Ontario differs from the national code. The Ontario Building Code’s rule is written per floor level that contains a bedroom: that level needs at least one compliant window, unless a door on the same level leads directly outside. The National Building Code asks for one in each bedroom.
In practice, many designers still give each basement bedroom its own egress window, because it’s safer and it’s what a family or tenant would expect, and your city may ask for it in a suite. If you’re building a legal basement suite, ask your city’s building department or your designer before you count on one window for two rooms.
A bedroom egress window is not the same as an escape window used to satisfy a suite’s exit or fire-protection design. That alternative can require a larger opening and well. Have the designer confirm which provision applies before ordering.
If your window doesn’t pass
Usually the existing opening has to be cut bigger, and in a basement that means cutting the concrete foundation wall. That’s structural work, so expect a building permit, a lintel above the new opening and a properly drained window well. It’s also why the window itself is the cheap part. Most of the cost is the cut, the well and the finishing around it.
“Can I do it myself?” People do, but a foundation cut is not a weekend window swap. A wrong cut can crack the wall, and a well without drainage turns into a funnel for water. If you hire it out, ask who handles the permit, who designs the opening, how the well drains and what happens to the wall inside.
Ask about one line on every quote: how the bottom of the well drains.
Where it goes wrong
- Big glass, small opening. A large slider that looks generous but only opens 300 mm wide fails.
- The well floods. A well with no drain or gravel collects snowmelt and rain right against your new window. Have the drainage route designed for the site. Any cover must open from inside without keys, tools or special knowledge.
- The bed goes under the window. Furniture or a built-in that blocks the window defeats the point. Plan the room in 3D first.
- The window is done last. Cut the opening before framing and insulation, so the wall is finished around it once.
One window, two plans (example)
Example, not a client project.
Picture a family adding a basement bedroom for a grandparent. Plan A keeps the old slider and adds a second, legal window later, after the drywall, the paint and the floor are done, so the wall is opened twice. Plan B cuts the opening first, installs a casement with a drained, covered well and a step below, and then frames and finishes the room around it. Same window, one round of dust instead of two.
What to do next
- Measure your window’s clear opening (the 5-minute check above).
- Decide which rooms will be bedrooms, now and later.
- If it fails, plan the new opening, the well and the drainage before framing.
- For a rental or family suite, check your city’s suite requirements and permit page.
How this goes with us
When a basement is getting a bedroom, the window is one of the first things we measure. If it passes, we design the room around it. If it doesn’t, the new opening, the lintel, the drained well and the permit are planned before framing, and they appear as their own lines in your itemized fixed quote. You see the room in a free 3D design with the window, the bed and the step where they’ll actually be.
Planning a suite? See what a legal basement apartment involves, why skipping the rules is costly in fines for renting an illegal suite, or ideas for a walkout, which gives the room a real door. For the whole lower level, start at basement renovation in Toronto.
Frequently asked questions
What size does an egress window need to be in Ontario?
The clear opening must be at least 0.35 m² (3.77 sq ft), with no side under 380 mm (15 in). It must open without tools and stay open on its own.
Does each basement bedroom need an egress window?
The Ontario Building Code requires at least one compliant window on each floor level that has a bedroom, unless a door on that level leads directly outside. Many designers give each bedroom its own anyway. Confirm with your city.
How much does an egress window cost in Ontario?
See legal basement rules by GTA city.
A new or enlarged basement egress window usually costs about $3,000–6,000 installed (GTA market range, 2026). Most of that is cutting the foundation, the well and the finishing.
Do I need a permit for an egress window?
Cutting a new or bigger opening in a foundation wall is structural, so expect a building permit. Replacing a window in an opening that already passes is usually simpler. Your city decides.
What is the maximum sill height for an egress window?
Above grade, 1,000 mm from the floor. Basements are exempt from that limit, but the window must still be usable, so a fixed step or built-in bench is common under a high basement window.
Can a window well be too small?
Yes. If the window opens into a well, leave at least 550 mm of clear space in front of it so a person can get out, and make sure the well drains. An outward-opening sash must not obstruct escape. Any cover must open from inside without keys, tools or special knowledge.
Sources
- Egress windows advisory (OBC 9.9.10.1), City of Ottawa Building Code Services (checked October 31, 2026)
- Markham builder tip: egress from dwelling units (PDF)
- OBC guide to a basement secondary suite (window well clearance), City of Oshawa
- Understanding egress openings, JELD-WEN Canada
- Installed cost range: Capable price ladder (GTA contractor price guides, 2026)
General information, not code advice. Your municipality’s building department decides what your house needs.
