A basement window can provide light, ventilation, or emergency escape. Sometimes it does all three. Sometimes it is a decorative rectangle that opens four inches and has excellent self-esteem. The code question is not the size printed on the carton. It is the clear opening a person can actually use.
The current Ontario numbers
Ontario's current Building Code adopts the National Building Code of Canada 2020 with Ontario amendments under O. Reg. 163/24. The City of Ottawa's current Building Code advisory for egress windows, reviewed under the 2024 Ontario code, summarizes a required egress opening as follows:
- At least 0.35 m² of individual unobstructed open area in the fully open position.
- No clear opening dimension less than 380 mm.
- Openable from inside without tools.
- Able to remain open without extra support.
- Clear travel through the window well, with the local reviewer confirming the applicable clearance and access details.
Area and minimum dimension apply together. An opening 300 mm high and 1,200 mm wide has 0.36 m² of area, but it fails the 380 mm minimum dimension. A very wide mail slot remains a mail slot.
Why window type changes the answer
Swipe to compare
| Window type | Useful feature | Common trap |
|---|---|---|
| Casement | Much of the frame can become clear opening | Hardware or sash may project into a tight well |
| Horizontal slider | Simple operation | Only part of the total frame opens |
| Awning | Good rain behaviour | Open sash can block the escape path in the well |
| Hopper | Compact ventilation | Often a poor clear opening and awkward escape path |
Measure with screens, guards, restrictors, and hardware considered in their normal installed condition. A removable screen may still need to be removable quickly from inside. A child-safety limiter on a required egress window needs an emergency-release arrangement that satisfies the applicable code. Safety features should cooperate, not arm wrestle.
The window well is part of the exit
The current Ottawa advisory calls for 550 mm clearance in front of the open window with continued egress out of the well, and says a well deeper than 1,000 mm needs a method to assist escape. Your municipality makes the project decision, so show the well width, depth, grade, cover, ladder or steps, and the sash swing on the permit drawing.
- The open sash must not consume the escape space.
- A well cover must be openable from inside without tools if it sits in the path.
- Snow, stored items, shrubs, and a locked gate must not turn the escape route into seasonal fiction.
- The well should be protected from soil collapse and coordinated with foundation waterproofing.
- Drainage must send water somewhere lawful and useful, not directly toward the new opening.
Sill height, steps, and fall protection
Access to the opening matters. The Ottawa advisory explains the general sill-height rule and basement exceptions, including the possible use of fixed steps or built-ins. It also discusses guard or window-restrictor rules where an openable window is low inside and high above exterior grade. Do not solve egress by accidentally creating a fall hazard on the other side.
Show fixed access clearly in the design. A loose stool is furniture, not an emergency-egress detail. A built-in cabinet can work only when its height, depth, strength, and path are accepted. This is a place for dimensions, not a note that says step here somehow.
Cutting the foundation is structural work
Enlarging a basement opening can require a building permit and structural design. The opening may need a lintel or other support, and the cut must avoid undermining the wall, footing, or adjacent openings. The City of Toronto lists new windows or doors where none existed before among work requiring a permit. Local rules determine your application.
Before excavation, request public utility locates and arrange private locates for owner-side services. Our Ontario One Call guide covers that step. Then coordinate excavation support, concrete cutting, lintel work, waterproofing, insulation, flashing, drainage, and interior finish as one detail. A saw cut is easy. A dry, supported opening is the actual product.
A bedroom window is not the whole exit plan
Whether a particular room or floor needs an egress window depends on the building layout, exits, suite arrangement, fire separations, sprinklers, and other code provisions. A legal secondary suite adds more questions about unit exits, interconnected alarms, and fire safety. Use the secondary-suite guide as the next checklist, not as a substitute for a code designer.
The clean approach is simple: confirm the permit path, design the whole escape route, select an exact window model, put its clear-opening data on the drawing, and inspect before covering. The window should be boring on a Tuesday and obvious during an emergency. That is excellent window work.
