Ontario Building Code sets a minimum window area for each room type based on floor area. Answer three questions to find the minimum for your room.
Step 1 — What room are you designing?
Step 2 — What is the floor area of the room?
Step 3 — Does the room have electric lighting?
Almost all rooms have electric lighting. Choose "No" only if there is truly no light fixture.
Ontario Building Code Table 9.7.2.3 requires a minimum area of natural light for certain room types in residential buildings. The requirement is expressed as a percentage of the room's floor area (for most rooms) or as a fixed area (for bathrooms).
Living rooms and dining rooms require windows equal to at least 10% of the floor area, regardless of whether the room has electric lighting. A 15 ft × 12 ft living room (180 ft²) needs at least 18 ft² of glass area.
Bedrooms require windows equal to at least 5% of the floor area. A 12 ft × 10 ft bedroom (120 ft²) needs at least 6 ft² of glass. Note that bedrooms also have egress requirements — minimum opening size for emergency exit.
Kitchens, bathrooms, and laundry rooms may not require any windows if electric lighting is provided. Check with your building department if you are unsure.
The "glass area" refers to the glazed (transparent) portion of the window — not the overall rough opening or frame size. Measure the visible glass, not the window unit.
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