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OBC DIVISION B · PART 9 · SECTION 9.7 — WINDOWS, DOORS AND SKYLIGHTS

How much window area is required for natural light in Ontario rooms?

Living and dining rooms need unobstructed glass area equal to at least 10% of the floor area they serve; bedrooms need at least 5%. Kitchens, bathrooms and utility spaces have no natural light requirement.

Code reference
Habitable rooms must be provided with windows or skylights giving minimum unobstructed glass areas expressed as a percentage of the served floor area: about 10 per cent for living and dining rooms and about 5 per cent for bedrooms and other finished rooms, with exemptions where adequate artificial lighting or other provisions apply.
OBC Division B, Subsection 9.7.2.

Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.

What this means in practice

Count glass area, not rough opening, and measure the floor area of the room served, including combined open spaces. This rule commonly constrains basement apartments, where small existing windows may not deliver 5% for a proposed bedroom, forcing window enlargement, which then also helps satisfy egress. Skylights count toward the requirement. Remember natural light is separate from ventilation and from bedroom egress: one window may need to satisfy all three, so size the largest governing requirement first when planning openings.

Plain language

Rooms you live in need real daylight: window glass equal to about a tenth of the floor area in living and dining rooms, and a twentieth in bedrooms. Kitchens and bathrooms can legally have no window at all.

Related questions

What standard must new windows meet in Ontario (NAFS)?What are the requirements for exterior doors on houses in Ontario?

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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.