01 · Introduction
Why natural light is a Code requirement, not a preference.
Every principal room and every sleeping room in an Ontario dwelling needs a minimum amount of daylight — measured in glass area, relative to the floor it serves. The rule is simple in principle and awkward in practice. This guide explains the principle, the measurements the Ontario Building Code actually asks for, and the missteps that cost people time on permit review.
Part 9 of the 2024 Ontario Building Code (OBC) sets out minimum window areas for dwelling units in Article 9.7.2.3. The logic is straightforward: habitable rooms should be usable without artificial lighting during the day. What makes the requirement tricky is that the Code measures glass area, not the overall window unit — and the percentage required depends on the room type and whether electric lighting is provided.
AD Polyline's OBC Residential Natural Light Calculator automates this computation and generates window configurations that meet or exceed the minimum. This article explains the rules behind the tool so you can use it — and defend your numbers — with confidence.
What's in this guide
02 · Key Terms
The vocabulary the Code uses.
Before looking at any percentage, it helps to be precise about what is being measured. The OBC's wording matters because the tool and the reviewer will both apply it literally.
Glass area (glazing area)
The transparent portion of the window — the actual pane of glass — excluding the frame, sash, mullions, and any opaque trim. This is what the Code counts.
Unit area
The overall dimension of the window as advertised (e.g. 24″ × 36″). Always larger than the glazing area because it includes the frame.
Unobstructed
Not blocked by structures, coverings, or permanent interior obstructions that reduce daylight. Skylights are excluded from this calculation.
Floor area served
The floor area of the room the window serves. In combination rooms (e.g. open-plan living/dining), this may extend into adjacent areas under OBC 9.5.1.4.
Principal room
A living room, dining room, or combination of these. Under OBC 3.7.2.1, principal rooms must have natural light.
Electric lighting
Permanent artificial lighting, wired to the building's electrical system. Some room types waive the window requirement when electric lighting is provided.
03 · The Rules
OBC Table 9.7.2.3 — minimum glass area by room.
The Code divides habitable rooms into five categories. Each has its own minimum percentage, and each treats electric lighting differently. These percentages apply to the glass area relative to the floor area served.
| Room type | Without electric lighting | With electric lighting |
|---|---|---|
| Living & dining rooms | 10% of floor area | 10% of floor area |
| Bedrooms & other finished rooms | 5% of floor area | 5% of floor area |
| Kitchen / kitchen space / kitchen alcove | 10% of floor area | Not required |
| Laundry / basement recreation / unfinished basement | 4% of floor area | Not required |
| Water-closet room (bathroom) | 0.37 m² minimum | Not required |
Summary based on OBC 2024 Table 9.7.2.3 — refer to the Code for the authoritative text.
Note — two rules can apply
The Code says the more restrictive requirement shall govern. For example, a large bedroom that also doubles as a home office must still meet the bedroom requirement at minimum; if another clause imposes a higher figure, the higher figure wins.
04 · The measurement
Glazing area vs. window unit area.
This is the single most common misunderstanding we see on permit reviews. A window that reads 24″ × 36″ on a supplier's spec sheet does not have 864 in² of glazing. The frame — typically around 2″ per side on a standard unit — reduces the transparent glass by several square feet.
Fig. 1 — A 24″ × 36″ window (6.00 ft² overall) provides only about 4.44 ft² of glazing once the frame is deducted. The Code counts the right-hand value.
The calculator uses an assumed 2″ frame on all sides to derive glazing from nominal dimensions — a conservative default that matches most vinyl and wood-clad residential units. If you are specifying narrow-frame aluminum or wide architectural sash, confirm the actual glass dimension from the manufacturer's data sheet.
Common mistake
Listing a window's rough opening or nominal size as the glazing area on your drawings. Reviewers catch this almost every time. Always dimension the glass, or include a note citing the supplier's clear-glass dimension.
05 · The exception
How electric lighting changes the requirement.
The Code recognizes that kitchens, bathrooms, laundry rooms, and unfinished basement recreation rooms do not strictly require daylight to function. For these rooms, providing permanent electric lighting waives the window requirement entirely.
This is particularly useful for:
- Basement conversions where grade-level windows are limited or impossible to add economically;
- Interior kitchens in open-plan dwellings where the kitchen is set back from an exterior wall;
- Bathrooms on interior walls, which is a near-universal condition in modern residential layouts.
Important — two room types are never exempt
Living rooms, dining rooms, and bedrooms always require their minimum glass area, even when electric lighting is provided. The 5% bedroom rule under OBC 9.7.2.3 is one of the most consistently enforced clauses in residential permit review.
06 · When to use the tool
Situations where the calculator earns its keep.
New builds
Sizing windows early in design development to balance daylight, elevation composition, and energy performance.
Additions & ADUs
Verifying that new rooms added to an existing house meet minimum daylight requirements before submission.
Basement conversions
Checking whether a proposed bedroom can meet the 5% rule, and sizing enlarged windows where needed.
Permit pre-check
Running a sanity check on window schedules before drawings go to the municipality.
Value engineering
Finding the smallest compliant window size to reduce cost without failing review.
Client conversations
Explaining to homeowners why a particular window can or cannot be downsized for budget.
07 · A worked example
Sizing windows for a 180 ft² living room.
Consider a typical principal room in an Ontario dwelling: a combined living/dining space with a floor area of 180 ft² (16.72 m²). The room has electric lighting, but because it is a principal room, the 10% rule still applies.
Step-by-step
1. Identify the room type: Living/dining → 10% of floor area required, regardless of electric lighting.
2. Calculate the minimum glazing: 180 ft² × 10% = 18.00 ft² of glass required.
3. Pick a window size: A single 24″ × 36″ unit provides only ≈4.44 ft² of glass — well short.
4. Iterate: Two 48″ × 48″ units or a larger picture window will clear the requirement. The calculator generates several compliant configurations and shows cost trade-offs.
Fig. 2 — A single small window falls well below the 10% rule; two larger units clear it with a modest buffer.
Common pitfalls
Counting the window unit, not the glass. As shown above, a 24″ × 36″ unit gives roughly 4.44 ft² of glazing, not 6.00 ft². Specify glass dimensions, or include the manufacturer's stated glazing area on your schedule.
Forgetting skylights are excluded. OBC 9.7.2.3 measures wall windows. Skylights do not count toward the minimum, though they absolutely help the space feel daylit.
Using a room's rough framing area. The Code uses finished floor area served. Deduct any built-in cabinetry or similar permanent reductions where they apply.
Assuming electric lighting waives everything. It waives windows only for kitchens, bathrooms, laundry rooms, and unfinished basement rec rooms. Bedrooms and principal rooms always need daylight.
Not leaving a buffer. Aim for 5–10% over the minimum. Measurement tolerances, site variations, and different product lines can shift actual glazing by a few square inches — enough to fail a close call at inspection.
08 · Disclaimer & references
Using this guide responsibly.
IMPORTANT NOTICE
This article and the associated OBC Residential Natural Light Calculator are provided for informational and preliminary design purposes only. They are not a substitute for the Ontario Building Code itself, for the judgement of a qualified designer or engineer, or for the approval of the Authority Having Jurisdiction.
All calculations are summaries based on the 2024 edition of the Ontario Building Code, Table 9.7.2.3. The Code is subject to amendment; always verify the current in-force text before relying on any figure for permit submission. Local municipalities may impose additional or more restrictive requirements.
Cost estimates produced by the tool are rough ranges only, based on typical Canadian retail pricing at time of publication. Actual prices vary by brand, glazing type, region, tax, and availability — confirm with your supplier.
AD Polyline makes no warranty, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of this content. Use at your own discretion and consult your designer for project-specific advice.
References. Ontario Building Code (O. Reg. 163/24), Division B, Part 9, Article 9.7.2.3 — Natural Light; Article 3.7.2.1 — Window Areas; Article 9.5.1.4 — Combination Rooms.
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