A habitable room wants window area of at least one eighth of its floor area. A 16 m² bedroom needs 2 m² of window; a 30 m² living room needs 3.75 m². It is the oldest rule of thumb in residential planning and it survives because it is close enough to right across a huge range of rooms — and because you can check it on a napkin.
Working it out
With a standard 900 mm sill and 2100 mm head, the glass is 1200 mm tall. So the width of window a room needs is:
width (mm) = floor area (m²) ÷ 8 × 1 000 000 ÷ 1200
| Room | Floor area | Glazing needed | As standard windows |
|---|---|---|---|
| Small bedroom | 10 m² | 1.25 m² | one 1050 mm window |
| Double bedroom | 16 m² | 2.0 m² | one 1700 mm window |
| Living room | 25 m² | 3.13 m² | two 1300 mm windows |
| Open-plan living/kitchen | 40 m² | 5.0 m² | a 4.2 m run, or a terrace door plus a window |
Two things the arithmetic hides:
- Orientation is not in the rule. 2 m² facing north is not 2 m² facing south. A north room deserves more than the minimum; a south room with the minimum may still overheat.
- One eighth is a floor, not a target. Rooms people actually like tend to land nearer one fifth, and a living room reads far better with light from two sides than with the same glass area on one wall.
What counts as “the room’s window”
Gridsnap decides which windows serve which room geometrically: it steps just off each face of the window and asks whether that point lands inside the room’s area polygon. A window in a shared wall therefore serves both rooms, and a window in a garden wall serves neither.
The glazing figure it sums is width × (head − sill) — the structural opening, frame included. That is the common reading of the rule in the codes that state it, but it does mean the real glass area is roughly 15–25 % less than the number being tested. If you are close to the line, add width.
Gridsnap checks this automatically
Two Pro rules, both in the plan check:
no-window— a bedroom or living room with no window at all: “‘Bedroom 2’ is a bedroom with no window”.daylight— glazing below one eighth: “‘Living’ has ~2.1 m² of glazing for 24.0 m² of floor — rule of thumb is at least 1/8 of the floor area”.
Both need the area to be named. Gridsnap’s own auto-detected rooms are
called Room 1, Room 2 and match nothing, so select the area and type
Bedroom or Living into Name in the properties panel. Both rules also
stay quiet until the plan has at least one window anywhere — while none are
drawn yet the warnings would just be noise.
When Gridsnap generates a plan from a brief, it works the same sum backwards: it sizes each room’s window run to (area ÷ 8) of glass at a 900 mm sill and a 2100 mm head, spreads it across every façade the room owns, and raises the sill to 1400 mm in wet rooms.
What this doesn’t do
- Only bedrooms and living rooms are checked. Kitchens, bathrooms, halls and studies are not — even though several national codes require daylight in a kitchen too.
- It is a rule of thumb, not a daylight calculation. No daylight factor, no sky component, no overshadowing by the building next door, no orientation, no glazing transmittance. A room with a deep balcony over its only window passes this rule and is dark.
- Rooflights count as nothing. Gridsnap has no rooflight entity, so a room lit from above reads as having no window.
- It is not ventilation. The openable area a room needs is a separate requirement, usually a smaller fraction, and Gridsnap does not model which part of a window opens.