The two numbers that matter are the sill height — how far the bottom of the opening sits above the floor — and the head height, the top. The normal domestic pair is a 900 mm sill and a 2100 mm head, giving 1200 mm of glass and putting the head in line with the doors. Gridsnap’s default window is 1200 mm wide at those heights, and its plan generator raises the sill to 1400 mm in bathrooms and other wet rooms, where you want daylight but not a view in.
Sill heights
| Room | Sill above floor | Why |
|---|---|---|
| Living room, bedroom, kitchen | 900 mm | Standing eye level looks out, furniture stands under it |
| Kitchen over a worktop | 900–1000 mm | The worktop is at 870–900 mm; the sill clears it |
| Bathroom, WC, utility | 1400 mm | Above head height when seated, and above a basin |
| Full-height / terrace glazing | 0 mm | A door-height opening you walk through |
| High strip glazing (a garage, a stair) | 1600–1800 mm | Light without a view either way |
The head stays at 2100 mm in almost every case, because it lines up with the door heads and gives one clean horizontal across the elevation. Raising a window head above the door heads is a deliberate architectural move, not a default.
Widths
| Constant Gridsnap uses | Value |
|---|---|
| Default width for a window you place | 1200 mm |
| Smallest width the generator will place | 600 mm |
| Largest width the generator will place | 2400 mm (3000 mm when a room needs the daylight) |
| Gap kept between two windows in a run | 500 mm |
| Distance kept clear of a wall end | 400 mm |
| Clear width above which “Auto” panes add a centre mullion | 1500 mm |
A window wider than about 2.4 m is normally built as a run of two or three units with mullions between, rather than one sheet — which is exactly what Gridsnap’s generator does when a room needs more glass than one window carries.
In the properties panel you can set anything: Width 300–5000 mm, Window height 200–3500 mm, and Bottom above floor (the sill) from 0 upwards. The panel’s own note spells out the relationship: “‘Bottom above floor’ is the sill height; window top = bottom + height.”
Panes
The Panes dropdown controls the mullions and transoms drawn in elevation and in 3D: Auto, Full glass, Divided in 2, Divided in 3, Divided in 2 (horizontal), Divided in 4 (cross). Auto puts a centre mullion in once the clear width passes 1500 mm and leaves narrower windows undivided, which is the behaviour every plan drawn before the setting existed already has.
Cills
Two separate things, each on or off per window, with the project default set in the ⚙ drawing settings:
- Interior board — the window board on the room side.
- Exterior cill — the weathering cill on the outside face.
Both default on. Which face is which is read from the plan, so an internal window reads indoors on both sides and a garden wall outdoors on both — that is why they are two switches rather than one three-way choice.
Gridsnap checks this automatically
Three Pro plan-check rules turn on sill and head heights, and it is worth knowing which number each of them reads:
blocks-window— keeps 600 mm clear in front of the glass. Only furniture taller than the sill counts, and never below 750 mm: a dining table in front of a French window is normal furnishing, a wardrobe is not. Something mounted above the head — a wall cabinet, a heat-pump head — blocks nothing.bed-under-window— a bed’s head must not stand under a window. A sill at or above 1200 mm clears the headboard and is exempt.tv-glare— a screen aimed within about 30° of a window closer than 4.5 m will wash out. Windows with sills above 1200 mm are exempt, since high strip glazing is not in the sightline.
The daylight rule uses the sill and head too — see how much window area does a room need.
What this doesn’t do
- Nothing checks escape or rescue windows. Codes that require a bedroom window of a minimum clear opening at a maximum sill height are not modelled.
- Nothing checks fall protection. A low sill above a drop needs a guard or laminated glass; Gridsnap draws the opening and says nothing about it.
- Glass area is the opening area. Gridsnap measures width × (head − sill). It does not subtract the frame, so the real glazed area is roughly 15–25 % less than the number the daylight rule works from.
- No opening direction. A window can be shown open in 3D, but there is no tilt-and-turn, top-hung or fixed-light distinction, and no restrictor.