Slimline systems push the frame down to the structural minimum — around 55mm on an opener and less on a fixed light — so the glass does as much work as possible. On a picture window or a run of fixed lights, the frame can almost disappear.
Every millimetre you take off the frame appears as glass. On a 1200 × 1200mm window, dropping the combined frame-and-sash section from 100mm to 70mm adds roughly 0.2m² of visible glass — about 15% more daylight through the same hole in the wall.
Fixed lights go further still, because there is no sash. If a window does not need to open — and many do not, once trickle ventilation is handled elsewhere — a slimline fixed light is both the cheapest option per square metre and the one with the most glass.
There is a structural floor. Glass is heavy: a double-glazed unit runs about 25kg per square metre, so a 2.5 × 1.5m pane weighs roughly 95kg. Beyond a certain size, the frame has to get deeper — not wider on the face, but deeper front-to-back — to carry the load and resist wind. A good supplier will tell you where that line is for your opening, rather than quoting a frame that will deflect.
| Job | Typical cost |
|---|---|
| Casement window, approx. 1200 × 1200mm supplied & fitted | £700 – £1,050 |
| Casement window, approx. 1800 × 1200mm supplied & fitted | £980 – £1,450 |
Ranges include VAT and installation. Final price depends on size, spec, access and site condition — a survey gives you a fixed number.
Around 55mm for a thermally broken opener on most residential systems, and less on a fixed light. Premium systems go slimmer still. Below that you run out of room for the thermal break and the glazing pocket.
Not on the face. Strength comes from the depth of the section, not its visible width — which is exactly why aluminium can be slim and strong at the same time, and why uPVC cannot.
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