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Flat roof windows are how extensions get their daylight: a sealed glazed unit on an insulated upstand, £1,000–£3,000 fitted for standard sizes in 2026. The category splits into four products at very different price points — here’s how to choose, what it really costs, and the details that keep them leak-free.
The four types
Fixed flat glass — the default: a double- or triple-glazed unit, flush and minimal, no moving parts. Cheapest per square metre of daylight; supply from roughly £400 for compact standard sizes (Fakro’s non-opening flat rooflights start around £670, per Eurocell’s comparison data).
Opening (usually electric) — the same unit with powered opening and typically a rain sensor, for ventilation in kitchens and bathrooms under flat roofs. Worth the £300–£800 premium in moisture rooms; skip it elsewhere.
Walk-on glass — structural glazing you can stand on, for terraces and floor-lights. A different engineering category with bespoke pricing — don’t budget it like a rooflight.
Polycarbonate domes — the budget end, common on garages and commercial roofs. Cheap and tough, but hazy with age and noisy in rain; for living spaces, glass earns its premium, as covered in the custom skylights guide.
(A roof lantern is the fifth option — a pitched glazed structure rather than a flat unit, bought for drama as much as daylight.)
What it costs
| Configuration | Typical 2026 cost |
|---|---|
| Fixed flat glass, standard size, supply | £400–£1,500 |
| Electric opening version, supply | +£300–£800 |
| Insulated upstand (if not building your own kerb) | £150–£400 |
| Fitted totals, standard sizes | £1,000–£3,000 |
Ranges per BookaBuilderUK’s 2026 data and Toughened Glass Systems’ price guide; run your own configuration through the cost calculator. Sizing note: flat-roof units use six-digit codes — width × height in centimetres (120090 = 120 × 90cm) — unlike the letter codes on pitched roof windows. VELUX’s flat range uses CF- prefixes for fixed and CV- for vented units, with matching upstands at 150mm and taller.
The three details that matter more than the brand
- The upstand. Glazing must sit proud of a flat roof — at least 150mm of kerb — so water drains around it. Buy the manufacturer’s insulated upstand or have the builder form one; either way, the waterproofing tie-in between roof membrane and kerb is where flat-roof windows live or die. Make the quote name who owns that joint.
- Glazing spec by orientation. Flat glazing faces the sky directly, so south-facing units cook without solar-control glass — specify it at purchase. For bedrooms under the glass, laminated inner panes and acoustic-minded glazing tame rain drumming.
- Condensation management. A cold glass ceiling over a kitchen is a condensation machine if the spec is wrong — insulated upstands, warm-edge spacers and decent glazing prevent it; our condensation guide covers diagnosis if you’ve inherited a problem.
Buying it
Standard sizes are dramatically cheaper than made-to-measure — design the opening around a catalogue size where possible (the bespoke maths rarely favours you). VELUX and Fakro anchor the branded market; specialist UK makers compete well on flat glass. You can browse flat roof skylights on Amazon to calibrate prices, though most buyers order through merchants or the installer. Planning permission is rarely an issue on extensions; building regs apply as ever — structural opening plus Part L glazing, self-certified by a competent-person-scheme installer.
Frequently asked questions
How much does a flat roof window cost in the UK?
£1,000–£3,000 fitted for standard-size flat glass units in 2026. Supply-only starts around £400–£670 for fixed units (budget domes less, premium and opening versions more), with installation, upstand and making-good accounting for the rest.
Do flat roof windows need an upstand?
Yes — a kerb (upstand) raises the glazing above the roof surface, typically to at least 150mm, so water drains around rather than over it. Most manufacturers sell matching insulated upstands; the waterproofing tie-in to it is the detail that decides whether the window ever leaks.
What's the difference between a flat roof window and a skylight?
'Skylight' is the umbrella word. A flat roof window is specifically a glazed unit for flat (or near-flat) roofs, mounted on an upstand — as opposed to roof windows that sit in the slope of a pitched roof.
Can you get flat roof windows that open?
Yes — electrically opening versions (often with rain sensors) handle ventilation for kitchens and bathrooms under flat roofs. Expect a £300–£800 premium over fixed; manual opening is rare because the window is usually out of reach.
Keep reading
Bespoke rooflights cost £1,000–£4,000+ supplied versus a few hundred for standard sizes. When made-to-measure is worth it — and when it isn't.
Most UK skylight installations cost £900–£2,500 per window in 2026, fully fitted. Full breakdown: supply, labour, scaffolding, flat roofs and lanterns.
Fakro, Keylite, RoofLITE+, Roto and Dakea compared against VELUX on 2026 prices, guarantees and quality — and which alternative fits which job.