A uPVC bay window costs £1,500 to £3,500 to replace in 2026, supply and fitted. Where you land depends mostly on the number of sections and whether the structure above the bay needs work. Aluminium and timber cost more.
A bay is not just a wide window. It projects from the wall and, on most houses, it carries the brickwork or the roof above it. That is why it costs more, and why the cheapest quote is not always the safe one.
Points to take away
- uPVC bay: £1,500 to £3,500 fitted. More sections, higher cost.
- The bay often supports the wall or roof above it. That load has to be carried during and after the swap.
- A quote that ignores the structural head or corner posts on a load-bearing bay is a red flag.
- First-floor bays need scaffold, adding £300 to £800.
- Roof, tiling or lead flashing above the bay can be a separate cost.
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Cost by number of sections
A bay is made of a wide central pane with angled or curved sections to each side. Prices are uPVC, supply and fitted, including VAT.
| Bay window | Typical fitted cost |
|---|---|
| 3-section (one centre, two returns) | £1,500 to £2,200 |
| 4-section | £1,900 to £2,800 |
| 5-section | £2,300 to £3,500 |
| Bow window (curved, 4 to 6 lights) | £2,000 to £3,600 |
| Two-storey or first-floor bay | add £500 to £1,200 for access and structure |
Aluminium is roughly 50 percent more. Engineered timber is often double. Coloured foil, A-rated glass and leaded or Georgian-bar designs add supplements as on any window.
Why the structure matters
On a typical bay, the wall or the pitched roof above it has no lintel spanning the full opening. The window frame carries that load, helped by:
- A structural head: a reinforced beam fixed along the top of the bay, inside the frame.
- Corner posts or jack posts: load-bearing uprights at the angles, often steel-reinforced, that transfer the weight down to the sill and the supporting brickwork.
When the old bay comes out, that load has to be held with temporary props (“acrow” props and needles), then transferred to the new frame’s head and posts. Skip this on a load-bearing bay and you risk cracking above the window, a dropped roof line, or worse.
A good quote states whether your bay is structural, and lists the head and posts. If it just says “supply and fit one bay window” for a suspiciously low price, ask the question before you sign.
The extras a cheap quote leaves out
- Scaffold for a first-floor or two-storey bay: £300 to £800.
- Roof or canopy work. Many bays have a small tiled or lead-covered roof. Disturbing it during the swap can mean re-tiling, new lead flashing or a new felt, £200 to £800.
- New lintel or head repair if the existing structure is failing: £300 to £700.
- Rot or spalled brickwork at the sill or corners, found once the old frame is out: variable, ask for a provisional sum.
- Internal making good: plastering the reveals and the head after fitting, £100 to £250 for a bay.
- A new internal sill or window board, especially if you are going from timber to uPVC.
Getting it right
- Get three quotes, at least one from a local FENSA or CERTASS registered installer.
- Ask each whether the bay is load-bearing, and make sure the quote lists the structural head and corner posts if it is.
- Confirm what happens with the roof and flashing above the bay.
- Match the spec: sections, glass rating, colour, opening lights. Our double glazing cost guide and how to compare quotes cover the rest.
Get a free quote and we will match you with vetted local installers who do bay window work.