Bay Window Costs UK — What to Charge to Supply and Fit a Bay Window in 2026

Quick Reference: Bay Window Costs UK 2026

These are typical supply-and-fit figures for a standard ground-floor bay, including removal of the old unit and basic making good. Upstairs bays, structural work and complex finishing push prices toward — and beyond — the top of each range.

Bay Window Styles and How They Affect Cost

"Bay window" covers several distinct shapes, and each one changes both your material cost and your fitting time. Knowing the style up front lets you price the frame correctly and warn the customer where one option costs more than another.

Splayed (Angled) Bay

The most common style on UK semis and terraces. Two angled side frames (usually at 30° or 45°) connect a flat front frame to the wall, creating a trapezoid projection. It's the cheapest bay to manufacture and fit because the angled corners use standard bay posts, and it's the shape most UPVC fabricators quote as their baseline. This is your reference price for everything else.

Square (Box) Bay

Side frames sit at 90° to the front, forming a rectangular box that projects straight out from the wall. Box bays look more substantial and are common on Edwardian and 1930s properties. They cost 10–20% more than an equivalent splayed bay because the right-angle corners need stronger corner posts, and the flat roof or lead-clad top above a box bay often needs more attention. The extra projection also means more structural load to think about.

Bow Window

A bow uses four, five or six equal-width frames set on a gentle curve, giving a rounded rather than angled projection. There are no sharp corners — every joint is a shallow angle. Bows generally cost the same as or slightly more than a splayed bay because of the extra frames and posts, and they take longer to set out accurately on site. Curved-glass true bows are far more expensive and are rare in volume replacement work.

Oriel Bay

An oriel projects from an upper floor and is supported on brackets or corbels rather than running down to the ground. These are almost always on period properties and frequently sit under building or conservation control. Treat oriel bays as a survey-first, price-after job — the structural support and matching of period detail make them the most variable to cost.

Materials: UPVC vs Aluminium vs Timber

The frame material is the single biggest driver of the supply price, and each one is a different trade-off between cost, lifespan and thermal performance. Quote the material the customer wants, but be ready to explain the difference.

UPVC

The default choice and the cheapest to supply and fit at £1,200–£2,500 for a standard ground-floor bay. Modern UPVC bays reach U-values around 1.2–1.4 W/m²K with a good A-rated double-glazed unit, and triple glazing can push below 1.0. Lifespan is typically 20–30 years before frames discolour or seals fail. UPVC is the right answer for most volume replacement work and the largest part of a fitter's bay turnover.

Aluminium

Aluminium bays cost £2,500–£4,500 supplied and fitted. The appeal is slim sightlines, large glass areas and a long structural life — 30–45 years is realistic. Thermally broken aluminium frames with quality glazing achieve U-values comparable to good UPVC (around 1.2–1.6 W/m²K), but cheaper non-broken systems perform far worse, so check the spec. Aluminium suits contemporary properties and customers who want a slim, modern frame.

Timber / Hardwood

Timber bays start around £3,000 and run to £6,000+ supplied and fitted, and bespoke hardwood bays for listed or conservation properties can go well beyond that. Timber gives the most authentic look on period and listed buildings — often the only material a conservation officer will approve — and engineered timber has good thermal performance (U-values around 1.4–1.6). The trade-off is maintenance: timber needs repainting or re-staining every 5–8 years, though a properly maintained timber bay can last 40–60 years. Price the survey and bespoke joinery carefully; these are not catalogue units.

What's Included in a Bay Window Price

A bay quote that only covers the frame and glass will lose you money. A complete supply-and-fit price for a bay should account for all of the following — list them on the quote so the customer sees the value and you don't get pushed on price by someone quoting frame-only:

• The window unit: frames, glazing, bay posts and corner couplers to the agreed spec.
• Bay pole or structural support: where the bay carries load from above (see below) — this is the part most often missed.
• Cill: a proper bay cill, often a wider or stepped profile, plus end caps and corner joints.
• Internal finishing: reveals, window board, plastering and making good around the new frame.
• External finishing: pointing, trims, mastic sealing and any rendering or brickwork repair.
• Plastering making good: bays almost always disturb internal plaster — budget for a plasterer or your own making-good time.
• Scaffold or access: for upstairs bays, an allowance of £150–£400.
• Removal and disposal: taking out the old bay and getting the waste off site legally.
• FENSA / building control certificate: notification so the work is compliant and registered.

The Structural Point: Why a Bay Needs a Bay Pole

This is the single most important thing to get right on a bay window, and the reason bays are not a beginner's job. On a huge number of UK properties — particularly Victorian, Edwardian and 1930s houses — the bay window carries structural load. The wall above the bay, an upper-floor bay, a section of roof, or a stacked bay on the floor above can all bear down through the bay frame onto the cill and brickwork below.

When you remove the old bay, you remove that support. If the load above is not held while the old unit comes out and the new one goes in, the structure above can drop, crack or — in the worst case — fail. This is why a bay pole (a structural steel post or reinforced jamb section) or a proper temporary support and acrow propping is essential on any load-bearing bay. Many modern UPVC bays use a galvanised steel bay pole bolted between cill and head to transfer that load through the new frame.

Allow £300–£800 for the structural support element when the bay is load-bearing. Never assume a bay is non-structural because it "looks" like a normal window — confirm it on survey. If you're ever unsure whether the support you're providing is adequate, get a structural engineer to specify it. A £150 engineer's note is cheap against the cost of a dropped lintel or a cracked first floor.

What Drives the Price Up

Two bays of the same width can differ by over £1,000 once the site conditions are accounted for. The main cost drivers to check on survey:

• Number of panes and posts: a five-frame bow costs more to make and fit than a three-frame splayed bay of the same width.
• Upstairs vs downstairs: first-floor bays need scaffold (£150–£400), add working-at-height time and are harder to handle into position.
• Structural load above the bay: any load-bearing bay needs a bay pole or support — add £300–£800 and more survey time.
• Rot or repairs to the structure: old timber bays and cills hide rot. If the sub-frame, cill or supporting brickwork is decayed, repairs can add hundreds before the new frame even arrives.
• Glazing spec: triple glazing, acoustic units, toughened or obscure glass, Georgian bars and decorative or leaded glass all add cost.
• Finishing standard: heritage detailing, matching period mouldings and full re-plastering cost far more than a basic trim-and-mastic finish.
• Access and disposal: awkward access, conservatories below, and old units that are heavy or contain hazardous material add time and cost.

FENSA, Building Control and Compliance

Replacement windows in England and Wales are controlled work under the Building Regulations. As an installer you either register the work through a competent person scheme such as FENSA or Certass, or you notify your local authority building control directly. The customer needs that certificate when they come to sell the house, so it is not optional.

For a like-for-like bay replacement, FENSA/Certass notification covers you. But be aware: if you alter the structural opening, change the load path, or the bay forms part of a larger structural change, full building control approval may be required rather than a self-certified notification. Where a bay is load-bearing and you're changing the support arrangement, check before you commit to a price — building control involvement adds cost and time. The notification itself is cheap (allow £15–£30 per certificate), but build it into the quote so it's never a surprise.

Worked Example: UPVC Splayed Bay, Ground Floor

Here's how a standard load-bearing UPVC splayed bay on a 1930s semi breaks down. The customer wants A-rated double glazing, white UPVC, a new bay cill and internal making good. It's a ground-floor bay carrying the wall above, so a bay pole is required.

• UPVC bay frame, glazing, posts and couplers (trade supply): £750
• Galvanised steel bay pole and fixings: £180
• Bay cill, end caps and trims: £90
• Labour (2 fitters, full day, fit + structural support): £560
• Plastering / making good (half day): £180
• Removal and disposal of old bay: £80
• FENSA certificate: £20

That's a cost base of around £1,860. Add a 30% margin and you're quoting roughly £2,420 — which sits at the top of the standard UPVC range and is exactly where a load-bearing bay with proper structural support should land. The fitter who skips the bay pole and quotes £1,500 either doesn't understand the job or is taking a structural risk they can't afford. Price the job that's actually in front of you, not the catalogue minimum.

How to Quote a Bay Profitably

Bays reward fitters who survey properly and punish those who quote off a photo. To protect your margin:

• Always survey on site. Confirm the style, measure each frame, and establish whether the bay is load-bearing before you give any number.
• Assume structural support is needed until you've proven otherwise, and price it in. It's far easier to remove a line than to add £500 after you've quoted.
• Probe for rot. On timber bays and old cills, check the sub-structure — rot found mid-job is unrecoverable money if you didn't allow for it.
• Itemise the quote. Frame, structural support, cill, finishing, scaffold, disposal and certificate as separate lines. This justifies your price against frame-only competitors.
• Allow for making good. Bays disturb plaster and pointing more than any other window job — budget the trade time or it eats your margin.
• Build in a contingency on period and load-bearing bays. A 10–15% buffer covers the structural surprise that turns up once the old frame is out.

Track your bay jobs separately from standard window swaps. They take longer, carry more risk and should earn a better margin — and the only way to know whether they actually do is to record the real cost and time against each one.