A bay window projects outward from the front or rear wall, forming a recess inside the room. Historic England lists it among the common features of the middle-class Victorian terrace, and replacing a draughty or failing original is one of the more involved double-glazing jobs a homeowner takes on. The whole projecting unit is priced as one bay, not per individual pane, so costs start considerably higher than for a flat run of windows. Use the bay window calculator to get a 2026 estimate tailored to your home.
What a bay window actually is
The defining characteristic of a bay is that it projects beyond the plane of the outer wall. A typical bay has three sections: a central pane flanked by two angled side panes. Bays can also be square (rectangular) or curved, and some larger bays have five sections. Whatever the configuration, the whole assembly sits on a sill or structural box at the base and is capped by a small pitched or flat-roof section above. That roofed box is a common failure point on older properties: lead or felt coverings degrade over decades, water tracks into the frame, and the timber structure rots. When it fails, a window replacement job becomes a structural repair job at the same time, which is one reason a bay window replacement carries a wider cost range than simple flat windows.
Inside the room, the projection creates a useful alcove. Many homeowners use it for a window seat, a radiator recess or simply to allow more natural light into the front room. It is this inward space, combined with the outward projection, that gives a bay its appeal and its character. The related curved style is the bow window, which uses four or more equal panes set at shallower angles to form a smooth arc rather than angular corners. Both are priced as one projecting unit, not per pane.
Bay window costs in 2026
Material is the primary variable. uPVC is the most affordable and accounts for the majority of bay replacements fitted each year. Aluminium commands a premium for its slimmer sightlines and longer service life. Timber is dearest of all and is largely specified for conservation areas and listed buildings, where councils will not always accept uPVC or aluminium in place of timber. All figures in the table below are fully fitted: they cover removal of the existing unit, supply and installation of the new one, sealing and making good, and the FENSA certificate. For a sense of how a bay replacement fits into the cost of re-glazing an entire home, the cost by house size guide shows how multiple windows stack up on a typical terrace or semi. These figures are cross-checked against MyJobQuote cost data.
| Material | Per bay (fitted) | Notes |
|---|---|---|
| uPVC bay | £1,250–£3,000 | Best value, most common choice |
| Aluminium bay | £1,690–£4,050 | Slim frames, long life |
| Timber bay | £2,380–£5,700 | Period and conservation homes |
Source: DGCC 2026 dataset, cross-checked against Checkatrade and MyJobQuote.
The wide range within each material bracket reflects several factors: the number of panes in the bay, the overall size of the unit, the angle of the corner sections (tighter angles require more precise cutting and sealing), and whether the structural box above the projection needs repair or replacement. A modest two-pane uPVC bay on a straightforward Edwardian terrace will sit near the lower end of the range. A five-pane timber bay on a period detached property, with a failing lead roof above that needs a roofing specialist to make good before the glazier can proceed, will push firmly towards the top.
A bay is never just a bigger window order. You have the angled corner frames to cut and seal precisely, the box above to open up and inspect, and a sill that has usually been taking water for years. Build in a contingency of a few hundred pounds, because that box above the bay almost never comes out clean on a Victorian house. Budget for the surprise and you will not be disappointed.
Double Glazing Cost Calculator Team
Which homes suit a bay window
Best Buy Bay windows are the right choice for Victorian and Edwardian terraces, semis and villas where the original front elevation already has a projecting bay. Replacing like-for-like keeps the character of the street, usually sits within permitted development rights and avoids the need for a planning application. Inter-war and 1930s semis were also commonly built with shallow-angled bays, and uPVC replacements for these are equally straightforward. If your home already has a bay and it is draughty, single-glazed, leaking or simply tired, a matched double-glazed replacement is a clear decision. The main choices are material (uPVC, aluminium or timber) and whether you want standard double glazing or an upgrade to triple glazing for better thermal performance. A FENSA-registered installer self-certifies that the new unit meets Building Regulations, including Part L, and issues you a certificate.
When a bay window is the wrong choice
It dependsIf your property does not currently have a bay, adding one is a structural alteration, and the Planning Portal says a new bay window "will be treated as an extension and may require permission". Whether it does turns on the permitted development limits: government guidance says an extension forward of the principal elevation needs a planning application. The additional cost and disruption make a from-scratch bay hard to justify on financial grounds in most cases. On modern homes built from the 1960s onward, the standard window style is a casement window, and a flat run of casements will look right, perform well and cost a good deal less than a retrofit bay. Equally, on Georgian and Regency frontages the historically correct style is a vertical sliding sash, which our sash windows guidecovers in detail. On those properties a bay would look out of place, and in a conservation area the council must pay special attention to preserving or enhancing the area's character or appearance when it decides the application.
Adding a bay to a flat-fronted 1970s house comes up more often than you would think, and the answer is always the same: you are creating work for a structural engineer, a builder and a planning officer before the glazing even comes up. If you want more light, a wider casement or a pair of French doors will do the job for a fraction of the cost and a fraction of the headache. Save the bay for a house that was built with one.
Double Glazing Cost Calculator Team
Bay window versus bow window
The two terms are used interchangeably in casual conversation but they describe two distinct shapes. A bay projects outward in straight angular sections, with clearly defined corner joints between the panes. A bow window curves outward in a continuous arc, using four or more equal-width panes set at shallower angles to approximate a circle segment. The corner joint is the easiest way to tell them apart: sharp, defined angles mean a bay; a smooth and flowing curve means a bow, and the bow window calculator prices that shape by section count rather than per pane. Architecturally, Historic England lists bay windows among the common features of the middle-class Victorian terrace (1837 to 1901). Both styles are priced per projecting unit rather than per pane, and both require more installation time than a flat window because of the angled sections and the projecting structure above that must be properly sealed and weatherproofed.
Oriel windows: the upper-floor bay
An oriel window is a bay window on an upper storey, carried on corbels or brackets rather than resting on the ground. Encyclopaedia Britannica defines it as "a bay window in an upper story, supported from below by projecting corbels, or brackets of stone or wood", and treats bay as the generic term: a bow is an arc-shaped bay, and an oriel is a bay that projects from an upper floor. Britannica says oriels became common early in the 15th century, were often placed over the gateways and entrances of late Gothic and Tudor manor houses and public buildings, and returned with the revivals of those styles in the 19th and early 20th centuries. The Great Gateway of Hampton Court Palace, from about 1520, has one.
The word now also sells a different product. Glazing firms use "oriel window" for a frameless glass box projecting from the wall, and Glideline, a maker, says they "are used on any floor". Most are cantilevered on concealed steel or aluminium frames tied into the building and project 400 to 900 mm, Glideline says, and most are fixed units; it advises bringing in a structural engineer early. Houzz UK reports the same from the trade: the support, timbers or steels in the base, is meant to be invisible.
| Source | Price | What it covers |
|---|---|---|
| The Oriel Studio, retailer (September 2026) | £2,347.38 plus VAT, four-sided, 1,200 x 1,350 x 300 mm, double glazed | Supply only, smallest listed size |
| Glideline, maker (2025) | £3,500 to £5,000 (1.2 x 1.2 m); £6,000 to £8,000 (1.8 x 1.8 m); £9,000 to £12,000 or more (2.4 x 2.4 m and up) | Supply only; Glideline says fitting adds at least 50%. VAT not stated |
| John Knight Glass, Wirral installer | £4,688 plus VAT (1,000 x 1,500 mm) to £6,878 plus VAT (1,800 x 1,800 mm); £8,152 plus VAT for its largest size; all 450 mm deep | Made and fitted, including the base; ground-floor new build with good access, North West |
| Daylight Glazing, installer (July 2026) | £3,000 to £5,000 into an existing opening; £7,000 or more for a new opening | Manufacture, structural glazing, labour and basic structural support. VAT not stated |
| Houzz UK (January 2024) | £6,000 to £10,000 | An architect's budget for a typical oriel in a modern home, not a price list |
Figures as each source states them. They are not comparable line by line: supply only against fitted, plus VAT against VAT not stated, existing opening against new. Research in lib/research/sections-batch-b-2026.md.
Read together, a modern glass oriel runs from about £2,350 plus VAT for a small unit, supplied only, to £8,000 to £12,000 or more for a large one. No UK source we found prices the other kind: replacing a traditional oriel, a uPVC or timber bay on an upper floor carried on brackets. Our inference, not a published figure, is to start from the bay rows above, which the bay window calculator prices by material, and add upper-floor access and any repair the corbels or brackets need.
Planning depends on whether the oriel is new. Everest, a national installer, says replacing an existing bay window does not need planning permission. For a new one, the Planning Portal says "a new bay window will be treated as an extension and may require permission". It does not name oriels, so treating a new oriel the same way is our inference. Government technical guidance on permitted development says any extension forward of the principal elevation needs a planning application, and an upper-floor window in a side elevation must be obscure-glazed and non-opening unless the opening part is more than 1.7 m above the floor. Conservation areas may add restrictions, and a listed building needs listed building consent. For building regulations, Glideline lists structure, fire safety, safety glass and thermal performance (Parts A, B, K and L); that is the maker's list.
The verdict on bay windows
For period properties that already have a projecting bay, replacing the glazing with a matched unit is a straightforward improvement and uPVC is the sensible default for the majority of homes. The higher cost compared with a flat window run reflects the extra structure involved, not any unusual complexity in the glass itself. Budget carefully, get the box above the projection inspected before you sign anything, and hold a contingency for any structural work that turns up. When you are ready to price the job, the bay window calculator will give you a 2026 fitted range in seconds. If you are still deciding between styles, our casement windows guide covers the standard flat alternative in full.
Get quotes for a bay window
A bay is priced per bay rather than per window, at £1,250 to £3,000 in uPVC, because the fitting work is structural. Tell us what you need and Leads 2 Trade will arrange quotes from installers covering your postcode.
Ask specifically what the price includes above the window. A bay carries load, so the quote should say who is responsible for the support over it, and a number that does not mention it is an incomplete number.
Frequently asked questions
A fitted uPVC bay window costs £1,250 to £3,000 per bay in 2026. Aluminium bays run £1,690 to £4,050 and timber bays £2,380 to £5,700. All figures cover the whole projecting unit including removal, installation, sealing and the FENSA certificate.
A bay window is a window that projects outward from the main wall, creating a recess inside the room. Britannica describes bays as rectangular, polygonal or arc-shaped: an arc-shaped bay may be called a bow window, and a bay on an upper storey carried on corbels or brackets is an oriel. It is priced as one unit per bay, not per individual pane.
A bay window projects in straight angular sections with clear corner joints, usually three panes. A bow window uses four or more equal-width panes set at shallower angles to form a smooth, curved arc. Historic England lists bay windows among the common features of the middle-class Victorian terrace.
A bay projects beyond the outer wall, which means the installer must handle angled corner sections, the sill below and the roofed box structure above the projection. If that box is failing, it often needs rebuilding before the new windows can go in. The extra structure and labour time are what push bay costs above those of a flat replacement.
Bay windows suit Victorian and Edwardian terraces, semis and villas where the original elevation already has a projecting bay. They are also common on 1930s inter-war semis. If your home already has a bay that is draughty, single-glazed or failing, replacing it like-for-like is a straightforward improvement.
Usually yes, if you replace like-for-like in style, configuration and material. Switching from timber to uPVC or changing the glazing bar pattern may require planning permission. Always check with your local planning authority before ordering, and use a FENSA-registered installer who knows the local requirements.
A bay window on an upper storey, carried on corbels or brackets rather than resting on the ground. Encyclopaedia Britannica defines it as a bay window in an upper storey, supported from below by projecting corbels, or brackets of stone or wood. Bay is the generic term; a bow is an arc-shaped bay. Glazing firms now also sell frameless glass boxes, on any floor, as oriel windows.
UK prices found are all for modern glass oriels. The Oriel Studio sells a small four-sided unit at £2,347.38 plus VAT, supply only. Glideline prices supply only at £3,500 to £5,000 for 1.2 x 1.2 m up to £9,000 to £12,000 or more for 2.4 x 2.4 m, and says fitting adds at least 50%. Daylight Glazing quotes £3,000 to £5,000 into an existing opening and £7,000 or more for a new one. No UK source prices replacing a traditional uPVC or timber oriel.
For a new one, possibly. The Planning Portal says a new bay window will be treated as an extension and may require permission. It does not mention oriels, so applying that to an oriel is our inference. An extension forward of the principal elevation needs an application, and upper-floor side windows must be obscure-glazed and non-opening unless the opening part is more than 1.7 m above the floor. A listed building needs listed building consent.