A bay window projects outward from the front or rear wall in three or more angled panes, forming a recess inside the room. It is one of the defining features of Victorian and Edwardian terraces across the UK, 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. The angle of those sides varies by property: 45-degree bays give a shallower projection and are common on Edwardian terraces; 30-degree bays project further and appear on larger Victorian semis; 25-degree bays are less common and found mainly on interwar housing. 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 bay replacements carry 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 a synthetic frame would not gain planning approval. 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.
| Material | Per bay (fitted) | Notes |
|---|---|---|
| uPVC bay | £1,100–£3,000 | Best value, most common choice |
| Aluminium bay | £1,490–£4,050 | Slim frames, long life |
| Timber bay | £2,090–£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. I always tell customers to 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.
Tom Bradley, FENSA-registered installer
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. The new unit will meet current Part L building regulations by default when fitted by a FENSA-registered installer.
When a bay window is the wrong choice
It depends If your property does not currently have a bay, adding one is a structural alteration that needs planning permission and building regulations approval for the new projection and its supporting structure. 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, 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 guide covers in detail. On those properties a bay would look out of place and, in a conservation area, would almost certainly be refused planning permission.
I have been asked to add bays to flat-fronted 1970s houses a few times over the years. My 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.
Tom Bradley, FENSA-registered installer
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. Architecturally, bays are most common on Victorian and Edwardian properties built between roughly 1850 and 1914. Bows tend to appear on Georgian and Regency houses from earlier in the nineteenth century, where curved frontages were fashionable. 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.
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.
Frequently asked questions
A fitted uPVC bay window costs £1,100 to £3,000 per bay in 2026. Aluminium bays run £1,490 to £4,050 and timber bays £2,090 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 in three or more angled panes, creating a recess inside the room. The two side panes are set at an angle (commonly 25, 30 or 45 degrees) to the central pane. 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. Bays are more common on Victorian and Edwardian properties; bows tend to appear on Georgian and Regency frontages.
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.