Bay Windows Installation and Replacement Cost Calculator

How this calculator works

This calculator estimates a realistic price range rather than a single figure, because real-world quotes genuinely vary that much. It starts from typical UK supplied-and-fitted price ranges drawn from published 2026 industry cost guides, then multiplies them by the number of units you enter and by adjustment factors for each choice you make below. The result is a low-to-high band that a fair quote is likely to fall within, not a promise of the exact price you will pay.

A standard three-panel uPVC bay window replacement typically costs around to fitted in 2026, with timber bays commonly to and aluminium to . Replacing an existing bay is much cheaper than creating a new one, because the lintel, bay roof and supports are already in place; forming a brand new bay where a flat window currently sits involves structural work and can cost considerably more.

Number of bay windows

Enter how many bay windows you are replacing or installing. Most homes with bays have one on the living room, and some Victorian and Edwardian properties have a matching bay on the bedroom above.

Number of sections

Bay windows are priced largely by the number of panels they contain, because every extra section needs its own frame, glass, seals, structural connectors and fitting time. Published guides suggest a 3-section uPVC bay averages around to for the windows themselves, rising to roughly for 4 sections and or more for 5 sections, before full installation costs are added.

Frame material

Frame material has a bigger effect on price than almost any other choice. uPVC is the baseline and accounts for around three quarters of UK installations. Aluminium typically costs 1.5 to 2 times as much as uPVC for the same opening, in exchange for slimmer sightlines and a longer lifespan. Timber usually costs around twice as much as uPVC and suits period properties, but needs repainting every five to eight years. Composite frames, with a timber core and aluminium outer skin, sit at the top of the market.

Specification level

Two windows that look similar from the street can sit at very different price points. Budget products use simpler profiles, standard hardware and basic glass. Mid-range covers the typical A-rated units most UK installers quote by default. Premium covers high-end systems with upgraded security hardware, better thermal performance, special finishes or designer profiles, and typically adds around 30 percent to the price.

Where you live

Labour rates are not uniform across the UK. Published cost guides consistently show installation prices in London and the South East running roughly 20 to 30 percent above the national average, driven by higher labour costs, travel, parking and congestion charges. The calculator applies an uplift of around 20 percent if you select London and the South East. Product prices themselves are broadly similar nationwide, so the difference mostly reflects fitting costs.

What else affects the price

Beyond the number of sections and the frame material, bay window prices depend on the shape, with square box bays generally cheapest, angled canted bays the standard option, and curved designs at a premium. Structural considerations matter more than with any other window style: bays often carry load from the wall or roof above, so a correctly specified lintel or jack studs are essential, and building control sign-off may be needed if structural elements change. Labour alone typically runs to for a two to three day replacement, and it is sensible to hold back an extra 10 to 20 percent contingency for hidden repairs uncovered when the old bay comes out.

Planning permission, building regulations and certification

Like-for-like window replacement on most houses does not need planning permission, but listed buildings always require consent and conservation areas often restrict materials and styles on visible elevations, so check with your local planning authority if either applies. Building regulations apply to every replacement window in a dwelling, setting minimum energy performance and safety glazing standards. Installers registered with a competent person scheme, such as FENSA or Certass, can self-certify compliance and issue a certificate after completion; otherwise you must arrange building control sign-off separately. Keep the certificate safe, as buyers’ solicitors routinely ask for it when you sell.