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Bay Window Calculator
Estimate bay window costs, sizes and U-values for angled, square and curved bay windows, with a Part L replacement-limit check.
Centre panel width
Each side panel
Window height
Override (0=auto)
Fitting labour (typically £600-1,200)
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How We Calculate This
Estimates bay window costs from type, dimensions, frame material and glazing.
The formula
Total = Base Price × Frame Multiplier × Glazing Factor × Size Factor + Additional Fitting
- The base price is an indicative installed figure (units, manufacture and standard fitting) that varies by bay type (angled/square/curved), set for a typical 3-panel bay of about 3.5 m²
- Aluminium frames add ~40%, timber ~20% on the base price (fitting labour does not change with frame material)
- Triple glazing adds ~30%
- Size factor is the glazed area divided by 3.5 m², so a typical bay scales to about 1.0 and larger bays cost proportionally more
- An additional fitting allowance is added on top for removal, making good and any structural/scaffold work — fitting labour alone is typically £600-1,200
Figures are indicative ballpark estimates anchored to 2026 UK cost guides (Checkatrade fitted £2,500-4,000 for a uPVC 3-panel bay, Expertsure, MyJobQuote). They are not a quote — bay prices vary widely with structural work, region and glass spec, so always confirm with a fitter.
Frequently Asked Questions
A standard 3-panel angled (canted) bay window in uPVC costs roughly £2,500-4,000 supply and fit (Checkatrade 2026). Square bays are similar to slightly higher, and curved/bow windows £2,500-5,000+ because they use more panels and curved frame sections. Aluminium frames typically add 30-40% and timber about 20% on the material. Fitting labour is usually £600-1,200. Prices depend on size, glazing spec and whether structural work is needed — always get a fixed quote.
Yes. Bay windows project beyond the building wall and need structural support. This is typically a gallows bracket, steel angle or cantilevered concrete slab. The support must be designed to carry the window weight plus any roof load above. Building Regulations approval is needed.
The same limits as any window under Approved Document L 2021 (Vol 1, Dwellings). Replacement windows in an existing dwelling must achieve a limiting U-value of 1.4 W/m²K (Table 4.2). New-build windows have a limiting (backstop) value of 1.6 W/m²K, but the notional/design target used in the SAP calculation is 1.2 W/m²K (Tables 1.1 and 4.1), so most new builds aim for 1.2. Bay windows can be harder to achieve good U-values due to multiple frames and angles increasing thermal bridging.
Like-for-like replacement of an existing bay window doesn’t need planning permission. However, if you are changing the size, shape or projection, you may need permission. Listed buildings and conservation areas have additional restrictions.
Bay windows have flat panels at angles (typically 3 panels at 30° or 45° for angled, or 90° for square bays). Bow windows have a gentle curve made of 4-6 panels. Bow windows cost more due to the curved frame sections and additional panels.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.