How much does a bay window cost?

An angled bay window in uPVC costs £2800-4000 supply and fit. Square bays cost £3200-4500. Curved/bow windows cost £4500-7000+. Aluminium frames add 30-40% to the cost. Prices depend on size, glazing spec and whether structural work is needed.

Do bay windows need a structural support?

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.

What U-value do bay windows need?

The same as any window under Part L: 1.4 W/m²K for replacements, 1.2 W/m²K for new build. Bay windows can be harder to achieve good U-values due to multiple frames and angles increasing thermal bridging.

Can I replace a bay window under permitted development?

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.

What is the difference between bay and bow windows?

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.

TradeCalculator

Bay Window Calculator

Calculate bay window costs, sizes and U-values for angled, square and curved bay windows with Part L compliance checking.

Bay Type

Angled most common

Frame Material

uPVC most affordable

Glazing

Triple for best U-value

Front Width

Centre panel width

Side Width

Each side panel

Height

Window height

How We Calculate This

Estimates bay window costs from type, dimensions, frame material and glazing.

The formula

Total = Base Cost × Frame Multiplier × Glazing Factor × Size Factor + Installation

Base cost varies by bay type (angled/square/curved)
Aluminium frames add ~40%, timber ~20%
Triple glazing adds ~30%
Size factor scales with total glazed area

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.