Bow vs Bay Windows Cost Difference Calculator

How this calculator works

This calculator compares typical UK supplied-and-fitted price ranges for the two options, drawn from published 2026 industry cost guides. Your inputs scale those base ranges: the calculator multiplies the per-unit range for each option by the number of units and by adjustment factors for the choices you make below, then shows both totals side by side along with the estimated difference between them. Because real quotes vary, everything is expressed as a range from a realistic low to a realistic high rather than a single figure.

Bow windows typically cost 40 to 60 percent more than comparable bay windows, with a uPVC bow at roughly to fitted in 2026 against to for a bay. The bow’s premium reflects its extra panels, each set at a slightly different angle, and the more complex manufacturing and setting out the curve demands.

Number of windows

Enter how many bay or bow windows you are comparing. Most projects involve a single feature window on the front elevation.

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 price, the two styles trade off differently. A bow projects further and curves smoothly, giving more internal floor space and more even light, and it suits period and cottage aesthetics. A bay is more angular and cost-effective for the space gained, and on Victorian and Edwardian houses the existing architecture usually dictates a bay in any case. Both need correct structural support above the opening, both cost more in timber or aluminium than uPVC, and both benefit from a 10 to 20 percent contingency for hidden repairs discovered when the old window 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.