Flush vs Non-Flush 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.

Flush windows typically carry a premium of around 20 to 30 percent over standard non-flush designs, with 2026 uPVC fitted ranges of roughly to against to . The flat, shadow-free finish of a flush window replicates traditional timber joinery, which is why the style has become the default on new builds and sympathetic replacements.

Number of windows

Enter how many windows you are comparing prices for. The difference between the two options scales with the number of windows, so a premium that looks small per window can become significant across a whole house.

Window size

Size is one of the biggest cost drivers for any glazed unit. A larger opening needs more frame material and a bigger, heavier sealed glass unit, and it usually takes longer to fit. The calculator uses three broad size bands based on common UK window dimensions: choosing the small band reduces the typical range by around 20 percent, while the large band increases it by around 40 percent. If your windows vary in size, run the calculator once per size band and add the results together.

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

Non-flush lipped sashes are cheaper to manufacture because the overlap simplifies sealing and tolerances, and from inside the two styles are indistinguishable. The flush premium is purely about the external face, so its value depends on how visible the windows are: on a flat-fronted cottage or a conservation street the difference is obvious, while on a rendered modern semi few people would notice. Both styles achieve identical energy ratings and security at the same specification, and both are available across uPVC, aluminium and timber with the usual material multipliers.

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.