How the fitted-price estimate works

Research coverage

As of 24 September 2026, the research register contains 298 unique source pages across 118 websites. It includes live supplier/product prices, fitted installer examples, national market guides, regional/local price evidence, official cost-pressure and regulatory sources, and lower-weight homeowner quote discussions. Duplicate URLs are removed and low-quality sources are not given the same influence as direct live prices or well-specified fitted examples. Some reviewed sources themselves aggregate large quote datasets — for example, one current provider says its 2026 window pricing is updated from 2,380 real UK quotes. We count that as one source page, not 2,380 separate sources.

Some source pages cover other products in the wider calculator, such as sash windows, doors and bifolds. The standard white uPVC casement model receives its own deeper like-for-like calibration.

1. Like-for-like evidence first

Supply-only, labour-only and fully fitted prices are tagged separately. A £200–£300 frame price is not treated as a £200–£300 fitted window. Exact size, opening layout, glazing and specification are preferred whenever a source publishes them. For standard casements, the actual pane layout is now modelled as well as overall size and opening sashes. Current made-to-measure supplier catalogues price side-by-side panes, top vents, stacked panes and mixed mullion/transom layouts separately, so the calculator distinguishes common arrangements such as a top opener over a fixed pane, a side opener beside a fixed pane, and mixed top-and-side-opener layouts.

2. Measurements are optional

If you know the width and height, enter them for a tighter estimate. If you do not, choose small, medium, large or not sure. The model uses a representative size and deliberately widens the result range to reflect the extra uncertainty.

3. Fitting, job size and region are included

Single-window jobs can carry more survey, travel, certification and mobilisation cost per window. Larger jobs receive an economy-of-scale adjustment. Regional factors are validated against 2026 local examples but kept broad where city-level data is too noisy to justify false postcode precision.

4. Seasonal market pressure

Installer workload changes through the year. Evidence supports winter bargaining opportunities, but headline “10–20% off” promotions are not assumed to represent the whole market. The production model therefore uses a deliberately small automatic adjustment, capped at about -2.5% in the quietest winter months and +1% in busier summer months. September 2026 is the current calibration baseline.

5. Current-price monitoring

Fresh September 2026 checks did not show a verified sudden retail-window price jump in the last week. However, PVC resin, raw materials, energy and labour remain under cost pressure, so the model is date-stamped and should be refreshed as supplier and installer prices change.

6. VAT

Consumer fitted estimates assume ordinary 20% VAT. GOV.UK explicitly lists secondary and double glazing among products that do not qualify for the energy-saving-products reduced/zero VAT treatment.

7. Blind validation

After the standard white uPVC casement model was frozen, it was checked against 7 new exact/size-specific fitted-price points from 4 previously unseen websites. The median difference between the model centre and the published price/range midpoint was 7.6%. 6 of the 7 published midpoints sat inside the calculator's predicted fair-price range.

The strongest point check was a King's Lynn 1200 × 1200 supplied-and-fitted example at £540 including VAT; the calculator returned about £557 for East Anglia, a difference of roughly 3.2%. This validation is deliberately kept separate from calibration. It is not described as invoice-level accuracy because most public sources publish market bands rather than completed-customer invoices.

Whole-house and premium-installer prices are also stress-tested separately. They confirm that installer business model can move a genuine quote well outside the competitive-market centre, especially where minimum charges, premium systems or sales overheads apply.

8. A range rather than false precision

The result is expressed as a low-to-high fitted market range with a model centre. Access, scaffolding, hidden structural work, listed-building requirements, unusual finishes and installer business models can move a genuine quote outside the range.