Here’s a change hiding in plain sight inside the Future Homes Standard: the way window and door U-values are calculated and declared is being overhauled. It’s not the headline the trade press ran with, but it lands squarely on assessors’ desks — because these are numbers you type straight into your SAP model. Grab a brew; this one’s worth two minutes.
What’s actually changing
Today, a window U-value is typically calculated on a standard reference size — a fixed benchmark pane that rarely matches what’s actually fitted. It’s a tidy simplification, but it flatters the paperwork: a real window is seldom that exact size, so the declared figure and the installed reality drift apart.
From 24 March 2027 in England (4 March 2027 in Wales), Approved Document L 2026 requires the declared U-value to reflect the actual size and configuration of every window and doorset installed — a far more rigorous, product-by-product approach calculated in line with BR 443 and EN ISO 10077-1/-2 (Specification Online, 10 June 2026).
Why the numbers go up
This is the bit worth internalising: smaller windows generally have a worse U-value, not a better one. Frames conduct heat more readily than the glazed unit, so the more frame you have relative to glass — as in a small window — the higher the whole-window U-value climbs.
In practice, a product currently declared at around 1.30 W/m²K on a standard reference size can recalculate closer to 1.40–1.50 W/m²K once the real dimensions and all the correct components are accounted for (British Woodworking Federation). Nobody’s product got worse overnight — the methodology just stopped rounding in the specifier’s favour.
What it means for assessors
A useful line to hold: you set the target, the supplier certifies the value. As the assessor you can specify the U-value a dwelling needs — say, windows at 1.30 W/m²K to make the fabric energy balance work — but you can’t calculate or certify the window’s U-value on the manufacturer’s behalf. That declared figure is the supplier’s responsibility, produced with the correct Future Homes Standard methodology.
For SAP 10.3 — still the sole compliance route at Future Homes Standard launch — the practical upshot is simple: the U-value inputs you use must reflect each window’s as-designed configuration, not a generic default or an optimistic reference-size figure. Ask suppliers for U-values calculated to the new method, and treat any old standard-size declaration with caution. To help the market get this right, BSI has just launched two new Certificates of Conformity for the underlying calculation data and software, and ran an introductory webinar on 1 July 2026 (Specification Online).
The bigger fabric picture
Tighter fabric standards mean every element of the heat-loss calculation gets more scrutiny — glazing today, and the junctions around it just as much. As walls and openings improve, the heat escaping through poorly-detailed thermal bridges becomes a bigger share of the total, and leaning on the default junction Psi-value of 0.15 W/m·K carries a real penalty in your assessment.
That’s where ΨMonkey earns its keep — a browser-based Psi-value calculator that does proper BS EN ISO 10211 thermal modelling to BR 497 conventions, without the desktop-software faff or the consultancy invoice. Get your window U-values honest and your junction Psi-values calculated, and your SAP figures have a lot less to prove.
Sources: New certification launched to help industry comply with the Future Homes Standard — Specification Online, 10 June 2026 · Changes to U-Values Explained — British Woodworking Federation · Approved Document L Volume 1 2026 (gov.uk)