New regulation forcing manufacturers and housebuilders to evidence the u-value for the exact configuration of each individual window supplied into new build, could drive a shift to triple-glazing.

The Future Homes Standard was published on Tuesday [24/3/26].

While notional u-values were held as outlined in Approved Document L at 1.2w/m²K for new build windows, the revision introduces a move away from the standard model of calculation to a specific assessment of each individual window supplied – something which may force developers to move to triple-glazed specifications to achieve compliance.

“The age of the ‘standard calculation window’ is effectively over for new build,”
said Kevin Jones, Head of Technical at the Glass and Glazing Federation (GGF).


“Under the Home Energy Model, every window has to be calculated to its actual size and configuration o arrive at an area weighted average. That includes all the items that sit within the opening. That’s a huge change in mindset and methodology for both housebuilders and their window and door suppliers.”

It comes as the Government attempts to close a gap in performance between theoretical building product performance – and what they deliver in real-life applications. 

At the heart of the change is the move away from SAP to the new Home Energy Model (HEM) for new homes.

This effectively means modelling each specific window configuration. That creates a challenge because most of windows supplied in a real-world context are smaller than the ‘standard’ window configuration (1230×1480 open/fixed) used to model u-values.

The smaller the glazed area, the harder it is to achieve a u-value of 1.2 W/m²K, especially where windows feature transoms, mullions, dummy sashes, astragal and Georgian bars, add-on cills, head vents and other components.

“Government has been very clear that the model of the home needs to reflect what is actually built,”
explained Kevin.


“That means everything in the window opening that affects thermal performance has to be taken into account. Up to now, most calculations have been based on relatively simple, clean configurations. Under the new regime, the real-world complexity of modern window designs is brought into the calculation.

“On a big, simple window with a high glass-to-frame ratio, you can often achieve compliance with a well‑specified double glazed unit. But when you start adding dummy sashes, transoms, Georgian bars and so on – particularly in smaller apertures – you’re making life much more challenging in thermal terms.”

Structural elements of the window or door which can negatively impact thermal performance, for example steel reinforcements and load bearing structural couplers, do not need to be included in the u-value calculation.

The shift also puts software and systems houses in an immediate race against time to create and integrate u-value configurators into their manufacturing processes before the end of the transition period.

The Approved Document states that changes ‘take effect on 24 March 2027 for use in England for building work that is not in connection with higher-risk building work’ and ‘24 September 2027 for use in England for building work that is in connection with higher-risk building work’.

The full transitional arrangements are noted in Circular Letter 01/2026 which can be found with the link below.

The Future Homes and Buildings Standards: Building Circular 01/2026 – GOV.UK

The Letter states that for non-high risk buildings (HRB) the new regulations will not apply to work where a building notice, an initial notice, or an application for building control with full plans approval has been given before 24^th March 2027, and the work starts before 24^th March 2028.

These changes to the 2010 and 2024 Regulations come into force on 24 September 2027 for HRB work (including a stage of HRB work) or work to an existing HRB and separate commencement and transitional arrangements apply in these circumstances. The Regulations do not apply to HRB work or work to an existing HRB work where a valid building control approval application has been submitted to the regulator before 24 September 2027, and the application has not been rejected before, on or after that date.  Where transitional protection begins because of the submission of an application for building control approval for a stage of HRB work then the protection will continue to apply to subsequent stages of HRB work, provided that the original application has not been rejected or lapsed.

For now, at least, for domestic replacement, the current regime, including the use of standard sizes and styles for U‑value declarations, essentially remains unchanged, however Kevin warned that an extension into window and door replacement, was the logical next step.  “Everything we’re seeing points to this being a staging post, not the final destination,” he said.


Kevin concluded: “The Future Homes Standard doesn’t just tweak the numbers – it changes the rules of the game for new build window and door specification

“Those who move early, understand the detail and invest in the right tools and relationships will be in a strong position.

“Those who wait until 2027 and hope for the best are likely to face difficult conversations, disrupted projects and squeezed margins. Now is the time for the trade to get on the front foot.”

For support, contact the GGF Technical team at technical@ggf.org.uk  or visit www.ggf.org.uk

Approved Document L can be found here.

For further press information please contact:

John Warren

07971829772

john@lascomarketing.co.uk