Reynaers Aluminium UK is urging developers, fabricators and installers to act now, as the Future Homes Standard ushers in a fundamental calculation shift that will reshape the specification of windows and doors to meet energy targets.

Reynaers’ technical experts believe the most critical implication of the Future Homes Standard is how energy performance must now be calculated, arguing that for new build homes under Approved Document L, the impact will prove to be far more significant than a series of regulatory revisions.

Published in March 2026, the Future Homes Standard outlines the changes to Part L and Part F of the Building Regulations.

Under the previous Part L regulations, the U-value of a window or door could be assessed using its actual size and configuration, a standard reference configuration, or the hot-box method. That standard configuration route has now been removed, meaning every window, door and patio door must be assessed using its actual size, or verified through hot-box testing.

The design implications are expected to be far-reaching, with any change to a single U-value significantly impacting the build.

With the need to meet energy emission targets, a factor in the specification of building projects, Reynaers urges developers and main contractors to act now to avoid costly changes or risk non-compliance down the line.

The Government has confirmed that compliance will initially be assessed using SAP 10.3, while the long-term intention is a transition toward the Home Energy Model (HEM). Once implemented, the HEM will introduce a fundamentally different approach to energy assessment.

Critically, projects specified for completion in 2027 are already in the design stages, with key specifications – such as fenestration systems – being fixed now. With the implementation of Approved Document L fixed for March 2027 or September of that year for high risk buildings, Reynaers believes the window for the market to respond is deceptively short when factoring in the time to assemble data sets, invest in simulation capability, complete training and build supplier relationships.

John McComb, technical director at Reynaers Aluminium UK, said:

“This transition marks a significant change in both the process and methodology used to demonstrate compliance in the new build residential setting, requiring considerably greater levels of design accuracy, coordination and performance forecasting from the earliest stages of a project.

“Given the long lead times associated with specification, procurement and construction within the new build sector, the implications of this new calculation methodology must be considered well in advance of formal implementation.”

Once key parameters are set, making early stage decisions effectively locked in, it will become difficult to reverse them without substantial financial and project delivery implications.

Reynaers says collaboration with manufacturers will be critical to navigating this transition successfully. With access to independently-verified thermal simulators in Belgium and in the UK, combined with ReynaPro technology, Reynaers is able to support both individual and area-average calculations across its entire product range.

The ReynaPro EOS system enables users to place orders efficiently with full traceability, providing instant access to detailed order breakdowns. This ensures stakeholders receive project-specific data quickly, while also reducing the risk of human error and improving the reliability of underlying datasets.

John McComb added:

“In commercial fenestration projects this kind of detail-oriented, project-specific calculation has been considered best practice and commonplace for many years and is something Reynaers is well versed in.

“Reynaers sees the Future Homes Standard as a welcome step towards achieving the UK’s net zero target and is committed to supporting the industry as it transitions to the New Homes Standard, with the products, software and technical expertise already in place to underpin the methodology vital to the new calculations.

“Whilst we are used to a similar approach within the commercial sector, our view is that the Future Homes Standard is not only a new target to meet within the residential new-build sector, but also a shift in the way residential fabricators and installers are used to working, and the market needs to be ready to implement that change effectively now.”