May 10, 2023

Tougher Part L requirements to come into force within days!

Anyone supplying windows and doors into new build – including home extensions – has only days to meet tougher standards of thermal performance. 
Nigel Headford, Director, Deceuninck Aluminium

The Part L new build transition period comes to an end on the 15th of June 2023.  

Unless a project breaks ground before the transition period ends, all windows and doors supplied to site need to meet the higher standards of performance introduced in last year’s update to Part L.

Coming into force almost exactly a year ago (June 2022) revised Part L building regs introduced a new notional u-value requirement for new build windows and doors of 1.2W/m2K – but with a 12-month stay of execution. 

Nigel Headford, Director, Deceuninck Aluminium, said that the end of the transition period could force manufacturers of legacy aluminium systems out of new build.

“There are a frightening number of older aluminium systems which can’t get down to u-values of 1.2W/m2K without using triple-glazed units, which comes with increased cost and weight, impacting on handling – and hardware.

“It also has ramifications for home improvement because under the regulations extensions are classed as new build, so the implications are significant.”

Deceuninck Aluminium continued with the rapid expansion of its product offer, with the launch of the new ultra-energy efficient Decalu88 Flush Casement Window in March.

Decalu88 Flush Casement

Flush inside and out, it achieves headline u-values of as low as 0.8W/m2k – a Passivhaus equivalent level of performance – and 1.2W/m2k with a standard double-glazed unit.

Designed by a UK design team, the system is built around a patented thermal break which helps to deliver the Decalu88 Flush Casement’s advanced thermal performance. 

In common with the Decalu88 Bi-folding Door, it also features a knock-in glazing bead with a flush finish pre-inserted gasket, saving time in fabrication and on site.

Nigel added that installers needed to get to grips with regulatory change and ask the right questions of their suppliers, as liability for compliance sat with them.

“The installer is responsible for compliance. Fabricators don’t know where ‘trade’ windows and doors end-up”, he said. 

“You could have a single job which includes a houseful of windows which are going into a pre-existing part of a single property, and two or three going into an extension as part of the same project. 

“In the pre-existing part of the home the u-value requirement would be 1.4W/m2K so double-glazing might be fine.

“Cross the threshold between the ‘old’ and new part of the building and the u-value will need to be 1.2W/m2K.

“The new Decalu88 Flush Casement makes compliance simple with u-values of 1.2W/m2K with a standard double-glazed unit and none of the complexity for either installers or the fabricators supplying them.

Nigel Headford, Director

“It’s also Future Homes proof getting down to u-values of as low as 0.8W/m2K if you step up to a triple-glazed unit – that’s in aluminium window that’s flush inside and out.”

For more information call 01249 816 969, email  or visit  

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