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Want to know how to prepare for the implementation of The Future Homes Standard in 2025? Access our latest webinar recording and slides containing all you need to know. Take me there!
8 Minute Read • Part L/Section 6

Future Building Standard – Tougher Requirements for Extensions

Part L/Section 6
Part L/Section 6

Future Building Standard – Tougher Requirements for Extensions


Pretty much all of the debate around the Future Homes Standard has been to do with new builds.

But the Government’s follow-up release, the Future Buildings Standard, talks about some significant changes to the requirements for people wanting to extend existing homes. This article covers some of the main points.

At time of writing, the Future Buildings Standard is still open to public consultation, so the following details are all subject to change. We’re expecting the requirements to be finalised in the latter half of 2021.

The first point to note is the definition of a ‘heavily glazed’ extension remains. That means there is one rule to follow if the glazing area in the extension is greater than 25% of the floor area, and another rule to follow if the glazing ratio is lower.

Extensions that aren’t heavily glazed need to meet U-Value targets, and any new or extended heating and ventilation systems need to be properly installed and commissioned.

The maximum U-Value targets for residential extensions are being reduced across the board:

Walls currently need to meet 0.28 in England – that’s dropping to 0.18! Wales is also proposing 0.18, but the current requirement there is to meet 0.21 so it’s not so much of a shock to the system.

Floors will need to meet 0.18 (from 0.22) in England, or 0.15 (from 0.18) in Wales.

Roofs will need to meet 0.15 (from 0.18) in England, or 0.13 (from 0.15) in Wales.

And finally, external doors and windows will need to achieve 1.40 (from 1.60).

Designers can use the area-weighted U-Value rule to prove compliance by trading one element against the other. For example, you may not need to achieve a wall U-Value of 0.18 if you use triple glazing as an offset.

Heavily glazed extensions:

As well as meeting compliance with the new U-Value targets, heavily glazed extensions also need to meet compliance with SAP.

Under current rules, the proposed extension must be shown to produce lower levels of CO2 than the extension based on a notional specification. As well as considering U-Values, this approach also looks at solar gains, heating, lighting, ventilation and renewable systems.

The process of comparing the actual extension to a notional one isn’t changing, but the targets are – and this is where things get a tad messy.

For England, the Government is proposing to retain the CO2 target, but introduce two additional targets, Primary Energy and Fabric Energy Efficiency (see our article ‘The New Targets’ 22 Jan 2021). To comply with Part L, all three targets will need to be met.

For Wales, the plan is to ditch the CO2 target, ignore the Fabric Energy Efficiency and just use the Primary Energy target.

We will publish a follow-up article later this year when the various requirements have been finalised, and what that means in terms of real-world compliance.

The above applies to extension projects only. We have separate articles that cover proposed changes regarding domestic renovations and conversions.

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