3 ways to comply with the standards for domestic extensions
So, you’re building an extension on your house. What do you have to do to make sure it complies with the standards? This blog explores the options…
When determining compliance for an extension, there are three routes to showing that it thermally complies with standards – these are:
- follow a package of target values laid out in the standards for domestic extensions.
To comply with this approach, the area of openings for the extension must be less than 25% of its floor area, plus a bit extra from the area of any windows or doors that will be built over as a result of the extension works.
Alternatively, you can
- show compliance by undertaking an area weighted U-value calculation for the proposed extension and for one built to the notional values and opening allowance, trading off improved thermal elements against slightly larger amounts of glazing.
This is known as a compensatory approach and can be useful where the glazed area is only a little over the glazing allowance.
The other approach, known as ‘design flexibility’, is to:
- use the SAP calculation procedure to show that the total CO2 emissions from the entire extended dwelling as it will become, are no greater than if the dwelling had been improved to the standards outlined (with the notional glazing allowance).
Upgrades to the existing dwelling, can be made to compensate for varying individual elements and greater glazing allowances. This approach is more complicated and requires a lot more information about the existing dwelling as well as the extension proposals.
The notional values are balanced around minimising excessive losses from highly glazed extensions, as even with the best available glazing, more heat will be lost from openings than would be lost through a well insulated wall or roof.
The ‘required’ values vary across the UK, with different targets required from 2014 for England, Scotland and for Wales. (In Scotland the target differ according to the existing performance of the dwelling):
Design flexibility, or compensatory approaches will require pushing the standards further to try and balance any poorer values or greater glazing requirements.
Ideally, when contemplating an extension, we should aim for a reduction, or at least no net increase, in carbon emissions and fuel bills; but failing that, it is not unrealistic to expect new elements to achieve a new build level of performance – and there is also often opportunity to improve the existing property at the same time that extension works are undertaken.
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