What can I do if the glazing in my extension exceeds to allowed 25%?

The glazing of any windows, doors and roof lights to an extension should be limited to 25% of the extension floor area (plus and doors/windows being built over)

The 25% rule is set by the government and is aimed at reducing global warming.

You can exceed the 25% rule if you demonstrate the increased glazed area does not increase the energy loss above that of a 25% or less glazed building (notional compliant extension). This can be achieved through compensatory measures such as increasing the insulation elsewhere in your home or even upgrading your boiler to a newer / more efficient model.

There are two ways to do this.

  • Employ the services of an energy assessor to provide a whole dwelling calculation (SAP), this may give you greater flexibility.
  • Providing an area weighted U Value calculation, to help you with this please you can download our ‘Area Weighted U-Value Calculator’ here. Download
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