In situations where the area of glazing proposed in an extension exceeds 25% of the extension floor area it is necessary to calculate the energy performance of the proposed extension combined with the existing dwelling and compare this to a second calculation which quantifies the energy performance that would be achieved should the extension be built with an area of glazing not exceeding 25%.

Assuming the highly glazed extension is less energy efficient (which is not always the case, for a variety of reasons) than the “notional” extension, compensatory measures will need to be determined and incorporated into the proposals to offset the excessive heat losses occurring through the extension glazing.

Compensatory measures could include upgrading the other thermal elements of the extension or may include upgrading thermal elements or services within the existing dwelling.