Overglazed Extension Calculations:
Covering England and Wales

Overglazed extension calculations are the Part L compliance calculations used when a domestic extension has more glazing than the default guidance allows. In practice, they show Building Control that the extra glass is being compensated for properly, either through a U-value trade-off route or a SAP / benchmark extension route, depending on the project and the jurisdiction.

The 25% rule is the default Part L limit on the opening area of a domestic extension. In England, the total area of windows, roof windows, rooflights and doors in the extension should not exceed 25% of the floor area of the extension, plus the area of any openings that no longer exist or are no longer exposed because of the extension. In Wales, the wording is very similar but uses 25% of the internal floor area of the extension.

No. It is the default guidance limit, not an absolute ban. Both England and Wales provide alternative approaches that allow more glazing if the design is compensated properly. That is exactly why overglazed extension calculations exist in the first place.

Yes, potentially. The issue is not whether a lot of glass is banned, but whether the extension can still be shown to perform no worse than the relevant benchmark or compliant extension. In real project terms, fully glazed or near-fully glazed designs usually mean you need the compensating-calculation route, tighter product choices and a more disciplined spec.

In England there are two main alternative routes. The first is the area-weighted U-value route for all thermal elements in the extension. The second is the SAP / notional extension route, where the dwelling plus proposed extension is compared against the dwelling plus a compliant notional extension. Which one works best depends on how far over the default glazing limit the design has gone.

In Wales, Section 13 gives two optional routes for more design flexibility. The first is the U-value trade-off approach. The second is the Equivalent Primary Energy and Carbon Emissions Target approach, which uses SAP 10 and a fully compliant benchmark extension or conversion. Those are the two live Welsh routes when the default extension rules are too restrictive for the design.

It is the trade-off method that compares the average thermal performance of the proposed extension against a compliant benchmark extension of the same size and shape. In England, the extension’s area-weighted U-value must not exceed that of an extension complying with paragraph 10.7. In Wales, the U-value trade-off approach requires the proposal’s area-weighted average U-value to be no greater than that of a fully compliant benchmark.

It is the whole-dwelling calculation route used to show that the dwelling plus proposed extension performs no worse than the dwelling plus a compliant benchmark or notional extension. In England, this route checks the dwelling primary energy rate, dwelling emission rate and dwelling fabric energy efficiency rate. In Wales, the equivalent primary energy and carbon route compares the proposal against a fully compliant benchmark extension or conversion.

It is a Building Regulations Part L issue, not a planning-permission test in itself. You may be under permitted development for planning and still need Building Regulations approval for the extension. In other words, a scheme can be lawful in planning terms and still need overglazed extension calculations for Building Control.

Design the glazing with the calculation in mind from the start. Measure the opening area properly, count any lost openings correctly, choose the right compliance route early, and lock the glazing and build-up specifications before orders are placed. The jobs that go smoothly are usually the ones where the calculation leads the design, not the ones where the calculator is asked to rescue it afterward.