Free tool
Extension glazing area check
Who runs your contract
Awais Sarwar, MSc — Accredited Energy Assessor
Awais is an accredited domestic and commercial energy assessor and the owner of Team EPC Ltd, which trades as Part L Consultant. He has surveyed and lodged EPCs across flats, houses, HMOs, managed portfolios and commercial premises throughout London and the Home Counties, and is currently studying an MSc in philosophy alongside running the business.
Off the clock: cricket, family time, an MSc in philosophy in progress and finding London's best food.
Awais Sarwar
Founder · Accredited energy assessor
The rule this uses
25% of the extension's floor area, plus the area of any windows or doors no longer in place
Above that, the extension must be shown to perform no worse than a compliant one, normally through a SAP comparison.
Area is only one of the tests. Thermal bridging, the existing heating system and any thermal elements being upgraded all affect the route your building control body will accept.
Background
The two routes for an extension
Most domestic extensions comply the simple way: every new element meets the limiting standard for its type, the glazed area stays inside the allowance, and no whole-building calculation is needed. This check tells you whether you are still on that road.
The other route is a comparison. We build a SAP model of the house as it is, a model with your extension, and a model with a notional extension that just meets the standard. If yours performs no worse than the notional one, the glazing can stay — the compensation usually comes from better U-values, improved junction details or an upgraded heating system.
Glass-heavy rear extensions with lanterns and bifolds routinely exceed the allowance, and it is rarely a problem. It just needs the calculation, and it is worth running before the windows are ordered rather than after.
Also useful
The other Part L tools
Part L 2026 transition checker
Enter your building control application date and your start on site, and see whether that building keeps transitional protection under the published 2026 dates.
Design-stage information checklist
Build a tailored list of the drawings, specification and services information a design-stage SAP or SBEM calculation needs for your project type.
Part L 2021 vs 2026 comparison
A side-by-side of the two standards as published: dates, scope and requirements, with a source against every line and nothing stated that we cannot cite.
Part L fee guide
Pick your project type and scale for the guide fee band we quote from, and what the fee does and does not include. Every job gets a written quote.
