Quick answer: an extension's opening allowance is 25% of its floor area, plus the area of any existing windows and doors that no longer exist or are no longer exposed because of the extension. A back door that now opens into the new room, a window bricked up to build it, patio doors taken out to form the opening — all of them are added. Measure them at structural opening size, mark them on the existing elevations, and keep the drawing: Building Control will want to see where the extra allowance came from.
Free to work out. Building Control-ready PDF £35 +VAT (£42 inc VAT).
There is a table for covered openings right under the glazing table.
Why the credit exists
The 25% limit is about heat loss, not aesthetics. An opening that the extension swallows has stopped losing heat to the outside — it now opens into a heated room — so the envelope has not gained as much glazing as the raw numbers suggest. Approved Document L recognises that by letting you add those areas to the allowance.
A worked example
A 20 m² rear extension across the back of a kitchen. The base allowance is 25% × 20 = 5.0 m² — one modest set of bifolds and you are done. But the extension takes out:
- French doors, now inside the extension — 3.6 m²
- Kitchen window built over — 1.4 m²
That is 5.0 m² of existing openings, so the allowance becomes 5.0 + 5.0 = 10.0 m² — 100% more than the number most people calculate, and enough to fit a 3 m bifold run and a window without any further calculation. Miss it and you are commissioning an area-weighted calculation you did not need.
What counts, and what does not
| Counts | Does not count |
|---|---|
| A back door or window now enclosed by the extension | A window elsewhere in the house you are replacing at the same time |
| An opening bricked up so the extension can be built | An opening that stays exposed to the outside after the works |
| Patio doors removed to form the link between old and new | Internal doors and openings, which were never in the envelope |
| A garage door enclosed where the garage becomes heated space | Rooflights in the existing roof that stay where they are |
How to evidence it
Mark the affected openings on the existing floor plan and elevations, with their structural opening sizes, and schedule them alongside the new glazing. GlazeMonkey prints them as their own lines in the report — each covered opening, its area, and the running total that produced the allowance — so the arithmetic is on the page rather than in an email.
One thing to be careful about
The credit applies to the openings, not to the wall. Building over an existing external wall does not earn you anything by itself; it is the windows and doors in it that count. And where a garage conversion is involved, the retained elements that become part of the heated envelope bring their own obligations — see renovating thermal elements.
Frequently asked questions
Do windows the extension covers up count towards the glazing allowance?
Yes. Approved Document L lets you add the area of any windows and doors that no longer exist, or are no longer exposed, because of the extension. They stopped losing heat to the outside, so the allowance grows by their area on top of 25% of the extension floor area.
Which openings qualify?
Openings in the existing external envelope that the extension removes or encloses: a back door that now opens into the new room, a window bricked up to build the extension, patio doors taken out to form the opening between old and new. What does not qualify is an opening somewhere else in the house that you happen to be replacing at the same time.
What if the opening is kept but now leads into the extension?
It still counts, because it is no longer exposed to the outside — that is the wording. A retained back door between the kitchen and a new garden room is inside the thermal envelope now, and its area is added to the allowance.
How do I measure them?
Structural opening size, the same basis as the new openings — the whole frame, not the glass. Take it off the existing elevations or measure on site before the wall comes down. Building Control may ask you to show them on the existing plans, so mark them up.
Does it make much difference?
Often the difference between passing on the reference route and needing a calculation. On a 20 m² extension the base allowance is 5.0 m²; a set of French doors and a window built over add 5.0 m², taking it to 10.0 m² — 100% more glass for free.
Next: what to do if you are still over · the U-values an extension must meet · how rooflights are counted.
Basis: Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), Section 10 — the area of windows, roof windows and doors in an extension.