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Check an extension's glazing against the 25% rule
The opening-area check, the Table 4.2 element checks and the full area-weighted comparison — the three tests an extension has to pass, run together on the areas you actually have.
Free to check. Building Control-ready PDF £35 +VAT (£42 inc VAT).
The quarterly planning statistics are not usually read for what they say about Building Regulations. This set is worth it, because of a single proportion that has held steady for years: about half of all the planning work local authorities in England get through is householder work.
What the April to June 2026 release says
The Planning applications in England: April to June 2026 statistical release was published by MHCLG on 24 September 2026. In the quarter, authorities in England:
- received 78,700 applications for planning permission, down 2% on the same quarter a year earlier
- decided 74,500 applications, down 8%
- granted 65,200 decisions, down 8%
Of those decisions, 37,700 were on householder developments — “developments to a residence which require planning permission such as extensions, loft conversions and conservatories”, in the release’s own words. That is 51% of all decisions, down from 54% in the quarter ending June 2025, with the volume itself down 14% year on year. Authorities granted 91% of them and decided 92% within 8 weeks or the agreed time.
Over the year ending June 2026 the pattern is the same: 145,600 householder decisions, 50% of all decisions, 90% granted.
So the softening is real — householder volumes fell faster than the total this quarter — but the share is not moving much. Half the planning system’s caseload, give or take, is someone adding a room to their own house.
And that undercounts the work
These figures only count extensions that needed planning permission. A great many do not: a single-storey rear extension within the permitted development limits needs no application at all, and the same release records that large householder extensions made up 57% of all prior approval applications reported under permitted development rights.
Every one of those extensions still needs Building Regulations approval. Planning and Building Control are separate consents, decided by different tests, and a granted planning permission carries no implication whatsoever that the scheme meets Part L. The homeowner who has just had a 91%-likely approval land in their inbox has cleared the consent they knew about and not yet met the one they did not.
The number nobody mentions at the planning stage
Approved Document L limits the total area of windows, roof windows and doors in an extension to 25% of the extension’s floor area — plus, and this is the part most often missed, the area of any existing windows and doors the extension covers over or removes. A back door that now opens into the new room, a kitchen window built over: both add to the allowance.
Stay inside it and the extension complies on its element U-values alone, with no calculation to submit. That is the quiet reward for getting the glazing right, and it is why the number matters before the bifolds are ordered rather than after.
The U-values that come with it
For new elements in an existing dwelling, Approved Document L Volume 1 (2021 edition incorporating the 2023 amendments) sets limiting values of walls 0.18, floors 0.18, roofs 0.15, windows and doors 1.4, and rooflights 2.2 W/m²K. A roof window in the plane of the roof is a window and takes 1.4, assessed vertically; a rooflight on an upstand takes 2.2, assessed horizontally.
A conservatory or porch is outside the energy efficiency requirements only while every condition holds — ground level, no more than 30 m², substantially glazed, thermally separated, no heating or heating on its own controls. Break one and it is an extension, with the 25% rule attached.
Go over the allowance and there are three routes rather than a refusal, the cheapest of which is usually to show the extension’s area-weighted U-value is no worse than a compliant one of the same size and shape. The full guide to building regulations for an extension works each of those through, along with what happens to the existing house when the work touches it.
Where this leaves the glazing
Modern extensions are designed around glass. A 24 m² rear extension has a 6 m² opening allowance before covered-over openings are added, and a set of bifold doors alone can be 10 m². That is not a failure — it is the ordinary starting point for most schemes, and the reason the area-weighted route exists.
GlazeMonkey runs the three tests an extension has to pass on the areas you actually have: the opening-area check including the covered-openings credit, the Table 4.2 element checks, and the full area-weighted comparison against the notional extension. It is free to use in the browser with no sign-up, so the answer arrives before the glazing is ordered rather than during a Building Control query.
The next quarterly release, covering July to September 2026, is due in December. If you have an extension where the glazing looks marginal and you would rather have it settled properly, get in touch.
Sources: Planning applications in England: April to June 2026 — statistical release, MHCLG, 24 September 2026 · Planning applications in England: April to June 2026 — technical notes, MHCLG · Planning applications statistics collection, GOV.UK · Approved Document L: Conservation of fuel and power, GOV.UK