Building regulations for an extension: the energy side, start to finish

Published 17 September 2026 — reviewed against the regs as they stand.

Quick answer: an extension’s glazing must not exceed 25% of its floor area, plus the area of any existing windows and doors the extension covers over — and that second part is routinely forgotten. Stay inside the allowance and the extension complies on its U-values alone: walls 0.18, floors 0.18, roof 0.15, windows and doors 1.4, rooflights 2.2 W/m²K. Go over it and you need an area-weighted calculation, a whole-dwelling SAP, or less glass.

Where Part L fits among the other regs

An extension picks up most of the building regulations at once. Structure, fire escape, stairs, glazing safety, ventilation, drainage and electrical work all apply, and your building control body will look at all of them. This guide is about Part L, the conservation of fuel and power — the part that decides how much glazing you can have and how well the new walls, roof and floor have to perform. It is also the part that most often produces the sentence “Building Control have asked for a calculation”, usually because of the glazing.

None of what follows is about planning permission, which is a separate consent with separate rules.

The number everything hangs on

Approved Document L caps the area of windows, roof windows and doors in an extension at 25% of the extension’s floor area.

That much is widely known. What is much less widely claimed is the rest of the sentence: you may add the area of any existing windows and doors that the extension covers over or removes. The logic is heat loss, not aesthetics — a back door that now opens into the new room has stopped losing heat to the outside, so the envelope has not gained as much glazing as the raw numbers suggest.

So the allowance is:

25% × extension floor area + the area of covered-over openings

Take a 24 m² rear extension. The base allowance is 25% × 24 = 6.0 m² — one modest set of bifolds and you are finished. But if the extension swallows an existing back door of 1.8 m² and builds over a kitchen window of 1.2 m², that is another 3.0 m², and the allowance becomes 9.0 m²: half as much again, and often the difference between needing a calculation and not.

Measure covered openings at structural opening size, mark them on the existing elevations and keep the drawing — your building control body will want to see where the extra came from. The covered-openings allowance works through it in detail.

What every new element has to hit

Whether or not you are inside the glazing allowance, each new element of the extension has a limiting U-value it must not exceed. These are the Table 4.2 values for new elements in an existing dwelling — tighter than the backstops that apply to a whole new house, which surprises people.

Two traps live in that table. Windows and doors are not area-weighted in an existing dwelling — each one has to meet 1.4 on its own, so a single heritage-style unit cannot be carried by better ones elsewhere. And the last two rows are a genuine distinction, not a synonym: a roof window sitting in the plane of the roof is treated as a window and assessed vertically at 1.4, while a rooflight on an upstand is assessed horizontally at 2.2. Getting it the wrong way round can turn a pass into a fail on paper. Rooflights and roof windows sets out which is which, and what U-values an extension needs has the full table with the exceptions.

If you need to know what a particular build-up actually achieves rather than what you hope it does, U-Monkey calculates it to BS EN ISO 6946, free.

Over the allowance? Three routes

Bifolds across the back, a lantern over the kitchen and a run of rooflights will clear 25% without much effort. That is not a failure — it just means the simple route is closed and one of three others has to be used.

1. The area-weighted U-value calculation. The usual answer, and the cheapest. You build a notional extension of the same size and shape, with its openings capped at the allowance and the excess area returned to wall or roof at the limiting U-values, then show that your design performs no worse overall. Because better-than-limiting glazing and fabric both count, a well-specified extension frequently passes with more glass than the 25% rule alone would allow. The area-weighted method has the formula and a worked example.

2. A whole-dwelling SAP calculation. House plus extension assessed together against the same house notionally extended. This route is often easier to pass than expected, because improvements you are making to the existing house count in your favour. It is more work and more cost, and it is the route to take when the glazing is not negotiable. That is a job we do every week — see SAP calculations for over-glazed extensions.

3. Less glass. Worth saying out loud. Moving from the allowance being exceeded by a little to not exceeded at all is sometimes one rooflight.

Which of the three you need is decided by the arithmetic, and the arithmetic is free: GlazeMonkey does the allowance, the covered-openings credit and the full area-weighted comparison on screen, and tells you what the glazing would have to be if you are short.

Conservatories, porches and the things that are not either

A conservatory or porch escapes the energy efficiency requirements entirely — but only while every condition holds at once: at ground level, no more than 30 m², substantially glazed (at least 75% of the roof and 50% of the external walls), glazing compliant with Part K, thermally separated from the house by the retained external wall, door or window, and unheated, or heated with its own independent controls.

Break one and it is an extension. Take the back wall out to open it into the kitchen and the thermal separation has gone; run a radiator off the house system and the heating condition has gone. An orangery or garden room that is not substantially glazed was never a conservatory in the first place — it is an extension, 25% rule and all. The conservatory exemption tests each condition in turn.

The part that catches people: work to the existing house

An extension rarely stops at the new bit. Take the roof off to tie in, re-render the flank wall, dry-line the room behind, bring a garage into the heated envelope — and you are into a different set of Part L rules, the ones that govern existing elements. Renovating more than half of a wall, roof or floor triggers an upgrade duty with its own thresholds, and a retained element that joins the heated envelope is triggered whatever proportion you touch.

Those rules are set out in building regulations when you renovate. The practical point here is that they are assessed alongside the extension, not instead of it — and both halves belong in one calculation.

What changes on 24 March 2027

Approved Document L Volume 1, 2021 edition incorporating the 2023 amendments, is the edition behind every figure on this page, and it governs work in England up to 24 March 2027. Work starting on or after that date falls under the 2026 edition, published alongside the Future Homes Standard.

If your extension is likely to start near that boundary, the commencement date is the thing to pin down early — it decides which edition your building control body assesses against, and it is much easier to establish at the start than to argue about later.

The bottom line

Count the glazing, then count the openings you are covering over, and compare the two totals against a quarter of the floor area. Inside it, specify to the Table 4.2 values and there is nothing to submit. Outside it, the area-weighted route is the cheap answer and passes more often than its reputation suggests — and the only way to know which side of the line you are on is to do the arithmetic before the glazing is ordered, not after it arrives.