Asked for heat loss calculations for your extension? Here is what they mean

Published 25 September 2026 — reviewed against the regs as they stand.

What building control is asking for

When building control asks for “heat loss calculations” on an extension, it is almost always about the glazing. Approved Document L, the guidance behind Part L of the Building Regulations, caps the total area of windows, roof windows, rooflights and doors in an extension at 25% of its floor area, plus the area of any existing windows and doors the extension removes or covers over. Inside that allowance an extension complies on the U-values of its parts alone, and there is nothing to calculate.

Over it, the extension can still comply, but somebody has to show that it does. Approved Document L gives two ways:

• An area-weighted U-value calculation (paragraph 10.9): the extension as designed is shown to lose no more heat through its fabric than the same extension built to the 25% limit.
• A whole-dwelling SAP calculation (paragraph 10.10): the house with your extension is shown to perform no worse than the house with a compliant, notional extension.

Building control bodies and their guidance notes call these different things — heat loss calculations, compensatory calculations, excess glazing calculations, “an area-weighted U-value calculation or SAP” — but they are all asking for one of the two. CNC Building Control puts the test plainly: you may exceed 25% if you “demonstrate the increased glazed area does not increase the energy loss above that of a 25% or less glazed building”.

What it is not is a room-by-room heat loss calculation to BS EN 12831, the kind a heating engineer runs to size radiators or a heat pump. That answers a different question — how many watts each room needs on a cold day — and it is not one of the Part L routes. If you are fitting a heat pump as well, you may need both; they are separate jobs.

First, check you are actually over

A surprising number of “over-glazed” extensions are not, once the allowance is worked out properly. Two things decide it.

Measure openings as openings. The area that counts is the structural opening — frame included, not just the glass — for every window, door, roof window, rooflight and lantern in the extension’s walls and roof.

Claim the openings you cover over. Any existing window or door that no longer exists, or is no longer exposed, because of the extension adds its area to the allowance. Take a 24 m² rear extension: 25% of it is 6.0 m², but if it swallows a 1.8 m² back door and builds over a 1.2 m² kitchen window, the allowance is 9.0 m². Mark those openings on the existing drawings, because building control will want to see where the extra came from. The covered-openings allowance works through it.

If the extension is inside the allowance after that, the answer to building control is the arithmetic itself — a schedule of openings against the allowance — not a heat loss calculation.

Route one: the area-weighted U-value calculation

This is the usual answer, and the cheaper one. Every element of the extension — the walls net of openings, the roof, the floor, and each window, door and rooflight — is multiplied by its area, the results are added up, and the total is divided by the total area:

area-weighted U = (U₁A₁ + U₂A₂ + …) ÷ (A₁ + A₂ + …)

The same sum is then done for a notional extension: the same size and shape as yours, with its openings cut back to the allowance and the area removed given back to wall or roof, and every element at the limiting U-values in Table 4.2 of Approved Document L — walls 0.18, floor 0.18, roof 0.15, windows and doors 1.4, rooflights 2.2 W/m²K. If your extension’s figure is no higher than the notional one, it complies.

Because glazing and fabric that beat the limiting values both count, this passes more often than people expect. GlazeMonkey’s worked example is a 24 m² extension with over 60% of its floor area in openings — bifolds, a side window and two rooflights — and it passes, because the glazing is at 1.0 and 1.2 W/m²K and the walls, roof and floor beat their limits. Put the same extension in with glazing at the limiting values and it fails. The area-weighted method sets out that example line by line.

One rule does not bend: every new element must still meet its own limiting U-value. The trade is between area and performance across the whole extension, not a licence for one poor element carried by good ones.

Route two: the whole-dwelling SAP calculation

When the area-weighted comparison cannot be made to pass, the SAP route brings the existing house into the sum. The dwelling is modelled twice: once with your extension as designed, and once with a notional extension of the same size and shape, its openings at the allowance, its door area the same as yours and the rest counted as windows. The first must not exceed the second on three measures — primary energy, carbon emissions and fabric energy efficiency. Where the existing house’s construction is unknown, standard values from SAP’s Appendix S are used.

Its advantage is that improvements to the existing house count in your favour: replacement windows, a topped-up loft, a better heating system. It is more work and it costs more, and it is the route that gets heavily glazed designs through. SAP calculations for over-glazed extensions is work we do every week.

What the document has to show

No form is prescribed. For the area-weighted route, whoever checks it will be looking for:

• The extension it applies to — the address, and the drawings it was worked from.
• The allowance — the extension’s floor area, 25% of it, and each covered-over opening with its size.
• The proposed extension, element by element — the area and U-value of the walls, roof, floor and every opening, and where each U-value came from: the manufacturer’s declared whole-window value or energy rating for the glazing, and a BS EN ISO 6946 calculation for the walls, roof and floor.
• The notional extension, built as above, totalling the same area as the proposed one.
• The two area-weighted U-values, and a clear pass.
• Confirmation that every new element meets its own limiting value.
• The date, and who produced it.

A SAP-route submission carries the same identification, the results for both cases on each of the three measures, and the assessor’s details.

Why they get sent back

1. Glass measured instead of openings. The frame counts.
2. Covered openings missed — or claimed with nothing on the existing drawings to show them.
3. A rooflight treated as a roof window, or the other way round. A roof window in the plane of the roof is a window at 1.4; a rooflight on an upstand takes 2.2. Swap them and the notional extension is wrong. Rooflights and roof windows sets out which is which.
4. A notional extension that shrinks. The glazing cut back to the allowance has to reappear as wall or roof. Delete it instead and the two sides no longer describe the same building.
5. Centre-pane glazing figures. The value that counts is the whole window, frame included, which is always worse than the glass alone.
6. Assumed fabric U-values. “Insulated to current regs” is not a U-value. U-Monkey calculates the real one for each build-up, free.

When it is due

No regulation sets a date. On a full plans application building control will want it before approving the plans; on a building notice, before it can sign the extension off. The time it is most useful is earlier than either: before the glazing is ordered, while a rooflight is still a decision rather than a delivery and a better glazing specification is a line on a quote rather than a refit.

What it costs

• An area-weighted calculation from a consultancy runs to roughly £95–£300; SAPgen lists £125.
• A whole-dwelling SAP is advertised from about £75 + VAT at the cheapest, with other providers starting at £275 or more, because the existing house has to be modelled as well as the extension.
• GlazeMonkey runs the area-weighted check free, as many times as you like, and charges £35 + VAT (£42 inc VAT) for the Building Control PDF.

Whichever you choose, ask what a revision costs. The glazing changes more than anything else on an extension’s drawings, and every change is a re-run.

Do it yourself, free

GlazeMonkey does the whole area-weighted route in your browser: the allowance, the covered-openings credit, the notional extension built the way Approved Document L describes it, and the comparison, live as you type. If you are short, it tells you what the glazing would need to be to pass. It is free to use; the Building Control-ready PDF is £35 + VAT, with a report ID building control can verify online. See an example report (PDF) first.

What you’ll need to hand: the extension’s floor area; the width and height of every window, door and rooflight at structural opening size; the same for any existing openings the extension covers over; and a U-value for each element — the whole-window figure from the glazing supplier, and the wall, roof and floor build-ups from your drawings, which U-Monkey turns into U-values if you don’t have them.

If it still will not pass, two options are left: the whole-dwelling SAP, or less glass — which is sometimes a single rooflight. The rest of an extension’s Part L, from the U-values to conservatories, is in building regulations for an extension.

Wales, Scotland and the 2027 changes

Wales runs the same idea with different numbers: windows and doors limited to 25% of the extension’s internal floor area plus the openings it does away with, an area-weighted or SAP route as the alternatives, and tighter limiting U-values for new roofs (0.13) and floors (0.15).

Scotland limits windows, doors and rooflights in an extension to 25% of its floor area plus any openings built over, and allows more where a “compensatory approach” shows the extra glazing causes no additional heat loss — or where a whole-dwelling SAP passes.

England’s Approved Document L 2026 takes effect on 24 March 2027. It keeps the 25% rule (renumbered as paragraph 10.8), both alternative routes and the same limiting U-values for new elements. An extension with its building notice or full plans application in before 24 March 2027 stays under the 2021 edition, provided the work has commenced by 24 March 2028 — and for an extension on new foundations, commenced has a legal meaning: its foundations and ground-floor structure complete, not the first spade in the ground.