Quick answer: the area-weighted U-value of an extension is Σ(U × A) ÷ ΣA across every element of its envelope. It is compared with a notional extension of the same size and shape whose windows, doors and rooflights are capped at 25% of the floor area (plus any openings the extension covers over) and whose elements are all at the Approved Document L limiting values — wall 0.18, floor 0.18, roof 0.15, windows and doors 1.4, rooflights 2.2 W/m²K. If your average is no worse than the notional one, the extension complies despite the extra glazing. Every new element must still meet its own limiting value.
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An extension normally complies simply by meeting the limiting U-values and keeping its windows, doors and rooflights inside 25% of its floor area, plus the area of any existing openings it covers over. Bifolds across the back and a lantern over the kitchen take most schemes past that. At that point building control bodies offer two routes — as CNC Building Control puts it, you may exceed the 25% rule if you "demonstrate the increased glazed area does not increase the energy loss above that of a 25% or less glazed building", by "providing an area weighted U Value calculation" or by employing an assessor for a whole-dwelling SAP. This page is the first of those.
The formula
Areas are the heat-loss areas of the extension: external walls net of openings, the roof, the ground floor, and every window, door, roof window and rooflight at its structural opening size — frame included, not just the glass. The wall shared with the existing house is not a heat-loss element and stays out of both columns.
How the notional extension is built
This is the part people get wrong, and it is where two calculations of the same extension end up disagreeing. The notional extension has the same floor, wall and roof areas as yours. Its opening area is cut to the allowance, keeping each type in the proportion you proposed, and the area removed is given back to the element it came out of — vertical openings become wall at 0.18, roof-plane openings become roof at 0.15. Every notional element sits at its limiting value. That way both columns describe a building of the same size, and the only difference is how much of it is glass.
A worked example
A single-storey rear extension of 24.0 m² with bifold doors, a side window and two rooflights. It swallows an existing back door (1.8 m²) and a kitchen window (1.2 m²), so the allowance is 25% × 24.0 + 3.0 = 9.00 m². The proposed openings total 14.52 m² — 60.5% of the floor area, and 5.52 m² over the allowance. So the reference route fails and the area-weighted comparison decides it.
| Element | Proposed A (m²) | Proposed U | U×A (W/K) | Notional A (m²) | Notional U | U×A (W/K) |
|---|---|---|---|---|---|---|
| External wall | 23.50 | 0.16 | 3.76 | 27.88 | 0.18 | 5.02 |
| Roof | 21.00 | 0.13 | 2.73 | 22.14 | 0.15 | 3.32 |
| Ground floor | 24.00 | 0.15 | 3.60 | 24.00 | 0.18 | 4.32 |
| External doors | 10.08 | 1.00 | 10.08 | 6.25 | 1.40 | 8.75 |
| Windows | 1.44 | 1.00 | 1.44 | 0.89 | 1.40 | 1.25 |
| Rooflights | 3.00 | 1.20 | 3.60 | 1.86 | 2.20 | 4.09 |
| Total / area-weighted U | 83.02 | 0.304 | 25.21 | 83.02 | 0.322 | 26.75 |
The excess opening area — 4.38 m² of it vertical, 1.14 m² in the roof — has been returned to wall and roof in the notional column, which is why the notional wall is 27.88 m² against a proposed 23.50 m². Both columns total the same 83.02 m².
Proposed 0.304 W/m²K ≤ notional 0.322 W/m²K — it complies. Fabric heat loss is 25.21 W/K against the notional 26.75 W/K. At 60.5% glazing that only works because the glazing is well inside the limit (1.0 and 1.2 against 1.4 and 2.2) and the walls, roof and floor are better than they need to be. Put the same extension in with glazing at the limiting values and it fails.
What moves the number
- Glazing U-value. The biggest lever by far, because the glazing carries the largest U-values in the sum. Going from 1.4 to 1.2 on a bifold run of 10.1 m² saves about 2.02 W/K on its own.
- Roof insulation. Cheap per m² and the roof is usually the largest opaque area. Going from 0.15 to 0.11 on 21.0 m² saves 0.84 W/K.
- Rooflights. The notional gives them a generous 2.2, so a rooflight specified at 1.2 or better earns credit rather than costing it — see rooflights and roof windows.
- Openings you build over. They enlarge the allowance directly, which shrinks the excess returned to wall in the notional column. Worth counting carefully.
What it costs elsewhere
An area-weighted calculation from a consultancy runs to roughly £95–£300 + VAT — SAPgen list £125, payable at booking, same day once they have your drawings. GlazeMonkey does the same calculation free on screen, as many times as you like while the design moves, and charges £35 + VAT only when you want it written up as a Building Control PDF with a verifiable report ID.
If it still fails
The area-weighted route runs out of road somewhere around half the floor area in glass, depending on the fabric. Past that, the answer is a whole-dwelling SAP calculation: the house plus your extension against the house plus a notional one, which lets improvements to the existing dwelling — replacement windows, loft insulation, a new heating system — pay for the glazing. That needs an accredited assessor, and it is work we do every week.
Frequently asked questions
What is an area-weighted U-value calculation?
It is one average U-value for a whole extension: every element — walls, roof, floor, windows, doors, rooflights — multiplied by its area, added up, and divided by the total area. Building Control asks for it when an extension has more glazing than the 25% allowance, because it shows whether the extra glass has been paid for elsewhere in the fabric.
What is it compared against?
A notional extension: the same size and shape as yours, but with its windows, doors and rooflights capped at the allowance and every element built to the Approved Document L limiting values. If your extension’s area-weighted U-value is no worse than the notional one’s, it complies.
Do the individual U-value limits still apply?
Yes. The area-weighted route lets you trade area against performance, but each new element still has to meet its own limiting value in Table 4.2 — you cannot offset a 0.25 wall with very good glazing. GlazeMonkey checks both, and tells you which element is the problem if one fails.
What if the area-weighted calculation fails?
Three ways forward, cheapest first: change the design (one rooflight fewer, or a solid panel in the bifold run) so you are back inside the allowance; improve the specification (better glazing, more roof insulation) until the average passes; or commission a whole-dwelling SAP calculation, which brings the existing house into the comparison so improvements to it count in your favour.
How much does an area-weighted calculation cost?
Consultancies charge roughly £95–£300 + VAT for it, usually with a day or two of turnaround. GlazeMonkey runs the calculation free on screen; you only pay £35 + VAT if you want the Building Control-ready PDF.
Is this the same as a SAP calculation?
No. The area-weighted method looks only at the extension and compares fabric heat loss. A SAP calculation models the whole dwelling, including heating, hot water and solar gain, and needs an accredited assessor. The area-weighted route is the quicker and cheaper of the two, and it is the one to try first.
Next: the U-values an extension has to meet · the allowance for openings you build over · work out a wall or roof U-value free.
Method and figures: Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), Section 10 and Table 4.2.