Quick answer: for an extension to a home in England, Approved Document L Table 4.2 sets the limiting U-values for new elements: external wall 0.18, ground floor 0.18, roof 0.15, windows and doors 1.4 (or WER band B / DSER band B–C), and rooflights 2.2 W/m²K. Every new element has to meet its own figure. Separately, the windows, doors and rooflights must not exceed 25% of the extension's floor area plus any openings the extension covers over — or the extension needs an area-weighted U-value calculation.
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Every element against the table, plus the glazing allowance, as you type.
Table 4.2 — limiting U-values for new elements in an existing dwelling
| Element | Limiting U-value (W/m²K) | What it is measured on |
|---|---|---|
| External wall | 0.18 | Net of openings. The wall between the extension and the house is not a heat-loss element. |
| Ground floor | 0.18 | Normally the footprint of the extension. |
| Roof | 0.15 | Flat or pitched. Measured on the slope for a pitched roof; insulation may sit at ceiling or rafter level. |
| Windows | 1.40 | Or Window Energy Rating band B. Whole-unit value, frame included — not the centre-pane figure. |
| External doors | 1.40 | Or Doorset Energy Rating band B; band C where more than 60% of the door is glazed. |
| Roof windows (in the plane of the roof) | 1.40 | A window, not a rooflight — assessed in the vertical position. |
| Rooflights (on an upstand or kerb) | 2.20 | Assessed in the horizontal position, which is why the number is higher. |
| Swimming pool basin | 0.25 | Rarely relevant, but it is in the table. |
Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), Table 4.2. Values apply to work starting before 24 March 2027.
Limits, not targets
These are the worst an element is allowed to be, and specifying exactly to them is a false economy on anything with a lot of glass. The moment your openings pass the 25% allowance, compliance is decided by an area-weighted comparison against a notional extension whose elements sit at precisely these values — so an extension built to the limits and glazed beyond the allowance fails by definition. Every 0.01 you take off the roof or the glazing buys headroom.
Three places extensions come unstuck
- Centre-pane values on the window schedule. Table 4.2 wants the whole-unit U-value — frame, spacer and glass together. A unit quoted at 1.2 centre-pane is often 1.5–1.6 as a whole window, which fails. Ask for the certified whole-unit figure.
- Rooflights judged as windows. A roof window in the plane of the roof is a window at 1.4; a rooflight on an upstand is 2.2, because it is assessed lying horizontally. Get those the wrong way round and you have either failed or given away headroom — the difference is worth understanding.
- Assuming the build-up performs. "100 mm of PIR in the cavity" is not a U-value. Bridging by timber or mortar, the fixings through the insulation and the actual block conductivity all move it. U-Monkey works it out to BS EN ISO 6946 free, with a PDF Building Control will accept.
Work to the existing house is a different table
If the job also re-roofs, re-renders, dry-lines or converts part of the existing house, those elements are judged against Table 4.3 — a threshold value and an improved value, with a feasibility test — not against the numbers above. See renovating thermal elements.
Frequently asked questions
What U-values does an extension have to meet?
Approved Document L Table 4.2 sets the limiting values for new elements in an existing dwelling: walls 0.18, floors 0.18, roofs 0.15, windows and doors 1.4, rooflights 2.2 W/m²K. Windows and doors may comply instead by energy rating. These are limits, not targets — a specification that only just meets them leaves nothing in hand if the extension is over-glazed.
Are these the same as the new-build U-values?
No. New dwellings are assessed against a notional dwelling through SAP, and the limiting values there are different and generally tighter. Table 4.2 is the set that applies to work on an existing home — extensions, and new elements built into an existing house.
Can I trade a worse wall against better glazing?
Not below the limiting value. Each new element has to meet its own figure in Table 4.2 whatever else you do. The area-weighted route lets you trade extra glazing area against better performance overall, but it never excuses an element that is worse than its limit.
Do I need a calculation to prove the U-values?
Building Control will expect the U-value of each build-up to be supported — a manufacturer’s figure for a window or doorset, and a calculation to BS EN ISO 6946 with BR 443 conventions for a wall, roof or floor. U-Monkey does those free, including the PDF.
When do these numbers change?
The 2026 edition of Approved Document L applies to work starting on or after 24 March 2027, under the usual transitional arrangements. Until then the 2021 edition incorporating the 2023 amendments is what applies to an extension.
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