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There's no single "good" U-value
The U-value you need depends on the element, the kind of work and where you are. Part L sets a limiting (worst-allowed) value for each element, and a new home is measured against a notional dwelling built to tighter figures. Roofs are held to the tightest targets, floors a little higher, walls higher still — so it's judged element by element, not as one figure.
The England figures
Approved Document L, Volume 1 (dwellings), in W/m²K:
| Element | Notional — Part L 2021 | Notional — FHS from 24 March 2027 (SAP 10.3 and HEM) | Limiting, new dwelling (unchanged in 2027) | New element in an existing dwelling | Retained element — upgrade if worse than → to |
|---|---|---|---|---|---|
| External wall | 0.18 | 0.18 | 0.26 | 0.18 | 0.70 → 0.55 (cavity fill) / 0.30 (internal or external insulation) |
| Floor | 0.13 | 0.13 | 0.18 | 0.18 | 0.70 → 0.25 |
| Roof | 0.11 | 0.11 | 0.16 | 0.15 | 0.35 → 0.16 |
| Party wall | 0 | 0 | 0.20 | — | — |
| Windows | 1.2 | 1.2 | 1.6 | 1.4 (or WER band B) | — |
| Doors | 1.0 | 1.0 | 1.6 | 1.4 (or DSER band B/C) | — |
| Rooflights | 1.7 | 1.7 | 2.2 | 2.2 | — |
| Air permeability, m³/(h·m²) at 50 Pa | 5.0 | 4.0 | 8.0 | — | — |
Approved Document L Volume 1, 2021 edition incorporating 2023 amendments — Table 1.1 (notional dwelling), Table 4.1 (limiting values, new dwellings), Table 4.2 (new elements in existing dwellings), Table 4.3 (retained elements); Approved Document L 2026 for the Future Homes Standard notional dwelling. Checked 2026-09-04.
What the Future Homes Standard changes — and what it doesn't
Approved Document L 2026 — the Future Homes Standard — comes into force on 24 March 2027 (24 September 2027 for higher-risk buildings). For walls, roofs and floors the numbers do not move: the limiting values are the same (now Table 3.1), and the notional dwelling keeps the 2021 fabric — walls 0.18, roofs 0.11, floors 0.13. The changes are a heat pump in the notional dwelling and a tighter air-permeability target. If you have seen a wall target of 0.15 or a floor of 0.11 quoted for the Future Homes Standard, that came from the 2019 consultation and was not adopted. U-Monkey checks against the edition you choose: work notified before 24 March 2027 and started before 24 March 2028 can stay on the 2021 edition.
Elsewhere the figures differ: Wales has been tighter since 2022, Scotland sets its own Section 6 maxima, and there are pages for Ireland and non-domestic buildings, and Northern Ireland follows its own Technical Booklets F1 and F2.
Don't memorise the numbers — check the build-up
The reliable way to answer "what U-value do I need" is to build up the actual element and see where it lands. U-Monkey traffic-lights your result live against the limiting and notional values for your nation and type of work, and its one-tap fix tells you how much more insulation gets you there — rounded to a thickness you can actually buy.
Meeting the U-value isn't always the end of it. Where a build-up steps outside the arrangements BS 5250 treats as safe — insulation along the rafters, a flat roof insulated above and below the deck — Building Control can also ask for a condensation risk analysis to BS EN ISO 13788. U-Monkey's free condensation risk calculator runs it on the same build-up.
Check any element
Build up a wall, roof, flat roof or floor from real UK products, read the U-value and the full working, and export a Building-Control-ready PDF — all completely free.
Frequently asked questions
What U-value do I need for Building Regs?
In England, a new dwelling's walls may be no worse than 0.26 W/m²K, roofs 0.16 and ground floors 0.18 — the limiting values — and the notional dwelling it is measured against uses 0.18, 0.11 and 0.13, so real designs land near those. An extension or a new element in an existing home needs walls and floors of 0.18 and a roof of 0.15. Wales, Scotland and Northern Ireland set their own figures. U-Monkey checks your build-up against the right set as you build it.
Does the Future Homes Standard change the U-values?
Not for walls, roofs and floors. Approved Document L 2026, in force from 24 March 2027, keeps the same limiting values (renumbered Table 3.1), and the Future Homes Standard notional dwelling uses the same fabric as the 2021 one: walls 0.18, roofs 0.11, floors 0.13 (SAP 10.3 Appendix R, Table R1). What changes is the heating (a heat pump in the notional dwelling) and the air-tightness target (4.0 m³/(h·m²)). The 0.15 wall and 0.11 floor figures still quoted online come from the 2019 consultation; they are not what was adopted.
Are the targets the same for every element?
No. Roofs are held to the tightest U-values, floors are a little higher and walls higher still, with windows and doors on their own scale. That's why there's no single 'good' U-value — it's judged element by element, which is exactly how U-Monkey shows it.
Do new builds, extensions and renovations have different targets?
Yes. A new dwelling has to stay inside the limiting values and meet its overall emission and energy targets, which in practice means fabric close to the notional values. An extension or new element in an existing home is judged element by element (Table 4.2). When you renovate an existing wall, roof or floor that is worse than a threshold (0.70 for walls and floors, 0.35 for roofs), Table 4.3 asks you to improve it: cavity walls to 0.55, internally or externally insulated walls to 0.30, floors to 0.25 and roofs to 0.16, where that is technically, functionally and economically feasible.
Is the calculator free?
Yes — U-Monkey is completely free, including the Building-Control-ready PDF. Build up any element and check it against the targets at no cost.
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