UK and Scottish building regulations for U-values in 2026
Current U-value limits and targets across England (Part L 2021), Scotland (Section 6), Wales (Part L), and Northern Ireland (Technical Booklet F). New build, extension, and renovation requirements explained.
UK building regulations on thermal performance are nation-specific. England, Scotland, Wales, and Northern Ireland each publish their own standards, and the targets differ meaningfully — particularly between Scotland and the rest of the UK, where Scotland has historically pushed tighter.
This guide sets out the current U-value requirements across all four nations for 2026, broken down by building type and work type (new build, extension, change of use, renovation of a thermal element). It's aimed at designers and building control applicants who need to know exactly what target they're hitting.
The four regulatory regimes
- England: Approved Document L Volume 1 (Dwellings) and Volume 2 (Buildings other than dwellings), 2021 edition, in force from June 2022
- Scotland: Technical Handbook Section 6 (Energy), 2023 edition
- Wales: Approved Document L — broadly aligned with England but with some Welsh-specific targets and future trajectory
- Northern Ireland: Technical Booklet F1 (Conservation of fuel and power in dwellings) and F2 (in other buildings), 2012 edition with amendments
Each document distinguishes between limiting U-values (the worst performance allowed for any one element) and notional or target U-values (the performance assumed in the reference building used to set the overall compliance target). You're free to trade performance between elements — a wall slightly worse than notional is fine if another element is better — but nothing can exceed the limiting value.
New build dwellings
The headline numbers for a new-build house.
England — Part L 2021
| Element | Limiting U-value | Notional U-value |
|---|---|---|
| External wall | 0.26 | 0.18 |
| Party wall | — | 0.00 (if sealed and fully-filled) |
| Pitched roof — insulation at ceiling level | 0.16 | 0.11 |
| Pitched roof — insulation at rafter level | 0.18 | 0.16 |
| Flat roof | 0.18 | 0.11 |
| Ground floor | 0.18 | 0.13 |
| Floor over unheated space | 0.18 | 0.13 |
| Windows (whole window) | 1.6 | 1.2 |
| Rooflights | 2.2 | 1.2 |
| External doors | 1.6 | 1.0 |
Scotland — Section 6 (2023)
Scotland's targets are similar to Part L 2021 but with some differences — particularly tighter window requirements.
| Element | Limiting U-value | Notional U-value |
|---|---|---|
| External wall | 0.22 | 0.17 |
| Pitched roof — ceiling level | 0.15 | 0.11 |
| Pitched roof — rafter level | 0.18 | 0.15 |
| Flat roof | 0.18 | 0.13 |
| Ground floor | 0.18 | 0.15 |
| Windows | 1.4 | 1.2 |
| Rooflights | 2.0 | 1.4 |
| External doors | 1.6 | 1.0 |
Scotland's wall notional of 0.17 vs England's 0.18 is a small difference but indicative: Section 6 has consistently pushed for slightly tighter fabric performance.
Wales
Wales published its own Part L 2022 in force from 23 November 2022. Notional and limiting values for dwellings are aligned closely with England's Part L 2021. Expect some divergence in future editions as Wales pursues its Net Zero Carbon strategy.
Northern Ireland — Technical Booklet F1
Northern Ireland's Technical Booklet F1 (2012 with amendments) runs behind mainland GB. Targets for new-build dwellings:
| Element | Limiting U-value |
|---|---|
| External wall | 0.30 |
| Pitched roof — ceiling level | 0.20 |
| Pitched roof — rafter level | 0.20 |
| Flat roof | 0.25 |
| Ground floor | 0.25 |
| Windows | 1.8 |
An update aligning NI with the 2021 mainland regulations is expected but not yet in force. For current projects, check the latest technical booklet on the Department of Finance website.
Extensions
Extensions use a different compliance route — the elemental method. You don't need to hit the notional targets; you just need to beat the limiting U-values for each new thermal element.
England — Part L 2021 (Approved Document L Volume 1, Table 4.2)
| Element | Extension limiting U-value |
|---|---|
| External wall | 0.28 |
| Pitched roof — ceiling level | 0.16 |
| Pitched roof — rafter level | 0.18 |
| Flat roof | 0.18 |
| Ground floor | 0.18 |
| Windows | 1.4 |
| Rooflights | 2.2 |
| Doors | 1.4 |
Scotland — Section 6
Scotland tightened extension targets in 2023:
| Element | Extension limiting U-value |
|---|---|
| External wall | 0.22 |
| Pitched roof — ceiling level | 0.15 |
| Pitched roof — rafter level | 0.18 |
| Flat roof | 0.18 |
| Ground floor | 0.18 |
| Windows | 1.4 |
Scotland applies new-build targets to extensions in many cases — a meaningful difference from England.
Renovations and retrofit
When you upgrade an existing thermal element (e.g. refurbish an external wall, insulate a loft), the regs apply threshold U-values that the upgraded element must meet if achievable without disproportionate cost.
England — Part L 2021 (Section 5 of Approved Document L)
| Retained element being renovated | Threshold U-value |
|---|---|
| Cavity wall | 0.55 (fully filled) |
| Solid wall | 0.30 internally / 0.30 externally |
| Pitched roof — insulation at ceiling | 0.16 |
| Pitched roof — insulation at rafter | 0.18 |
| Flat roof | 0.18 |
| Floor | 0.25 |
| Windows | 1.4 |
Scotland — Section 6
Scotland's Section 6.2 (Extensions, conversions and alterations) sets similar retrofit thresholds with slightly tighter windows (1.4) and generally higher expectations on fabric.
The "where technically and economically feasible" clause
Retrofit requirements have always included a get-out: if hitting the target would be disproportionately expensive or technically impossible, you can justify a lesser upgrade. But you have to show the reasoning in writing to Building Control. Blanket assumptions that "we can't afford it" don't count — you need to present a cost-benefit or physical-constraint argument.
Common legitimate reasons for deviation:
- Solid-wall insulation that would reduce internal floor area below acceptable limits
- Listed building consent restrictions on external insulation
- Structural constraints that prevent adequate insulation thickness
Non-domestic buildings
Commercial and institutional buildings have their own targets, broadly similar to dwellings but with element-specific variations. See Part L 2021 Volume 2 (England) or Section 6 Non-domestic (Scotland) for the full tables. For reference, a new office external wall target is 0.26 (limiting) and 0.18 (notional) — the same as housing.
The future: Part L 2025 and beyond
England's Future Homes Standard is expected to replace Part L 2021 from 2025 (with transitional arrangements running into 2026). Under the Future Homes Standard, fabric U-value targets won't move dramatically — roughly a 5–10% tightening — but the big change is that gas boilers will effectively be phased out of new housing in favour of heat pumps. The fabric targets are sized to ensure heat pumps can meet the heating demand at acceptable efficiency.
Scotland's New Build Heat Standard came into force in April 2024 with similar effect: new dwellings must use a zero-direct-emissions heating system. That practically means heat pumps or district heating.
For U-value purposes, expect:
- External wall notional to drop towards 0.15 W/m²K by 2025–2026
- Windows to drop to 1.0 W/m²K notional
- Party walls fully sealed and fully filled, or treated as external walls
- Air permeability targets tightening to 5 m³/h/m² at 50 Pa
The compliance pathway
For new-build housing in England:
- Use SAP 2021 software (or equivalent) to calculate the target emission rate (TER), target primary energy rate (TPER), and target fabric energy efficiency (TFEE) for the notional dwelling
- Calculate the same metrics for your actual design (using your chosen U-values, M&E spec, renewables, thermal bridging approach)
- Ensure your design meets or beats all three targets (DER ≤ TER, DPER ≤ TPER, DFEE ≤ TFEE)
- Submit SAP output, U-value calculations, thermal bridging assessment, and design-stage compliance certificate to Building Control
- Repeat the calculation at practical completion with as-built data
Small extensions can use the elemental compliance route — simpler, no full SAP calc required, just ensure each element meets limiting U-values.
Common compliance mistakes
- Using declared insulation λ values instead of design values. Declared values are factory test conditions. Design values include safety factors. Always use design λ for compliance calcs.
- Forgetting the thermal bridge y-value. If you don't calculate it, SAP applies the default 0.15 W/m²K which usually tanks your compliance.
- Ignoring the correction for wall ties. Small but can push a borderline wall over the limit.
- Wrong surface resistances. Heat flow direction matters — getting Rsi or Rse wrong gives the wrong U-value.
- Optimistic window U-values. The whole-window U-value (frame + glass + spacer + fixings) is what matters. The centre-of-pane value from the glazing manufacturer is only part of the picture.
- Retrofit that fails the threshold without justification. "Not economical" is not automatic — you have to document the reasoning.
Jurisdiction quick reference
| Building in... | Primary regulation | Nearest equivalent elsewhere |
|---|---|---|
| England | Part L 2021 (2022 edition in force) | Wales Part L 2022 |
| Scotland | Section 6 (2023 edition) | ≈ Part L 2021 but slightly tighter |
| Wales | Part L 2022 | Follows England closely |
| Northern Ireland | Technical Booklet F (2012 + amendments) | Behind mainland by one regulatory cycle |
Frequently asked questions
Which target U-value applies to my project? Check: (a) where the building is (England, Scotland, Wales, NI), (b) what type of building (dwelling, non-domestic), (c) what work (new build, extension, material alteration, renovation of a thermal element). The table you want is in the relevant Approved Document L, Section 6, or Technical Booklet F for that combination.
What happens if my design marginally misses a limiting U-value? You can't. Limiting U-values are hard limits — Building Control will not sign off a project that exceeds them. You either improve that element or, for some categories, use a compensating performance elsewhere through the SAP calculation route.
Are Passivhaus targets tighter than Part L? Significantly. Passivhaus walls target 0.10–0.15 W/m²K (vs Part L notional 0.18), windows 0.80 or better (vs 1.2 notional), airtightness 0.6 ach @ 50 Pa (vs 5 m³/h/m² @ 50 Pa). Passivhaus is a voluntary high-performance standard — not a regulatory requirement.
Does my retrofit have to hit new-build targets? No. Retrofits use threshold U-values which are less stringent than new-build notional values. They're set at the point where further insulation becomes marginally cost-effective or physically impractical.
How do I know if my U-values are compliant? Upload your construction to our AI U-value calculator. It returns the calculated U-value and flags compliance against the relevant Part L / Section 6 / Technical Booklet F limits for the element type.
Further reading
- What is a U-value?
- How to calculate U-values — the ISO 6946 method
- Psi values and thermal bridging explained
- Condensation risk in construction
Primary source documents:
- Approved Document L — https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l
- Scottish Technical Handbook Section 6 — https://www.gov.scot/publications/building-standards-technical-handbook-2023-domestic/
- Northern Ireland Technical Booklet F — https://www.finance-ni.gov.uk/publications/guidance-buildings-regulations-ni