In 2026, the UK U-value targets for building fabric insulation are 0.18 W/m²K for walls, 0.11 W/m²K for roofs, and 0.13 W/m²K for floors in new dwellings, as set by Approved Document L of the Building Regulations (gov.uk, 2026). These figures represent the maximum heat loss permitted through each element.
The target U-values apply to new-build homes and major renovations where building control approval is required. For existing homes undergoing retrofit work, the targets are slightly less stringent: 0.30 W/m²K for walls, 0.16 W/m²K for roofs, and 0.18 W/m²K for floors. These apply when replacing or upgrading insulation, not for minor repairs. The values assume standard construction methods and do not apply to listed buildings or properties in conservation areas where alternative compliance routes exist.
Wall insulation U-value target
For new homes, external walls must achieve a U-value of 0.18 W/m²K or better. This typically requires 150mm to 200mm of cavity or external wall insulation, depending on the material used. For retrofit projects, the target is 0.30 W/m²K, which can often be met with 100mm to 120mm of mineral wool or rigid foam board. The Energy Saving Trust advises that upgrading cavity wall insulation from 50mm to 100mm can reduce heat loss by up to 40% (Energy Saving Trust, 2026). Solid walls need internal or external insulation to reach these figures.
Roof insulation U-value target
New-build roofs must achieve 0.11 W/m²K, requiring around 300mm to 350mm of mineral wool or 200mm of rigid foam insulation in a cold roof design. For warm roofs, where insulation sits above the rafters, 150mm to 200mm of high-performance material is typical. In retrofit situations, the target is 0.16 W/m²K, which can be met with 270mm of mineral wool laid between and over ceiling joists. The BRE notes that upgrading loft insulation from 100mm to 270mm can save around £200 per year on heating bills for a typical semi-detached home (BRE, 2026).
Floor insulation U-value target
New-build ground floors must achieve 0.13 W/m²K, which typically requires 150mm to 200mm of rigid insulation board below a concrete slab or between timber joists. For retrofit projects, the target is 0.18 W/m²K, achievable with 100mm to 150mm of insulation. Suspended timber floors can be insulated with mineral wool or rigid foam between joists, while solid floors may require insulation laid above the existing surface. The government’s Standard Assessment Procedure (SAP) uses these U-values to calculate a home’s Energy Performance Certificate rating (gov.uk, 2026).
A worked example
Upgrading a 1930s semi-detached house in Manchester to meet the 2026 retrofit U-value targets costs roughly £8,250 after the 0% VAT saving. The property has uninsulated solid brick walls and a cold loft with 50mm of old mineral wool. To hit 0.30 W/m²K for walls, the owner installs 100mm of external wall insulation at £6,500. The loft gets topped up to 300mm of mineral wool for £750, achieving 0.16 W/m²K. Floor insulation beneath the ground-floor suspended timber reaches 0.18 W/m²K for £1,000. Based on Energy Saving Trust figures, the combined upgrades save £540 per year on heating bills. The total upfront cost is £8,250, giving a payback period of roughly 15 years. Over 25 years, the cumulative saving reaches £13,500, well above the initial outlay.
| Item | Figure |
|---|---|
| Upfront cost after grants | £8,250 |
| Yearly savings | £540 |
| Payback period | 15 years |
| 25-year lifetime savings | £13,500 |
What homeowners often get wrong
The most common mistake is assuming the same U-value targets apply to both new builds and existing homes. This leads to overspending on unnecessary insulation thickness or missing out on permitted lower standards. Here are the three biggest errors homeowners make.
- Chasing new-build targets for retrofits Many homeowners aim for 0.18 W/m²K on existing walls, but the 2026 retrofit target is 0.30 W/m²K. Installing 200mm of insulation where 100mm suffices wastes £2,000 to £3,000 and can cause moisture issues in solid walls.
- Ignoring the floor U-value requirement People often insulate walls and roof but skip the floor, thinking it is optional. The 2026 retrofit target of 0.18 W/m²K for floors is mandatory for major renovations, and missing it can void building control approval and add £200 per year to heating bills.
- Assuming all insulation materials are equal Homeowners pick the cheapest option without checking the declared U-value per thickness. Using 100mm of low-density mineral wool instead of rigid foam board may only achieve 0.35 W/m²K, failing the 0.30 target and requiring a costly redo that adds £1,500 in labour.
Quick reference
- New-build wall U-value target for 2026 is 0.18 W/m²K, requiring 150mm to 200mm of insulation.
- Retrofit wall U-value target for 2026 is 0.30 W/m²K, achievable with 100mm to 120mm of standard insulation.
- Roof U-value target for retrofits is 0.16 W/m²K, typically needing 270mm to 300mm of loft insulation.
- Floor U-value target for retrofits is 0.18 W/m²K, adding roughly £1,000 to a typical project cost.
- Listed buildings and conservation area properties are exempt from these targets but must still meet minimum energy efficiency standards.
Frequently Asked Questions
For new dwellings, the 2026 targets are 0.18 W/m²K for walls, 0.11 W/m²K for roofs, and 0.13 W/m²K for floors, per Approved Document L. Retrofits have higher allowances: 0.30 walls, 0.16 roofs, 0.18 floors.
Yes, but only for major renovations or when replacing insulation, with less stringent figures: 0.30 walls, 0.16 roofs, 0.18 floors. Listed buildings and conservation areas have alternative routes.
Walls need 150-200mm for new builds and 100-120mm for retrofits. Roofs require 300-350mm of mineral wool to reach 0.11 W/m²K. The Energy Saving Trust says upgrading cavity wall insulation to 100mm can cut heat loss by 40%.
