Last reviewed July 2026 against Approved Document L Volume 1 (2021 edition, 2023 amendments). This guide sets out the building regs u-values for every element of a new or existing dwelling, the difference between limiting and notional values, and typical build-ups that pass.
Table of Building Regulations U-Values for New Dwellings in England
| Limiting U-Value (W/m2K) | Notional U-Value (W/m2K) | |
| External walls | 0.26 | 0.18 |
| Floor | 0.18 | 0.13 |
| Roof | 0.16 | 0.11 |
| Windows | 1.6 | 1.2 |
| Roof windows | 1.6 | 1.2 |
| Rooflights | 2.2 | 1.7 |
| Doors | 1.6 | 1.0 |
| Party walls | 0.20 | 0.0* |
Within Approved Document L (Part L) there are two sets of u-values that apply to new build dwellings:
- Limiting U-Values
- Notional Building U-Values
What Is a U-Value?
A u-value measures how fast heat escapes through a building element, measured in W/m²K. The lower the u-value, the better the insulation. For construction elements it’s calculated using every layer and material in the build-up, from the outer finish through to the internal plaster. Building regulations set a maximum (limiting) u-value for each element and a lower target (notional) value, and understanding the difference between the two makes complying with building regulations far easier.
Notional & Limiting Explained
The limiting u-value is the maximum u-value an element can be, a hard cap. Designing to this value is not an automatic SAP fail but it probably won’t pass overall.
The notional values should be your target.
During a SAP Calculation, a notional dwelling is created that passes SAP. This dwelling uses the notional u values and your design is weighed against it to determine a pass or fail.
In short, the notional building is your benchmark. You’ll need to hit its fabric heat loss (TFEE), carbon emissions (TER) and primary energy (TPER) targets.
The reason the two sets exist is to give design freedom and to fall short of mandating a recipe for all building projects. You can ‘borrow’ from elements and make up in others as long as no element exceeds the back stop limiting values. Need a little leeway on the walls? No problem, just make sure your floor and roof are under.
U-Value Requirements for a New Dwelling
The building regulations u-value requirements for a new dwelling are set out in Approved Document L Volume 1. Every element has a limiting (worst acceptable) value and a lower notional value that SAP uses to set your target:
- Walls – limiting 0.26, notional 0.18 W/m²K
- Floors – limiting 0.18, notional 0.13 W/m²K
- Roofs – limiting 0.16, notional 0.11 W/m²K
- Windows and glazed doors – limiting 1.6, notional 1.2 W/m²K
- Opaque doors – limiting 1.6, notional 1.0 W/m²K
- Rooflights – limiting 2.2, notional 1.7 W/m²K
- Party walls – limiting 0.20 W/m²K, notional 0 (filled and sealed cavity)
The minimum u-value you need for building regs is the limiting figure, but designing every element to the limiting value will almost certainly fail SAP overall, so treat the notional values as the real requirement. All of these are area-weighted averages, so you can combine different build-ups across a single element – our area-weighted u-value calculator works this out for you.
Common Build Ups That Pass Building Regs
We’ve created a quick reference guide for the most common elements to show what you’d need to reach the notional u-values. We’ve used reference materials in Elmhurst’s u-value calculator. Specific products will have a different thermal resistance and may not achieve the figures below.
Wall U-Values (Limiting 0.26, Notional 0.18 W/m2K)
A typical build up, starting from the outer leaf would be:
- Standard Brick
- Air Gap (50mm)
- 120mm Polyurethane Insulation
- Lightweight Block
- Dot & Dabbed Plasterboard
- Skim
Thermal bridging has been taken into account for the mortar and dabs. It assumes plastic wall ties, which have no thermal bridging effect on u-values. This build up achieves 0.17 W/m2K.
Floor U-Values (Limiting 0.18, Notional 0.13 W/m2K)
Floor u-values differ with overall size and the length of the exposed perimeter. A semi will be different than a detached. Let’s assume a 7m x 5m semi detached and look at a floor build up that would meet the notional 0.13 W/m2K target:
- Concrete Slab (100mm)
- 150mm Polyurethane Insulation
- Screed (75mm)
This build up achieves 0.13 W/m²K.
Roof U-Values (Limiting 0.16, Notional 0.11 W/m2K)
For a standard pitched roof with a cold loft space and horizontal insulation at ceiling level:
- Plasterboard
- 150mm Timber Frame*
- 150mm Mineral Wool Insulation Between Joists
- 250mm Mineral Wool Cross Laid Above Joists
*Thermal bridging via the timber has been taken into consideration. We’ve assumed a 50mm width at 500mm centres and noggins every 1.2m.
This build up matches the 0.11 W/m2K notional value.
Airtightness
U-values aren’t the only fabric metric. Airtightness is important too.
| Limiting | Notional | |
| Air permeability | 8 m³/(h.m²) @ 50Pa | 5 m³/(h.m²) @ 50Pa |
All new dwellings must be pressure tested. Like u-values, the notional figure (5) is your real target. Hitting 8 and hoping for the best will mean compensating elsewhere.
Note: Below 5 is good for energy. Below 3 triggers mechanical ventilation requirements under Part F.
U-Values for Extensions, Conversions & Existing Dwellings
The values above apply to new build dwellings only.
Conversions (e.g. barn to dwelling, commercial to residential) have different u-value requirements based on the existing structure.
Extensions don’t use the notional dwelling comparison. They still need to meet specific u-values to avoid a full home SAP calculation as well as keeping the glazing under thresholds too. We’ve got a page dedicated to extension building regulations compliance.
New and replacement elements in existing dwellings, including extensions, follow a separate set of limiting u-values under Table 4.2 of Approved Document L:
| Element | Limiting U-Value (W/m2K) |
| Wall | 0.18 |
| Floor | 0.18 |
| Roof | 0.15 |
| Windows | 1.4 |
| Rooflights | 2.2 |
| Doors | 1.4 |
Building Regs U-Value FAQs
What u-value do I need for building regs?
For a new dwelling the limiting (maximum) u-values are 0.26 for walls, 0.16 for roofs, 0.18 for floors, 1.6 for windows and doors and 2.2 for rooflights. In practice you should design to the lower notional values – 0.18 walls, 0.11 roofs, 0.13 floors and 1.2 windows – because SAP compares your whole dwelling against a notional building built to those figures.
What is the minimum u-value for a wall under building regulations?
The limiting u-value for an external wall in a new dwelling is 0.26 W/m²K, tightening to 0.18 W/m²K for a wall in an existing dwelling or extension. The notional target for a new-build wall is 0.18 W/m²K.
What is the minimum u-value for a roof or floor?
For a new dwelling the limiting roof u-value is 0.16 W/m²K (notional 0.11) and the limiting floor u-value is 0.18 W/m²K (notional 0.13). In an existing dwelling the roof limit tightens to 0.15 W/m²K.
Are these the latest building regs u-values?
Yes. These figures come from Approved Document L Volume 1, 2021 edition incorporating the 2023 amendments, which is the current standard in England as of 2026. The Future Homes Standard is expected to tighten them further but is not yet in force.
What is the difference between limiting and notional u-values?
The limiting u-value is a hard cap that no element may exceed. The notional u-value is the figure used in the notional dwelling that SAP measures your design against, so it is the value you should actually aim for.
Do extensions have different u-value requirements?
Yes. Extensions don’t use the notional dwelling comparison. They follow the existing-dwelling limiting u-values in Table 4.2 – 0.18 for walls and floors, 0.15 for roofs and 1.4 for windows – and must keep glazing under the area thresholds to avoid a full SAP calculation.
