Quick Answer
Part L Building Regulations were updated in June 2022, cutting carbon emissions from new homes by 31% compared to 2013 standards. The key changes are tighter U-values (walls 0.18, roofs 0.11, floors 0.13 W/(m²·K)), a switch from SAP 2012 to SAP 10, mandatory BREL reports at design and completion stages, and stricter air permeability targets. Extensions and refurbishments are affected too. The Future Homes Standard in 2026 will tighten things further still.
Table of Contents
What Part L Actually Is
Part L is the energy efficiency section of the UK Building Regulations. It covers conservation of fuel and power in buildings, which in practice means insulation thickness, window performance, heating system efficiency, and how airtight a building is.
There are four approved documents under Part L:
- L1A: New dwellings (houses, flats)
- L1B: Existing dwellings (extensions, refurbishments)
- L2A: New non-domestic buildings (commercial, industrial)
- L2B: Existing non-domestic buildings
Most tradespeople are primarily concerned with L1A and L1B. L1A applies to new-build homes; L1B applies to any extension, conversion, or change of heating system in an existing property.
The regulations set two types of U-value targets. The notional U-value is what the SAP calculation uses as its reference standard for a compliant building. The limiting U-value is the worst a single element can perform; no individual element can exceed this threshold, even if the rest of the building over-compensates elsewhere.
Who Does Part L Apply To?
If you build new homes, you deal with L1A. If you do extensions, loft conversions, or replace heating systems, you deal with L1B. Roofers, bricklayers, insulation contractors, electricians wiring new builds, and heating engineers replacing boilers all need to understand Part L. It affects every trade on a new build or refurbishment project.
The 2022/2023 Changes in Plain English
The updated Part L came into force on 15 June 2022. There was a transitional period for projects with building regulation applications already submitted. Those had until November 2023 to complete under the old rules. By late 2023, all new projects in England had to comply with the updated standards.
Here is what actually changed:
Tighter U-values
The notional and limiting U-values were significantly tightened. If you are building or specifying insulation, these are the numbers that matter:
| Building Element | Old Notional (2013) | New Notional (2022) | New Limiting |
|---|---|---|---|
| External Walls | 0.30 W/(m²·K) | 0.18 W/(m²·K) | 0.26 W/(m²·K) |
| Roofs | 0.20 W/(m²·K) | 0.11 W/(m²·K) | 0.16 W/(m²·K) |
| Ground Floors | 0.25 W/(m²·K) | 0.13 W/(m²·K) | 0.18 W/(m²·K) |
| Windows & Doors | 2.0 W/(m²·K) | 1.2 W/(m²·K) | 1.6 W/(m²·K) |
In practice, hitting 0.18 for walls means moving from a 100mm cavity filled with mineral wool to either a 100mm PIR-filled cavity (e.g. Kingspan K8 or Recticel Eurothane) or a 150mm mineral wool cavity (e.g. Dritherm 32). Some builders are switching to full-fill PIR, while others are extending cavity widths. There is no single right answer. It depends on the design and the structural constraints.
The Switch to SAP 10
SAP 2012 (Standard Assessment Procedure) has been replaced by SAP 10 as the calculation methodology. SAP 10 uses updated carbon factors for grid electricity, which now reflects a cleaner national grid. This means electric heating and heat pumps look better in SAP 10 than they did in SAP 2012, while gas boilers look worse. If you are specifying heating systems, this shift matters.
Stricter Air Permeability
The maximum permitted air permeability for new dwellings remains at 10 m³/(h·m²) at 50 Pa as a hard limit, but the notional building target in SAP 10 is now 8 m³/(h·m²), and in practice most SAP assessors are targeting 5 to stay comfortably within compliance. This means proper attention to membrane taping, window installation details, and service penetrations. These were often the weak points on older builds.
Mandatory Air Pressure Testing
Pressure testing is now required for the majority of new dwellings. Previously, small developments could opt out. Under the 2022 rules, air pressure tests are required unless the developer can demonstrate through a robust quality-management process (and by achieving a rate of 5 or better on tested dwellings) that untested dwellings will comply.
How It Changes Your Quotes
The 2022 changes have direct cost implications for builders, extension contractors, and anyone quoting structural envelope work. Here is what to factor in:
More Insulation, Thicker Walls
A standard 100mm mineral wool cavity fill no longer hits the notional U-value for walls. To achieve 0.18 W/(m²·K) with a standard 100mm cavity, you need PIR insulation (thermal conductivity around 0.022 W/mK) rather than standard mineral wool. PIR costs more per board than mineral wool. Budget roughly 30 to 40% extra on the insulation material cost for cavity walls.
Alternatively, some projects widen the cavity to 150mm and use Dritherm 32 or similar. That requires wider wall ties and adjusted brick courses, adding labour time and coordination on site.
Floor Space Reduction
Thicker wall constructions eat into floor area. Research from the Federation of Master Builders estimated that the average new extension loses around 1.8 square metres of usable floor space due to the increased wall thickness needed under the 2022 rules. For a small single-storey kitchen extension, that is not trivial. Mention this when scoping work with clients, as they often have a room size in mind based on the external footprint.
SAP Calculations Are Now Standard
For new dwellings and some larger extensions, a SAP assessment is required before building regulations approval. SAP assessors typically charge between £250 and £600 per dwelling for a standard assessment, or more for complex projects. If your client needs this and has not budgeted for it, the job stalls. Add SAP costs to your pre-contract checklist and factor them into your quote breakdown.
Watch Out for Non-Compliant Specifications
Architects and designers sometimes issue drawings with insulation specs that worked under the 2013 rules. If you are building to those specs, the job will fail building regulations sign-off. Check U-value calculations before pricing. If the spec looks thin, query it in writing before you start, not after the walls are up.
Where to Win on Quote Accuracy
Builders who understand the new U-value requirements can price more accurately and avoid abortive work. Use AI tools to cross-check specifications: upload a drawing or job description and ask whether the insulation specification is likely to hit the Part L notional U-values. It takes two minutes and can flag problems before you commit to a price.
SAP Calculations and BREL Reports
Two new documentation requirements came in with the 2022 update that affect how work is signed off.
SAP 10 Calculations
For new dwellings, a SAP 10 assessment is required to demonstrate compliance with the Target Emissions Rate (TER) and Target Primary Energy Rate (TPER). The assessment is done by an accredited SAP assessor. You submit a design-stage SAP at planning/building regs application, and an as-built SAP once the property is complete (for the EPC). The as-built SAP uses actual measurements from the pressure test and installed specifications.
SAP assessors need accurate specifications: insulation lambda values, glazing specifications, heating system efficiency, and the pressure test result. Give them vague information and you get a vague (and often wrong) SAP result.
BREL Reports
Building Regulations England Part L (BREL) reports are now required for new dwellings in England. These replace the older Compliance Report. There are two stages:
- Design-stage BREL: Submitted with the building regulations application. Shows the intended specification meets the notional building requirements.
- As-built BREL: Submitted on completion. Confirms the building as actually constructed meets the requirements. Must include actual air permeability test results and evidence of low-energy lighting compliance.
Building control inspectors increasingly ask to see BREL reports as part of their sign-off process. If you are a main contractor managing the building control process, make sure your SAP assessor knows both reports are needed and what evidence they require from you (specification sheets, pressure test certificate, lighting schedules).
SAP for Extensions
Extensions do not always need a full SAP calculation, but they do need to meet limiting U-values under L1B. The simplified approach in L1B allows extensions to comply by meeting elemental U-value standards rather than running a SAP calculation. However, if the extension is large relative to the existing dwelling, or if it significantly changes the heating demand, a SAP may still be required. Check with building control early.
The Future Homes Standard Is Coming
The 2022 Part L update was described by the government as an "interim" step. The destination is the Future Homes Standard, which is expected to require new homes to produce 75 to 80% fewer carbon emissions than a 2013 new build. Draft legislation was consulted on in 2023, with final regulations due to be published in December 2025 and coming into effect from December 2026.
What does this mean in practice? The Future Homes Standard will almost certainly require:
- Heat pumps or other low-carbon heating as the primary heat source (gas boilers will not comply in new homes)
- Even thicker fabric insulation or higher-performing systems to compensate for the lower heating output of heat pumps
- Mechanical ventilation with heat recovery (MVHR) in most new-build homes
- Solar PV panels as standard or near-standard on new homes
Gas Boilers Out of New Builds by 2026
Under the Future Homes Standard, new homes will not be able to use gas boilers as the primary heating system. For heating engineers, this is a significant shift; the new-build market will move almost entirely to heat pumps. If you are working in new-build heating, upskilling in heat pump installation and commissioning is not optional, it is essential. MCS accreditation for heat pumps will become far more valuable than it already is.
For builders and extension contractors, the good news is that the Future Homes Standard applies to new dwellings, not existing ones. Extension work under L1B will still be governed by the limiting U-value approach, though those limits are likely to tighten as well. Watch for updated Approved Documents when the legislation is finalised.
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What Tradespeople Are Saying
Frequently Asked Questions
Yes. Extensions and conversions are covered by Approved Document L1B (Conservation of Fuel and Power in Existing Dwellings). The requirements are less stringent than for new builds, but limiting U-values still apply. Any extension must meet the limiting U-values: walls no worse than 0.28 W/(m²·K), roofs 0.18, floors 0.22. New windows in an extension must achieve at least 1.4 W/(m²·K) for the whole window unit. Doors (if more than 60% glazed) must achieve 1.4; less-glazed doors achieve 1.8.
SAP (Standard Assessment Procedure) is the government-approved method for calculating the energy and environmental performance of dwellings. Under Part L 2022, a SAP 10 assessment is required for all new dwellings. The assessor produces a design-stage SAP for the building regulations application, and an as-built SAP for the Energy Performance Certificate (EPC) on completion. SAP assessments are not usually required for simple extensions unless the project significantly affects the whole-building heat loss. They typically cost between £250 and £600 per dwelling.
A BREL (Building Regulations England Part L) report is a compliance document submitted to building control demonstrating that a new dwelling meets the Part L energy efficiency requirements. Two reports are required: a design-stage report (submitted with the building regulations application) and an as-built report (submitted on completion, including the actual air permeability test result). The BREL report is produced by an accredited SAP assessor. As the main contractor or builder, you are responsible for giving the assessor accurate as-built information: the actual insulation products used, pressure test results, and installed heating and lighting specifications.
If a dwelling fails the pressure test (result worse than 10 m³/(h·m²) at 50 Pa), it cannot receive building regulations completion and cannot be occupied. The testers will often provide a report identifying where leakage is occurring. Common culprits are around window frames, at floor/wall junctions, around service penetrations (pipes, cables), and at loft hatches. These areas need to be remediated and re-tested. Prevention is far better: tape membrane joints and service penetrations during construction rather than trying to fix them after plasterboarding. Some testing companies offer mid-construction visits to identify problems early.
The Future Homes Standard is expected to come into effect from December 2026 (final legislation was due to be published in late 2025). It will require new homes to produce 75 to 80% fewer carbon emissions than a 2013 new build. In practice this means the end of gas boilers in new-build homes. Heat pumps or other low-carbon heating will be required. Fabric standards will tighten further and mechanical ventilation with heat recovery (MVHR) will become standard in most new builds. Extensions and existing homes are not directly covered by the Future Homes Standard, though related updates to L1B are expected.
