What is Part L?

The energy efficiency section of the UK Building Regulations

Part L is the energy efficiency section of the UK Building Regulations. It sets how much energy a new or altered building is allowed to use, and it says how that energy performance has to be proved on site.

If you're building or renovating a home in England or Wales, Part L applies to you. So does its evidence trail - a mix of calculations, commissioning certificates, and photographs taken at the right stages of construction.

What Part L actually covers

Part L of the Building Regulations is set out in Approved Document L. It splits into four parts depending on what you're building and whether it's new or existing:

  • Approved Document L1A - new dwellings
  • Approved Document L1B - work to existing dwellings (extensions, renovations, thermal-element replacements)
  • Approved Document L2A - new buildings other than dwellings
  • Approved Document L2B - work to existing non-dwellings

Each covers the same core things: fabric energy efficiency (walls, floors, roof, windows), heating and hot water systems, ventilation, lighting, and low-carbon technologies. The target U-values and system efficiencies differ depending on which document applies.

Who needs to comply with Part L

The short answer: pretty much anyone doing building work in England or Wales, plus in Scotland and Northern Ireland under their own regional equivalents. Specifically, Part L applies to:

  • All new-build dwellings
  • Extensions above certain thresholds
  • Major renovations
  • Change-of-use conversions
  • Replacement of thermal elements (new windows, re-roofs, new insulation)

Your building control body - either the local authority or a private Approved Inspector - signs off Part L compliance before the building can be handed over or occupied.

What evidence you need

Two things:

  1. Calculations. A qualified assessor runs the SAP (Standard Assessment Procedure) calculation for dwellings, or SBEM for non-dwellings, and produces a compliance report.
  2. Photographic evidence. The builder takes photos at specified construction stages to prove what's hidden in the walls, under the floor, and in the roof was actually built to the design. For dwellings, this evidence is set out in Appendix B of SAP.

The photos need GPS coordinates, a date and time, and a clear link to the plot and the construction stage they're supposed to cover. Zip files of hundreds of un-labelled JPEGs don't cut it any more. An OCDEA reviewing the evidence will ask for missing or unclear photos, and until they're satisfied the BREL report won't be signed off.

Buildsnpper walks you through Appendix B section by section on your phone. Every photo gets GPS and a timestamp automatically. Generate a tidy report in one tap and send it straight to your assessor. From £6.67 per month, 14-day free trial. See how it works.

Part L and the Future Homes Standard

Part L isn't static. It gets tightened every few years as the UK moves towards net zero. The biggest shift is the Future Homes Standard, which is rolling out through 2025 and 2026. In practice this means:

  • Higher fabric energy efficiency standards
  • No new gas boilers in new-build dwellings by the end of the transition
  • Low-carbon heating (typically heat pumps) as the default
  • Tighter airtightness expectations
  • More emphasis on the Performance Gap - the difference between designed and as-built performance - which means more scrutiny of your Appendix B photos

Frequently asked questions

What is Part L of the Building Regulations?

Part L is the energy efficiency section of the UK Building Regulations. It sets how much energy a building is allowed to use and how that has to be proved on site through calculations and photographic evidence.

Who needs to comply with Part L?

Anyone building a new home, extending or renovating an existing one, or replacing thermal elements like windows, insulation, or roof coverings in England or Wales. Scotland and Northern Ireland have equivalent sections in their own regulations.

What's the difference between L1A, L1B, L2A and L2B?

L1A covers new dwellings. L1B covers work to existing dwellings. L2A covers new non-dwellings. L2B covers work to existing non-dwellings. The target values and evidence requirements differ between them.

What evidence does Part L require?

Two things: a SAP or SBEM calculation run by an accredited assessor, and photographic evidence from site showing how the building was actually constructed. For new dwellings the photo requirements are set out in Appendix B of SAP.

Is Part L changing under the Future Homes Standard?

Yes. The Future Homes Standard is the UK Government's plan to make new homes zero-carbon-ready. It tightens Part L's fabric and systems standards, phases out new gas boilers in new builds, and raises the expectations on photographic evidence during the 2025-2026 transition.

What happens if I fail Part L compliance?

Your building control body won't sign off the building, which blocks handover and occupation. In practice, the OCDEA assessor will flag missing or inadequate evidence before sign-off so you can fix it - but delays and extra site visits cost time and money.

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