The Ultimate Guide to Part L Compliance

A plain English, practical guide to Part L of the UK Building Regulations for builders, site managers, and assessors

Part L Compliance Pillar Guide

Last updated 13 April 2026 · 15 min read

Part L is the section of the UK Building Regulations that sets the rules for energy efficiency in buildings. If you're building or altering a home in England or Wales, it affects you. If you're an assessor signing off new builds, it's most of what you do. And with the Future Homes Standard transition live through 2025 and 2026, the bar is moving up.

This guide is everything a builder or assessor needs to know about Part L in one place. Plain English. No waffle. Links out to deeper explanations where you need them.

What Part L actually is

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 how that energy performance has to be proved on site. In practice, that means three things:

  1. Calculations - a qualified assessor models the expected energy performance before and after construction
  2. Evidence - builders capture photographs at specified construction stages to prove what's hidden inside the walls, floors, and roof
  3. Sign-off - Building Control signs off the dwelling as compliant before it can be handed over or occupied

All three are equally important. A perfect design with bad site evidence still fails. A perfect site with no calculation also fails. Part L is about both the design and what actually got built.

The four parts of Part L

Part L is set out in what's called Approved Document L, and it splits into four parts depending on what you're building and whether it's new or existing:

DocumentCoversTypical users
Approved Document L1ANew dwellingsSME housebuilders, self-builders, developers
Approved Document L1BWork to existing dwellings (extensions, renovations, thermal-element replacements)Renovators, extension builders, retrofit specialists
Approved Document L2ANew non-dwellings (offices, shops, industrial)Commercial builders, developers
Approved Document L2BWork to existing non-dwellingsCommercial fit-out, refurbishment contractors

Each part covers broadly the same things - fabric, heating, ventilation, lighting, renewables - but the target U-values, system efficiencies, and evidence requirements differ. Most new-build housing work happens under L1A. Most domestic renovation work happens under L1B.

This guide focuses on L1A (new dwellings), because that's where the heaviest evidence requirements land and where builders most often get tripped up.

What Part L covers on site

For a new dwelling, Part L covers six core areas:

  • Fabric - walls, floors, roofs, windows, doors. The insulation quality and thermal-bridging details that determine how much heat escapes.
  • Heating and hot water - the boiler or heat pump, hot water cylinder, system efficiency, and the controls that operate them.
  • Ventilation - natural, mechanical, or mechanical with heat recovery (MVHR). Under the Future Homes Standard, MVHR becomes more common because homes are airtight enough to need it.
  • Lighting - the proportion of low-energy fittings installed.
  • Renewables and low-carbon technologies - solar PV, solar thermal, heat pumps, heat networks.
  • Airtightness - verified by a pressure test on the finished dwelling.

Every one of these areas needs evidence. Most of that evidence is photographic, captured as each stage of construction gets buried or covered up. The rest is commissioning certificates, test results, and product data sheets.

The seven-step Part L compliance process

Here's what Part L compliance actually looks like from start to finish for a new dwelling.

  1. Step 1 - Design-stage SAP calculation. Before construction starts, an OCDEA runs the SAP calculation on the design. This shows the dwelling should meet Part L based on its specified fabric, heating, and renewables. The output is submitted to Building Control with the Part L application.
  2. Step 2 - Pre-construction planning. The builder, assessor, and site manager agree what photographic evidence is needed, by plot and by Appendix B section. The assessor may supply a specific list. This is also when you set up any evidence-capture tools - spreadsheets, shared drives, or a purpose-built app like Buildsnpper.
  3. Step 3 - Capture evidence at each stage. As construction progresses, builders photograph each Appendix B section before it's covered up. Foundations first, then cavity-wall insulation, floor insulation, roof insulation, glazing, airtightness membranes, heating and ventilation plant, renewables. Each photo needs GPS coordinates, a date, and a timestamp.
  4. Step 4 - Flag changes from the design. If the specification changes mid-build - different boiler, different insulation product, window changes - notify the OCDEA. They update the SAP model so the as-built calculation reflects what actually got installed.
  5. Step 5 - Airtightness test. Once the dwelling is weather-tight and internal finishes are substantially complete, a pressure test measures how airtight the building is. The test result feeds the as-built SAP calculation.
  6. Step 6 - As-built SAP and BREL report. The OCDEA updates the SAP calculation with what was actually installed and produces the BREL report. This bundles the design SAP, as-built SAP, photographic evidence, commissioning certificates, and airtightness results into one document.
  7. Step 7 - Building Control sign-off and EPC. Building Control reviews the BREL and signs off Part L compliance. The OCDEA lodges the Energy Performance Certificate (EPC) on the Energy Performance of Buildings Register. Handover can complete.

Every step depends on the previous one. Skipping or weakening any of them delays handover and adds cost.

Buildsnpper handles Steps 2 through 6 from the builder's side. Section-by-section capture mapped to Appendix B, GPS and timestamps automatic on every photo, one-tap report straight to the assessor. From £6.67 per month, 14-day free trial. See how it works.

The Appendix B photographic evidence requirement

Appendix B of SAP is where the photographic evidence requirements live. It splits the evidence into sections that match construction sequence. Different assessors and accreditation bodies group them slightly differently, but the sections you'll photograph are:

  1. Foundations, DPC and ground-floor insulation. Photographs taken before the slab or suspended floor is cast or covered. Must show the insulation continuity, edge detail, and damp-proof course.
  2. External walls and cavity-wall insulation. Photographs taken while cavities are still open. Must show the insulation continuity, any cavity closers at openings, and the wall-to-roof and wall-to-floor junctions.
  3. Floor and floor insulation. Photographs taken before the screed or final flooring covers the insulation. Must show continuity, thickness, and any edge details.
  4. Roof and loft insulation. Photographs taken before the final ceiling or roof covering closes the insulation in. Must show the insulation depth, continuity at joists and rafters, and ventilation gaps where required.
  5. Windows, doors, and glazing. Photographs showing installation, insulation around the reveal, and any sealing tapes or membranes.
  6. Airtightness measures. Photographs of the airtightness barrier - membranes, tapes at service penetrations, sealed joints around boxes and sockets, and any specific details needed to hit the design air permeability.
  7. Heating, hot water, and ventilation systems. Photographs of the plant installation - boiler or heat pump, hot water cylinder, mechanical ventilation or MVHR unit, controls. Plus commissioning certificates that typically sit alongside the photos.
  8. Renewable and low-carbon technologies. Photographs of any solar PV, solar thermal, heat pump, or heat-network installation. Plus commissioning and warranty paperwork.

What makes a photo count as valid evidence

  • Clear subject. Sharp focus, decent light, the construction feature centred. Blurry wide shots don't cut it.
  • GPS coordinates embedded. The metadata proves the photo was taken on the actual plot.
  • Date and timestamp. Proves the photo was taken at the relevant construction stage, not reconstructed later from a phone gallery.
  • Linked to the correct plot. On a multi-plot site, photos have to stay separated.
  • Filed against the correct Appendix B section. A DPC photo filed under roof insulation creates work for the assessor.

Who does what: DEA, OCDEA, and Building Control

Three different roles sit around Part L compliance. They get confused because the acronyms look similar.

DEA (Domestic Energy Assessor)

Handles existing dwellings only. Visits a property that's being sold or rented and produces an EPC using RdSAP (a simplified version of SAP). Not qualified to sign off new builds.

OCDEA (On Construction Domestic Energy Assessor)

Handles new-build dwellings. Runs design-stage and as-built SAP, reviews the Appendix B photographic evidence, and produces the BREL report that signs off Part L. Every OCDEA is also a DEA; not every DEA is an OCDEA.

Building Control

Either the local authority Building Control body or a private Approved Inspector. Reviews the OCDEA's BREL report and signs off Part L compliance. Handover can't complete without their sign-off. Building Control doesn't review the photographic evidence itself - that's the OCDEA's job - but they rely on the BREL that summarises it.

For a new-build dwelling, you need an OCDEA. Not a plain DEA. Check accreditation on the public registers held by Elmhurst Energy, Stroma Certification, Sava, Quidos, or ECMK before a project starts.

The Future Homes Standard transition

The Future Homes Standard (FHS) is the UK Government's plan to make new homes zero-carbon-ready. It's being rolled out in phases through 2025 and 2026 as a package of updates to Part L and Part F. The big shifts:

  • No new gas boilers in new builds. Once the full standard is in force, heat pumps or heat networks become the default. Gas is out for new dwellings.
  • Higher fabric standards. Lower U-values on walls, roofs, floors, and windows. More insulation, better thermal bridging.
  • Tighter airtightness. Lower air permeability rates. Better membrane and tape work on site.
  • MVHR becomes typical. As airtightness improves, mechanical ventilation with heat recovery becomes the sensible choice for maintaining indoor air quality.
  • More scrutiny of the Performance Gap. The Performance Gap is the difference between a home's designed and as-built energy performance. FHS pushes assessors to check more carefully that the as-built evidence actually matches the design.

For builders, FHS means more photographic evidence per plot, more detailed evidence on things like thermal-bridging junctions, and stricter review by OCDEAs. The evidence bar is rising at the same time as the construction standards. A plot that scraped through Part L 2022 with patchy photos won't scrape through under FHS.

For assessors, FHS means more photos per dwelling to review, greater Building Control scrutiny of weak evidence, and harder conversations with builder clients whose evidence packages are thin.

Common Part L mistakes

Things builders and assessors hit most often:

  • Photos taken with location services off. No GPS means the photo can't be verified. Can't be added later.
  • Photos imported from the phone gallery. Timestamps and location data often stripped or unreliable. If metadata is critical, take the photo through a dedicated app.
  • Missing a stage. Missing the cavity-wall insulation before close-up, or the airtightness membrane under plasterboard, can mean opening the work back up. Or, more commonly, a conservative as-built SAP and a worse EPC band.
  • Multiple plots mixed together. A single zip with 300 photos across five plots. The assessor has to sort it. They charge for the time.
  • No link between the photo and the stage. A photo with no context is just a photo. The assessor needs to know what they're looking at.
  • Using a DEA instead of an OCDEA. A DEA can't sign off Part L on a new build. The site has to have an OCDEA-accredited assessor from the start.
  • Ignoring the Performance Gap. Evidence that's clearly different from the design (say, a cheaper boiler model than specified) has to be flagged to the assessor so the as-built SAP reflects reality. Otherwise the BREL isn't valid.
  • Waiting until the end. Trying to retrofit evidence after handover is the most expensive way to do Part L. Capture as you go, not at the end.

Tools and technology

You can run Part L compliance with a spreadsheet, a phone, and a shared drive. People do. But the failure modes listed above are common enough that purpose-built tools now exist.

Any tool worth using for Part L evidence should do at least this much:

  • Prompt you through each Appendix B section in order
  • Capture GPS, date, and timestamp on every photo automatically
  • Keep plots separated
  • Generate a report in a format your assessor can use
  • Work offline on construction sites
  • Be cheap enough to roll out across every device on site

Buildsnpper does all six. One licence per device, £6.67 per month on the annual plan. One-tap report generation. Free assessor portal on the other end.

The tool matters less than the discipline. Whatever you use, use it consistently from day one of the project.

Frequently asked questions

Is Part L different in Scotland, Wales, and Northern Ireland?

Yes. The overall approach is similar but each nation has its own regulations. Scotland uses Section 6 of the Scottish Building Regulations. Wales has its own version of Part L aligned with England but separately published. Northern Ireland uses Part F of its own regulations. This guide focuses on England, where Part L of the Building Regulations applies directly.

How often does Part L change?

Part L has been updated in 2022 and is being updated again through the Future Homes Standard transition in 2025 and 2026. Expect further updates as the UK moves toward net zero by 2050. The SAP methodology version used for calculations also updates periodically.

Do I need Part L for a garden office or garden room?

Possibly. If the structure is heated, permanently occupied, and above a certain size, it can fall under Part L. Small outbuildings often don't. Your Building Control body or Approved Inspector will confirm what applies.

Who pays for the OCDEA?

The builder or developer typically pays. The cost is usually built into the project cost and passed on to the eventual purchaser. On a self-build, the self-builder pays directly.

Can Building Control reject my Part L submission?

Yes. If the BREL isn't consistent, the evidence is weak, or the as-built doesn't match the design, Building Control can refuse sign-off. In practice the OCDEA flags problems before the BREL is submitted, so outright rejection at Building Control is rare - but delays while evidence gets reworked are common.

Is Buildsnpper approved by Building Control or accreditation bodies?

Building Control and accreditation bodies don't formally approve individual tools. They approve the evidence - photos with GPS, dates, timestamps, mapped to Appendix B and assessed by an OCDEA. Buildsnpper's report format was designed in collaboration with BREL assessors to meet those requirements, and reports generated through it are accepted by Building Control bodies and OCDEAs across the UK. We recommend showing your assessor a sample report at the start of a project.

Glossary definitions

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