Trades master copy
Last reviewed: March 2026
What Part L is
Part L is the energy-efficiency bit of Building Regulations. For dwellings it covers insulation levels, glazing performance, airtightness, heating and hot water efficiency, plus the SAP / EPC numbers in the background.
The aim is: new homes and major works should use less energy and produce less carbon, by hitting minimum fabric standards (U-values), limiting air leakage, and using reasonably efficient heating and controls.
This guide is a summary to make Part L easier to use on site. It does NOT replace Approved Document L - Volume 1: Dwellings (current edition) or the SAP and air-test guidance.
You must read and follow the full Approved Document L and your project's SAP reports for detailed requirements.
This guide is written for England. Wales, Scotland and Northern Ireland have their own versions of building regulations - the principles are similar but the documents and approval routes differ, so check local requirements if you're working outside England.
Where it applies on your jobs
Part L touches almost every domestic job you do:
- New builds - houses and flats: full SAP, U-values, thermal bridge details, airtightness test, efficient heating, often with some renewables.
- Extensions - anything with new walls/roof/floor and significant glazing: you must meet limiting U-values and overall energy targets for the extension.
- Loft conversions - upgraded roof insulation, possibly wall/floor upgrades, and efficient heating/lighting to the new space.
- Window and door replacements - even in existing houses, replacement windows/doors must hit current Part L performance standards.
- Renovations to thermal elements - re-rendering, re-roofing, dry-lining external walls often triggers minimum insulation upgrades.
Volume 1 is for dwellings; non-domestic is Volume 2. Always check which one your job falls under if it's a mix (e.g. flat over a shop).
Key "trigger points" - what gets checked
These are the moments where Part L quietly becomes a big deal:
New external elements (walls, roofs, floors)
- New build and extensions must meet or beat the limiting U-values in AD L (for the 2021 regs uplifted in June 2022).
- Poor insulation or thermal bridging can blow the SAP numbers even if you pass on paper U-values.
Glazing and door replacements
- Replacement windows and doors must meet the limiting U-values in current AD L - typically 1.4 W/m²K maximum for replacement windows/doors in existing dwellings; new-build SAP/notional specs often assume around 1.2 W/m²K or better for windows in new dwellings.
- Excess glazing in an extension is limited unless you compensate elsewhere in the design.
- If you're also doing Part F work (background vents), you may need trickle vents sized to the newer standards.
Airtightness
- New homes must be air-tested - you're typically aiming for around 5 m³/(h·m²) at 50 Pa or better in practice to keep SAP happy (exact target comes from the design).
- Gappy work around windows, service penetrations and loft hatches can sink the air test and blow compliance.
Heating, hot water and controls
- Boilers, heat pumps, cylinders and controls must meet minimum efficiency and control standards in AD L.
- Extending old, inefficient systems into big new areas can be a problem - the SAP assessor may require upgrades.
SAP calculations and the "notional" dwelling
- For new dwellings the SAP model compares your design against a notional building - you must meet or beat targets for primary energy, emissions and fabric energy efficiency.
- For extensions/renovations, you still have to meet element-by-element limits and sometimes show overall performance via SAP/other methods.
Quick reference table - common jobs
Single-storey rear extension
- New walls/roof/floor must meet limiting U-values from AD L.
- Total glazing area usually limited to a % of floor area unless compensated elsewhere.
- Heating and hot water design must be efficient and properly controlled in the new space.
Loft conversion in existing house
- Insulate roof to current standards (warm or cold roof build-up).
- Check if you're triggering upgrades to existing thermal elements (e.g. exposed walls).
- Heating/emitter sizing and controls for the new storey.
Replacing all windows and external doors
- New windows/doors must meet the limiting U-values in current AD L - typically 1.4 W/m²K for replacement windows/doors in existing dwellings; new-build designs often assume better performing glazing.
- If you're also dealing with Part F, you may need trickle vents sized to the new ventilation rules.
New-build house
- Must achieve SAP design targets for primary energy, CO₂ emissions and fabric energy efficiency.
- Air-tightness test result must be at or better than the design assumption.
- All major elements (walls, roof, floor, windows, doors) must meet or beat notional U-values and thermal bridging details.
External wall insulation to existing solid wall
- After upgrade, wall should hit the renovation U-value target in AD L where reasonably practicable.
- Details at reveals, eaves and thresholds must avoid cold bridges and moisture traps.
Routes to compliance for trades
There's no "Part L registration scheme", but there is a fairly standard route for small domestic jobs:
Follow Approved Document L Volume 1 and the project SAP spec
For new builds, you'll normally be given a SAP design summary with target U-values, airtightness, heating spec and any renewables. Build to that, not just to minimums.
For extensions and window replacements, use the limiting U-values and glazing rules in AD L as your baseline.
Coordinate early with the SAP assessor / designer
If the client wants "more glass", "cheaper windows" or "less insulation", get the SAP person to re-run the numbers before promising anything.
Trade-offs are possible (e.g. better roof/wall insulation to allow more glazing), but they need checking in the model.
Build for airtightness, not just insulation thickness
Detail and tape around windows, doors, services and junctions in line with the drawings/spec.
Aim to beat the design air-test figure, not just scrape a pass - a bad test result can kill SAP and require retrofit sealing later.
Use compliant products and details
Insulation boards, cavity batts and membranes must be installed to manufacturer guidance (gaps and slumps hit performance hard).
Windows/doors should come with declared U-values/energy ratings; don't swap spec without checking the numbers.
Also worth having on your radar: the Future Homes Standard is expected to tighten these requirements further for new homes (including moving away from gas boilers) in the next few years - keep an eye on gov.uk and your SAP assessor for when those changes actually land.
If you drift off the design (cheaper glass, thinner insulation, messier airtightness), you can pass Building Control on structure but fail the SAP/Part L sign-off at the end - which is when it hurts most.
Who is responsible for what
On a typical domestic project:
- The designer/SAP assessor sets the strategy - U-values, airtightness target, heating/ventilation spec, renewables, and SAP calculations.
- The builder/main contractor is responsible for actually delivering that spec on site: correct insulation, correct windows, careful airtightness, and not "value-engineering" it to death without new SAP sign-off.
- Specialist trades - window installers, insulators, M&E contractors - are responsible for fitting to spec and providing evidence (product data, commissioning certs, air-test reports).
- The client/owner ends up with the SAP/EPC and Building Control sign-off (or not) - delays and remedial work hit them directly.
Blunt version for smaller builders:
If you're running the job and you swap materials, cut insulation, accept leaky window fitting or ignore the SAP spec because the client "wants it cheaper", that's on you. When the SAP fails or Building Control refuses to sign off, it's your problem to fix.
Simple rule to drum into your team
If a change affects insulation thickness, window/door spec, airtightness details, heating/ventilation kit, or the amount of glass, treat it as a Part L issue first. Check with the designer/SAP assessor or Building Control before you tell the client "yeah, that'll be fine".
On-site checklist (Part L)
Before you start
- Get the latest SAP design summary / Part L spec for the job (U-values, airtightness target, heating/ventilation spec).
- Mark up where each insulation type and thickness goes, and what windows/doors are specified.
- Agree with the client in writing that any spec changes will need checking for Part L/SAP impact.
While you're working
- Install insulation to the full thickness, tight-butted, no big voids or cold bridges at junctions.
- Fit windows and doors with appropriate tapes/sealants as detailed - don't leave big foam gaps behind trim.
- Keep a list of any "on the day" changes (different products, moved openings, dropped insulation) so they can be checked against SAP.
When you finish
- Get the air-test done once the envelope is complete and sealed - not before everything is finished.
- Make sure the SAP assessor has accurate as-built information: actual U-values, air-test result, heating/ventilation kit, controls, any renewables.
- Don't hand the keys over or let the client book the removal van until you know the SAP / Part L sign-off and EPC are on track.
Sources
Based on:
- Approved Document L: Conservation of fuel and power - Volume 1: Dwellings (2021 edition implemented June 2022, plus later updates).
- Government and industry summaries of the 2021/2022 Part L uplift (U-values, SAP and airtightness changes, and Future Homes Standard direction of travel).
- Practical guidance from building control and energy/SAP specialists on domestic extensions, windows and new-build dwellings.
This guide was last reviewed March 2026. SiteKiln does not provide legal, financial or tax advice. All content is for general information purposes only. Always seek professional advice for your specific situation.
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Working in Wales? The building rules are different. See our Working in Wales guides.
Working in Scotland? Building standards work differently. See our Working in Scotland guides.
Working in Northern Ireland? The system uses Technical Booklets. See our Working in Northern Ireland guides.
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