— reviewed against the regs as they stand.
Quick answer: Part L is the section of the Building Regulations (England) covering the conservation of fuel and power — the energy performance rules for new and existing buildings. As of August 2026 the edition in force is Part L 2021 (SAP 10); the Future Homes Standard edition applies to new schemes from 24 March 2027. Compliance for a new dwelling is demonstrated by a SAP calculation passing four tests at once.
Part L for new dwellings
For new build dwellings, Part L sets the energy efficiency requirements that developers, builders and designers must meet, demonstrated at both design stage and as-built. The core compliance target is CO₂-based: the Dwelling Emission Rate (DER) must be equal to or lower than the Target Emission Rate (TER) of a notional dwelling, alongside a primary energy target (DPER vs TPER, introduced in Part L 2021 / SAP 10).
SAP calculations are the approved methodology for showing compliance — pass the SAP and you’ve demonstrated the dwelling is designed and built to standard.
The four things a new-build SAP actually checks
It’s easy to think of Part L as “the carbon test,” but a new dwelling has to clear four separate hurdles at once under SAP 10:
| Test | Metric | What it means |
|---|---|---|
| Primary energy | DPER ≤ TPER | The headline target in Part L 2021 — total primary energy demand. |
| Carbon | DER ≤ TER | CO₂ emissions per m² per year. |
| Fabric efficiency | DFEE ≤ TFEE | Fabric-only heat demand — can’t be bought back with renewables. |
| Limiting standards | Backstop values | Individual U-values, air tightness and fixed building services must each sit within their limits. |
Miss any one and the dwelling fails, so a good design balances all four from the start rather than chasing the carbon number and tripping over the fabric or a limiting U-value. The notional dwelling that sets these targets is demanding under SAP 10 — it even assumes solar PV is installed — which is why fabric improvements alone rarely get a modern dwelling through without PV or a heat pump.
Part L for existing dwellings
Work to existing homes — extensions and conversions — falls under the requirements for existing buildings (formerly Approved Document L1B). This covers extensions, ‘material change of use’ (barn and garage conversions), and changes to a building’s energy status, such as an unheated space becoming heated.
New thermal elements must meet minimum u-value standards, and conversions must show the converted dwelling performs no worse than if the existing building had simply been improved. You can check a wall, roof or floor build-up against the limiting values with the free U-value calculator before committing to a specification.
Over-glazed extensions
Where an extension’s glazing (all openings: windows, doors, rooflights, lanterns) exceeds 25% of its floor area, u-value compliance alone isn’t enough — notional vs proposed SAP calculations are required to show ‘reasonable provision’. The whole-dwelling calculation route gives useful design flexibility, since improvements to the existing house can offset the extra glazing.
Which edition of Part L applies to my project?
This is where most confusion lives, because the paperwork still refers to old document names. The picture in England right now:
- Part L 2013 — introduced fabric energy efficiency; now revoked, with its transitional arrangements withdrawn.
- Part L 2021 — the edition in force today, assessed with SAP 10, applying to work from 15 June 2022. It targets a 31% CO₂ improvement over 2013 as a stepping stone to net zero.
- Approved Document L 2026 — the Future Homes Standard, published 24 March 2026 and in force 24 March 2027.
On the document structure: since the 2021 edition, Part L is split into Volume 1: Dwellings (covering new and existing homes — the old L1A and L1B combined) and Volume 2: Buildings other than dwellings. So if a consultant mentions “L1A,” they mean the new-dwelling half of Volume 1. Which volume you are under decides which calculation you need: Volume 1 work is assessed with SAP (see our residential compliance services), Volume 2 work with SBEM (see our commercial services).
What changes under the Future Homes Standard (2026)
The biggest shake-up to Part L in over a decade lands with Approved Document L 2026. The headlines for dwellings:
- In force 24 March 2027, with a transitional buffer — projects notified before that date can build to current standards if work starts before 24 March 2028.
- Roughly a 75% cut in carbon emissions versus a 2013 baseline home — effectively the end of the road for new-build gas boilers, with heat pumps the default.
- A new functional requirement (L3) making on-site renewable electricity (solar PV) a requirement for new dwellings.
- SAP 10.3 is the only compliance methodology at launch; the Home Energy Model (HEM) follows later and eventually replaces it.
Tighter fabric makes thermal bridging matter more, not less — as walls and roofs improve, the heat lost through poorly-detailed junctions becomes a bigger share of the total. Designing now? The SAP 10 decisions — good fabric, PV, low-carbon heating, calculated Ψ-values — are exactly the ones the Future Homes Standard will require.
In Scotland
The equivalent requirements sit in Standard 6.1 of the Scottish Building Standards.
The bottom line
Part L is really four tests dressed as one, and the target moves with your dwelling’s shape — so the earlier you model it, the cheaper compliance is. Whether you’re on Part L 2021 today or planning ahead to the Future Homes Standard, the fundamentals are the same: good fabric first, then renewables. Unsure which requirements apply to your project? Ask us — it’s quicker than reading the Approved Documents — or get your SAP calculations under way.
Common questions
Which version of Part L applies to my project?
Part L 2021 (assessed with SAP 10) is the edition in force in England today, applying to work from 15 June 2022. Approved Document L 2026 — the Future Homes Standard — comes into force on 24 March 2027, with transitional arrangements for projects notified before then. The earlier 2013 standards have been revoked.
What's the difference between Part L1A and L1B?
They're the old Approved Document names: L1A covered new dwellings and L1B covered work to existing dwellings. Since the 2021 edition both sit inside 'Approved Document L, Volume 1: Dwellings', which covers new and existing homes together; Volume 2 covers buildings other than dwellings.
Do I need a SAP calculation for an extension?
Not always. A modest extension can often comply on element U-values alone. But where glazing exceeds 25% of the extension's floor area — or you want design flexibility — notional-versus-proposed SAP calculations are required to show 'reasonable provision'.
When does the Future Homes Standard come into force?
Approved Document L 2026 was published on 24 March 2026 and comes into force on 24 March 2027. SAP 10.3 is the compliance methodology at launch, with the Home Energy Model following later. Projects notified before 24 March 2027 can build to current standards if work starts before 24 March 2028.