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.
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.
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.