Building Regulations Part L Explained

Updated 20 September 2026 — 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 September 2026 the edition in force is Part L 2021, assessed with SAP 10.2; the Future Homes Standard edition, assessed with SAP 10.3, applies to new schemes from 24 March 2027. A new dwelling complies by passing four SAP tests at once, then proving it as built with photographs, a compliance report and an EPC.

What Part L is

Requirement L1 of Schedule 1 to the Building Regulations 2010 says, in essence, that reasonable provision must be made for the conservation of fuel and power in buildings. Approved Document L is the government’s guidance on how to meet it, and it is where the numbers live. Since the 2021 edition it comes in two volumes: Volume 1: Dwellings, covering new and existing homes, and Volume 2: Buildings other than dwellings. The old names — L1A for new dwellings, L1B for existing ones, L2A and L2B for other buildings — were retired in 2021, although the paperwork and half the industry still use them.

Which volume you are under decides which calculation you need. Volume 1 work is assessed with SAP; Volume 2 work with SBEM.

Which edition applies to my project?

This is where most of the confusion lives. The picture in England:

• Part L 2013 — introduced the fabric energy efficiency target. Fully revoked: its transitional window closed on 15 June 2023.
• Part L 2021 — the edition in force today, applying to work from 15 June 2022. It targets roughly a 31% carbon improvement over 2013 as a stepping stone to net zero. Assessed with SAP 10.2: the Notice of Approval of 24 March 2026 brought in SAP 10.3 for the Future Homes Standard but lets buildings notified before 24 March 2027 (and started before 24 March 2028) stay in the earlier version.
• Approved Document L 2026 — the Future Homes Standard, published 24 March 2026 and in force from 24 March 2027 (24 September 2027 for higher-risk buildings). Projects with a building notice or full plans submitted before 24 March 2027 can build to the 2021 standards if work starts on site by 24 March 2028.

If your scheme straddles a date, the question is when it was notified and when it starts, not when it finishes.

The transitional protection works building by building, not site by site: starting one plot does not protect the rest of the scheme. And “started” has a legal meaning since October 2023 — for a new building it is the point at which the foundations and the structure of the ground floor are complete, not site clearance or the first pour.

Part L for new dwellings: the four tests

It is easy to think of Part L as “the carbon test”. A new dwelling has to clear four hurdles at once:

The targets come from a notional dwelling the same size and shape as yours, built to a reference specification: walls 0.18, floors 0.13, roof 0.11 and windows 1.2 W/m²K; air permeability 5; a gas boiler at 89.5%; waste water heat recovery; and — the one that surprises people — solar PV sized at 40% of the ground floor area divided by 6.5. That is why fabric improvements alone rarely get a modern dwelling through without PV or a heat pump. The limiting values are the other half — nothing worse than a wall at 0.26, a floor at 0.18, a roof at 0.16, windows and doors at 1.6, rooflights at 2.2, or an air permeability of 8 — and a dwelling built merely to those will fail the targets by a distance. The full detail is on the U-values guide.

Proving it: notices, the BREL report and the photographs

The 2021 edition put a process around the calculation, and this is the part people searching for “Part L compliance” usually need.

Before work starts, the regulations require the target rates, the as-designed rates and a list of the specifications the calculation depends on to be notified to Building Control — by the day before work begins — on the design-stage BREL report (Building Regulations England Part L compliance report). Within five days of completion, the as-constructed rates and any changes to that specification list are notified on the completion-stage BREL, signed by the assessor and the builder. The homeowner gets a copy of each.

The completion BREL is accompanied by photographic evidence — new in 2021, and the thing most often missing at the end. Every dwelling needs its own set, showing the insulation continuous and properly installed at six stages: the foundations and ground floor; each external wall type; each roof type; each opening type including the reveals and cavity closers; the airtightness details; and the building services, including pipework insulation. They are normally taken by the builder, need to be date-stamped and located, and are reviewed by the SAP assessor against the specification before the report is signed. There is no way to photograph insulation that has been plastered over, so the site team needs to know on day one. The photographic evidence guide breaks the six stages into thirteen details, says when on the programme to catch each one, and has a free plot checklist to print.

Then the EPC, produced from the as-built SAP by an accredited on-construction assessor and given to the owner within five days of completion, and the home user guide for the occupier. Building Control cannot issue the completion certificate without the EPC.

The other things Part L 2021 asks of a new dwelling

Air tightness. Every new dwelling is tested — sample testing ended in 2022 — and the measured result goes into the as-built SAP.

Lighting. Fixed internal lighting should have lamps of at least 75 lumens per circuit-watt, with local controls per space; fixed external lighting needs daylight-responsive switch-off, and occupancy control where the efficacy is 75 or below. The notional dwelling assumes 80.

Heating design. A design flow temperature of 55°C or lower is what the notional dwelling assumes, which is why radiators are getting bigger and why heat pumps fit the standard so naturally.

Thermal bridging. Junction heat losses are assessed separately from U-values, as Ψ-values, and SAP’s default values are punitive — every Table K1 default alongside the Table R2 notional value shows how punitive, junction by junction. Calculating your actual junctions — which the free ΨMonkey psi value calculator does to BS EN ISO 10211 — is usually the cheapest improvement available; the alternative is to build to Recognised Construction Details, the scheme that replaced Accredited Construction Details under SAP 10.

Overheating is a separate requirement. Since June 2022 a new dwelling also has to pass Part O — glazing limits and free-area rules on the drawings, or TM59 modelling — assessed alongside the SAP, not inside it.

Part L for existing dwellings

Work to existing homes — extensions, conversions, renovations, replacement windows — falls under Volume 1 too, but without the target-rate machinery. New thermal elements must meet the limiting values for new elements in existing dwellings: walls 0.18, floors 0.18, roof 0.15, windows and doors 1.4, rooflights 2.2. Retained elements being renovated must be brought from a threshold to an improved value where that is feasible and pays back within 15 years — RenoMonkey checks an element against both tests free in your browser, and prints the payback justification Building Control asks for. And every window and door in an existing dwelling has to meet 1.4 on its own — they are excluded from area-weighting. Building regulations when you renovate sets out the triggers, the thresholds and the payback test in full.

Extensions comply on those U-values provided the windows, doors and rooflights stay within 25% of the extension’s floor area plus the area of any existing openings the extension covers (check yours here). Exceed it and there are two routes to showing reasonable provision — an area-weighted U-value calculation, which GlazeMonkey runs free against the notional extension, or a whole-dwelling SAP in which improvements to the existing house offset the extra glazing. A conservatory escapes Part L only if it is at ground level, no more than 30 m², thermally separated from the house and not on the house’s heating system. The whole of the extension side, including the credit for openings the extension covers over, is in building regulations for an extension.

Conversions — a material change of use creating a dwelling — bring Part L in full, but with no target emission rate; compliance is element by element, and a SAP is the usual way to show the trade-offs when a retained stone wall cannot reach the improved value.

Part L for other buildings

Volume 2 does the same job for offices, retail, industrial and everything else that is not a dwelling, with SBEM as the calculation, a BRUKL report in place of the BREL, its own limiting standards, and the same design-stage and as-built duties. The commercial EPC is a separate duty on completion. Volume 2’s 2026 edition lands on the same dates as Volume 1’s.

What changes under the Future Homes Standard

The biggest shake-up to Part L in over a decade lands with Approved Document L 2026 — retitled “Energy and greenhouse gas emissions”. For dwellings:

• In force 24 March 2027, with the transitional buffer above.
• Roughly a 75% cut in carbon emissions against a 2013 baseline home — effectively the end of new-build gas boilers, with heat pumps the default.
• A new requirement L3 making on-site renewable electricity — solar PV — a legal requirement for new dwellings, rather than something the notional dwelling merely assumed.
• A tighter air permeability in the notional dwelling. The limiting U-values do not change; anyone expecting new backstops will find the same numbers, renumbered.
• SAP 10.3 is the compliance methodology at launch — with new fuel factors: electricity 0.086 kgCO₂e/kWh and a primary energy factor of 1.969, gas 0.214 and 1.12 (what changed); the Home Energy Model follows later and eventually replaces it.

Tighter airtightness and a heat-pump baseline make thermal bridging matter more, not less. Designing now? The SAP 10 decisions — good fabric, PV, low-carbon heating, calculated Ψ-values — are exactly the ones the Future Homes Standard requires.

Wales and Scotland

Wales has its own Approved Document L, 2022 edition, in force since 23 November 2022, with its own tighter carbon target for new homes and its own approval of the SAP methodology; the process — design-stage and as-built calculations, testing of every dwelling, the EPC — is the same shape as England’s.

Scotland has no Part L. The equivalent is Section 6 (Energy) of the Building Standards Technical Handbook, with revised standards for building warrants from 1 February 2023, assessed with SAP 10 under a target emissions rate of its own.

The bottom line

Part L is four tests dressed as one, followed by a paper trail — and the target moves with your dwelling’s shape, so the earlier you model it, the cheaper compliance is. Whether you are on Part L 2021 today or planning for the Future Homes Standard, the fundamentals are the same: good fabric first, then renewables, and photographs of the insulation before it disappears. Unsure which requirements apply to your project? Ask us — it is quicker than reading the Approved Documents — or get your SAP calculations under way.