Part L 2025 cost impact on UK residential new build and extensions
What the 2025 Part L amendments add to build costs, by project type and spec, with a live index-adjusted benchmark.
The 2025 Part L amendments push residential build costs up by an estimated 4 to 9% depending on project type. Here is where the money actually goes.
Sources: Approved Document L, Volume 1 (Dwellings), 2025 edition. Live cost figures from ONS Labour Cost Index for construction and MHCLG Monthly Building Materials. Latest print: N/A.
Part L 2025 is the interim step towards the Future Homes Standard, which is expected to land in late 2026. The scope applies to new dwellings and material changes of use in England, with Wales Part L and Scotland Section 6 following similar principles on different timings. This article is England-focused.
What changed in Part L 2025
- Higher fabric performance targets. U-values tightened for walls (around 0.18), roofs (around 0.13), floors (around 0.11) and windows (around 1.2 W/m²K).
- Air-tightness of 5 m³/(h·m²) at 50 Pa as the practical standard. Tighter than the 2013 Part L default and no longer achievable with casual detailing.
- Low-carbon heating bias. Gas boilers are effectively phased out for new build via the primary energy calculation; heat pumps become the default.
- On-site renewable expectation. Solar PV or equivalent contribution is baked into the SAP 10 calculation for most detached new builds.
Source: Approved Document L, Volume 1 (Dwellings), 2025 edition. HM Government. Confirm specific values against the current live edition on gov.uk before designing to them.
Where the cost actually goes
| Work package | Change vs 2013 Part L | Indicative uplift | Notes |
|---|---|---|---|
| Insulation and fabric | Tighter U-values | +£15 to £25/m² | Additional cavity width, rigid board specs |
| Heating | Heat pump over gas boiler | +£3,000 to £6,000 per dwelling | Includes hot water cylinder and controls; labour heavier too |
| Renewables | PV array (typically 3 to 4 kWp) | +£5,000 to £8,000 per dwelling | SAP requirement for most detached new build |
| Air-tightness and testing | Improved detailing and third-party test | +£2 to £4/m² | Lost time on site if first test fails |
| MVHR or extract | Background ventilation upgrade | +£1,500 to £3,000 per dwelling | Replaces trickle-vent-only provision in many houses |
| Certification overhead | SAP 10 calc, Part L designer, EPC B minimum | +£500 to £1,500 per dwelling | Added design fee, not a site cost |
Figures are indicative industry ranges at time of writing and vary by dwelling size, spec and region. Blended ONS labour and MHCLG materials inflation applies on top; see the hero for current YoY values.
Labour and materials since 2020, for context
Source: ONS Labour Cost Index for construction, MHCLG Monthly Building Materials. Series rebased to 100 at the earliest period in each dataset so the cumulative shape is comparable. The Part L uplift figures sit on top of this underlying movement.
Project-type impact, ranked
New build dwelling: +7 to +9%
Expect the top of the uplift range on typical residential new build versus pre-2025 spec. Heat pump plus PV plus fabric are the biggest lines. New-builders have the simplest compliance path because there is no legacy fabric to work around.
Rear extension: +4 to +6%
Fabric upgrade is the main cost, because the extension fabric must hit the new U-values. Heating is often served by the existing boiler unless extended area justifies a heat pump upgrade. Air-tightness detailing matters at the junction with the existing building.
Loft conversion: +3 to +5%
Insulation between and below rafters becomes thicker, eating headroom, which can cascade into structural changes. Rooflights to Part L 2025 spec are meaningfully more expensive than 2013-standard units.
Full refurbishment or change of use: +2 to +10%
Variable. At the top of the range where the change of use triggers new-build equivalent standards, at the bottom where only elemental thermal upgrades are required.
Common non-compliance pitfalls
- SAP 10 calc not kept current through design changes, triggering late-stage redesign.
- Thermal bridging at junctions ignored, failing the overall elemental check despite individual U-values passing.
- Air-tightness test assumed to pass first time; no contingency for remedial detailing.
- PV under-specified for the dwelling size, pushing the SAP calc into a fail.
- Hot water cylinder sized incorrectly for the heat pump, creating a call-back after occupation.
What this means for your quote
- Price the heat pump and cylinder as a single MEP line. Not as a swap of a gas combi. The labour content is materially heavier, the trade pool is tighter, and the commissioning workload is higher.
- Carry a specific Part L contingency line of 2 to 3% on new-build quotes until your supply chain is reliably delivering to the new detailing standards. Roll this down once you have two or three completed jobs under the new regime.
- On extensions to older properties, flag the air-tightness interface with the existing fabric as a design risk in the quote. It is the most common driver of re-work and it is fairer to the client to surface it up front than as a variation mid-build.
Figures are at time of writing and represent blended industry ranges. The Future Homes Standard, expected late 2026, will tighten requirements further, particularly around embodied carbon. Check the Approved Document L current edition on gov.uk before designing to specific values. For the live labour and materials figures this analysis rests on, the Cost Tracker is the free tool.