UK Part L Building Regulations — Definitive Guide for Homeowners 2026
UK Building Regulations Approved Document Part L ("Conservation of Fuel and Power") is the residential energy-performance code under Schedule 1 of the Building Regulations 2010 (SI 2010/2214). The Approved Document is published in two parts — Part L Volume 1 (Dwellings) and Part L Volume 2 (Buildings other than dwellings) — by the Department for Levelling Up, Housing and Communities (DLUHC, now MHCLG). The 2021 Part L update came into force on June 15, 2022 ("interim Part L"), and the Future Homes Standard 2025 further tightens requirements with phased implementation through 2025 and beyond. Compliance is verified through SAP (Standard Assessment Procedure) calculation for new dwellings and through targeted requirements for renovation work.
What it governs
Part L Volume 1 covers four categories: thermal envelope (U-values for walls, roof, floor, windows, doors), fixed building services (boilers, heat pumps, hot water systems), low and zero-carbon technology (solar PV, heat pumps, MVHR), and energy efficiency calculation methodology (SAP).
The 2022 update introduced tighter elemental U-value targets:
- External walls: 0.18 W/m²K (down from 0.28)
- Roofs: 0.11 W/m²K
- Windows + doors: 1.4 W/m²K
- Air permeability: target 5 m³/(h·m²) at 50 Pa for new dwellings (tested at completion via the Air Tightness Testing & Measurement Association (ATTMA) protocol)
Future Homes Standard 2025 targets a further 75-80% reduction in CO₂ emissions versus 2013 Part L baseline, requiring heat-pump primary heating in most new homes and tighter envelope plus MVHR. The phased implementation runs through 2025-2027.
For renovation work, Part L imposes targeted retrofit requirements when extending a dwelling, replacing a thermal element (wall, roof, window), or upgrading a fixed building service. Each scope has its own elemental compliance pathway documented in Approved Document L1B.
Homeowner implications
For a UK homeowner — London, Birmingham, Manchester, Bristol, Leeds, Newcastle — Part L applies to:
- New dwellings: full compliance via SAP calculation submitted to building control
- Extensions: thermal-envelope compliance for the new portion + glazing constraint (extension glazing area ≤ 25% of new floor area unless compensated elsewhere)
- Renovations: targeted requirements when replacing thermal elements or fixed services. Replacing a boiler triggers 2025 phase-out of new gas boilers. Replacing a window triggers Part L glazing U-value compliance.
Energy Performance Certificate (EPC) at sale or rental is governed by the Minimum Energy Efficiency Standards (MEES) Regulations 2015, tied to Part L performance levels. From 2025, rental properties typically need an EPC C rating to be lawfully let.
Contractor implications
For a UK contractor (general builder, M&E contractor, window installer), Part L compliance is documented through SAP calculations (for new dwellings) or through targeted compliance certificates (for retrofit work). Glazing installers register through FENSA or CERTASS — Competent Person Schemes that self-certify Part L compliance for window replacements without separate Building Control inspection.
Heating engineers self-certify boiler installations through Gas Safe Register for gas work and through MCS for heat pumps and renewables. Electricians self-certify through NICEIC, ELECSA, or NAPIT under Part P.
The transition to Future Homes Standard 2025 has accelerated heat-pump installer demand. Most large UK developers shifted to heat-pump primary heating in 2024-2025 in anticipation of the phased rollout.
How AskBaily uses it
Every AskBaily London + UK metro match runs:
- Project type detection (new-build, extension, renovation)
- For glazing scope: FENSA/CERTASS member verification
- For heating scope: Gas Safe (gas) or MCS (heat-pump/renewables) member verification
- For electrical scope: Part P-registered competent-person scheme membership (NICEIC/ELECSA/NAPIT)
- Cross-link to our UK Building Regs canonical
- Cross-link to our UK CDM 2015 canonical
- Surface a flag on homeowner-facing scope card noting Part L compliance pathway
Recent changes 2024–2026
DLUHC (now MHCLG) consulted on the Future Homes and Buildings Standards in 2023 with publication in 2024 + 2025. The 2025 Future Homes Standard mandates approximately 75-80% lower CO₂ emissions versus 2013 Part L, with heat-pump primary heating expected to be the primary compliance pathway in new dwellings.
The 2024 SAP 10.2 to SAP 11 transition updates the calculation methodology with revised emissions factors reflecting the decarbonising electricity grid. SAP 11 implementation is phased into 2025-2026.
Frequently asked questions
Does Part L apply to my window replacement? Yes — replacement windows must meet Part L glazing U-values (1.4 W/m²K for new). FENSA/CERTASS-registered installers self-certify compliance.
Will Part L 2025 ban gas boilers? Yes — new gas boilers in new dwellings are essentially phased out under Future Homes Standard. Replacement gas boilers in existing dwellings are still permitted under transitional arrangements until 2035.
What's a SAP calculation? Standard Assessment Procedure — the methodology for calculating new-dwelling energy performance and CO₂ emissions. Required for new dwellings.
Where do I find the Approved Document? gov.uk Approved Document L.
What's the difference between Part L and EPC? Part L is the building-regulations compliance code at design + construction. EPC is a marketing-and-rental rating issued post-completion under MEES.