Part L (Conservation) | ConcreteMaths.com

The complete UK practitioner’s guide to Approved Document L — Conservation of Fuel and Power. Covers U-value targets, SAP 10.2, air permeability, thermal bridging, Approved Documents L1A, L1B, L2A & L2B, and the Future Homes Standard pathway.

What is Part L of the Building Regulations?

Part L is the section of theBuilding Regulations 2010(England) covering theconservation of fuel and powerin buildings. It sets minimum energy efficiency standards for all new buildings and for most significant works to existing buildings. Compliance is mandatory — failure to comply is a criminal offence under the Building Act 1984.

Part L is given effect through fourApproved Documentspublished by the Ministry of Housing, Communities and Local Government (MHCLG). The current edition for England is the2021 edition, which came into force on15 June 2022with a significant tightening of energy performance requirements. It is freely downloadable from GOV.UK.

Key Facts — Part L England 2021

In force from:15 June 2022 (England). All applications for Building Regulations approval submitted on or after this date must comply
Primary energy target:New dwellings must achieve a Primary Energy Rate not exceeding a Target Primary Energy Rate, determined using SAP 10.2
Tightening vs 2013 edition:Approximately 31% improvement in energy efficiency for new homes compared to the 2013 Part L standard — a significant single-step uplift
Transitional arrangements:Projects with Full Plans Building Regulations applications made before 15 June 2022 and started on site before 15 June 2023 could comply with the 2013 edition — these transitional projects are now essentially all live under the 2021 edition
Free download:All four Approved Documents are free atgov.uk

New Dwellings (L1A)

Covers all new homes — houses, flats, and apartments. Requires SAP 10.2 energy working, achievement of fabric energy efficiency targets, air permeability testing, and a Predicted Energy Assessment (PEA) for sale.

Existing Dwellings (L1B)

Covers extensions, material alterations, and material changes of use to existing houses and flats. Sets minimum U-value thresholds for new elements and replacement windows, doors, and roof lights.

New & Existing Non-Domestic (L2A & L2B)

Covers new commercial, industrial, and public buildings (L2A) and works to existing non-domestic buildings (L2B). Uses SBEM (Simplified Building Energy Model) or DSM (Dynamic Simulation Model) instead of SAP.

The 4 Approved Documents — What Each Covers

Part L compliance is demonstrated through four separate Approved Documents, each addressing a different building type and work category. All four were updated in June 2022 as the current England edition.

The primary reference for all new house and flat construction. Requires compliance with five criteria: (1) Target Primary Energy Rate; (2) Target Fabric Energy Efficiency rate; (3) minimum fabric performance standards (limiting U-values); (4) maximum air permeability of 8 m³/(h·m²) with mandatory pressure testing for developments of 2+ dwellings; (5) overall CO₂ emission rate not exceeding target.

SAP 10.2 is the mandatory working resource. An Energy Performance Certificate (EPC) and a Predicted Energy Assessment must be provided on completion.

Applies to extensions, loft conversions, material alterations, and change of use in existing dwellings. For extensions, new elements must achieve minimum fabric U-values (limiting values in Table 4). Replacement windows, doors, and rooflights must meet minimum WER/DER ratings or U-value thresholds. A whole-house SAP assessment is not required for typical domestic extensions but may be required for material changes of use.

Key rule: extensions must not make the overall energy performance of the existing dwelling significantly worse. Compensating measures may be needed where the extension cannot achieve limiting U-values.

Applies to all new non-domestic buildings — offices, retail, schools, hospitals, warehouses, and industrial. Uses SBEM (Simplified Building Energy Model) or an approved DSM resource to demonstrate that the building does not exceed its Target Emission Rate (TER) and Target Primary Energy Rate. Sets limiting fabric U-values, maximum air permeability (5 m³/(h·m²) at 50Pa for office/retail), and minimum efficiency requirements for building services (heating, ventilation, cooling, lighting).

Applies to extensions, material alterations, and change of use to existing non-domestic buildings. Sets limiting U-values for new building fabric elements, minimum performance standards for replacement building services, and consequential improvements requirements for large extensions to buildings over 1000m². Consequential improvements require the owner to upgrade other parts of the building fabric or services to bring overall energy performance closer to current standards.

Scotland, Wales & Northern Ireland Have Different Part L Equivalents

The Approved Documents described above apply only inEngland. Wales, Scotland, and Northern Ireland have their own equivalent energy efficiency standards — see theNations sectionbelow. The U-values, SAP methods, and air permeability targets differ between the four nations. Always confirm which nation’s standards apply to your project before commencing design.

U-Value Requirements 2026 — England

U-values measure the rate of heat transfer through a building element in watts per square metre per degree Kelvin (W/m²K). Thelowerthe U-value, the better the thermal performance. Part L 2021 (England) sets two types of U-value limits:notional building U-values(used in SAP/SBEM to work out the target) andlimiting U-values(minimum fabric performance that must be achieved regardless of other measures).

Notional vs Limiting U-Values — Key Distinction

Thenotional valueis used in the SAP or SBEM software to work out the energy performance target for the building — it defines the target performance level. Thelimiting valueis the absolute worst-case U-value any individual element can achieve, regardless of other energy measures. Both must be satisfied: you cannot offset a poor wall U-value with better glazing. Each limiting U-value is an independent minimum requirement.

U-Value Must Be Determined, Not Assumed

U-values for building elements must be determined using the method in BS EN ISO 6946 (for building components), BS EN ISO 10077 (for windows and doors), or BS EN ISO 13370 (for ground floors). Simple rule-of-thumb values from manufacturers’ literature are not sufficient for Building Regulations compliance working — a proper U-value working or third-party certified value is required.

SAP 10.2 — Standard Assessment Procedure

SAP (Standard Assessment Procedure) is the UK government’s approved methodology for working out the energy and environmental performance of dwellings.SAP 10.2is the current version required for Part L 2021 compliance in England. It is used to produce Energy Performance Certificates (EPCs) for all dwellings and to demonstrate compliance with Building Regulations for new homes.

SAP 10.2 Key Changes from SAP 2012

Carbon factors updated:The CO₂ emission factor for grid electricity was significantly reduced (from 0.519 to 0.136 kgCO₂/kWh) reflecting the decarbonisation of the electricity grid — making electric heating and heat pumps significantly more competitive in SAP working
Primary energy introduced:SAP 10.2 introduces a primary energy metric alongside CO₂ emissions — Part L 2021 uses primary energy as the primary compliance metric
Fabric Energy Efficiency (TFEE):A fabric-only metric that must be achieved regardless of services/renewables — prevents over-reliance on PV panels to compensate for poor fabric
Heat pumps:SAP 10.2 significantly improves the determined performance of heat pump heating systems due to the updated electricity carbon factor and improved COP working methodology
Overheating:SAP 10.2 includes a simplified overheating assessment — a full Overheating assessment under Approved Document O is also required for all new dwellings

The Three Compliance Checks in SAP 10.2 (Approved Document L1A)

Target Primary Energy Rate (TPER)

The dwelling’s determined Primary Energy Rate (PER) must not exceed the Target Primary Energy Rate (TPER). The TPER is determined by running SAP with the notional dwelling specification (notional U-values, notional heating system, notional renewables). The actual dwelling must match or beat this target. This is the primary compliance metric under Part L 2021.

Target Fabric Energy Efficiency (TFEE)

The dwelling’s Fabric Energy Efficiency (FEE) — which measures heat loss through the building fabric alone, excluding services and renewables — must not exceed the Target Fabric Energy Efficiency (TFEE) in kWh/m²/year. This ensures the building envelope itself is well-insulated and cannot be compensated entirely by renewable energy systems.

Limiting U-Values & Air Permeability

Every individual building element must achieve the limiting U-value specified in Table 4.1 of Approved Document L1A (see U-values section above). Additionally, the air permeability must not exceed 8 m³/(h·m²) at 50Pa. For developments of two or more dwellings, pressure testing is mandatory for a minimum percentage of units (all units if ≤2, or a sample for larger developments with a robust airtightness programme).

SAP Software Resources

SAP 10.2 working must be produced using DCLG-approved SAP software. Common resources include Elmhurst Energy SAP, Stroma FSAP, and Benchmark SAP. Only figure from approved software is acceptable for Building Regulations compliance — spreadsheet working are not sufficient.

Design SAP vs As-Built SAP

Two SAP working are required: adesign stage SAPsubmitted with the Building Regulations application, and anas-built SAPsubmitted on completion using actual construction data (air test results, as-built insulation, confirmed services). The EPC is generated from the as-built SAP.

Who Can Produce SAP?

SAP working for Building Regulations compliance and EPC production must be carried out by anaccredited energy assessorregistered with a DCLG-approved accreditation scheme (e.g. Elmhurst, ECMK, Stroma). Unaccredited SAP working are not accepted for EPC lodgement.

Air Permeability & Airtightness Testing

Air leakage through the building envelope is a major source of heat loss and accounts for a significant proportion of a dwelling’s space heating energy use. Part L 2021 sets amaximum air permeability of 8 m³/(h·m²) at 50 Pascalsfor new dwellings — and requires physical pressure testing to verify compliance on site.

Air Permeability vs Airtightness — The Terminology

Air permeability:The UK Building Regulations metric — measured in m³ of air leakage per hour per m² of envelope area at a test pressure differential of 50 Pascals (m³/h/m² @50Pa)
Air changes per hour (ACH):Used in Passive House design — measured in air changes per hour at 50Pa (n50). For a typical dwelling, 8 m³/h/m² ≈ 5–8 ACH depending on volume/surface area ratio
Passive House target:n50 ≤ 0.6 ACH — approximately equivalent to 0.5–1.0 m³/h/m² under Part L testing, significantly better than the Part L maximum
Current best practice:Many leading housebuilders target 3–5 m³/h/m² — well below the 8 m³/h/m² maximum — as the air permeability value directly feeds into the SAP working and affects EPC rating

Pressure Testing Requirements — Part L 2021

Mandatory Testing Threshold

Pressure testing is mandatory for all developments of2 or more dwellings. Single one-off dwellings may use a default design value of 15 m³/(h·m²) in SAP if not tested — but this significantly penalises the SAP score, so almost all new homes are tested in practice.

Test Standard — ATTMA TS1 / BS EN ISO 9972

Tests must be carried out toATTMA Technical Standard TS1(Measuring Air Permeability of Building Envelopes) orBS EN ISO 9972. Tests must be conducted by a suitably qualified and registered tester (ATTMA registered or equivalent). The fan pressurisation (blower door) method is used — pressurising and depressurising the dwelling to 50Pa and measuring leakage flow rate.

Sample Testing on Larger Developments

For developments with a robustairtightness programmeagreed with Building Control, sample testing is permitted: a minimum of one test per dwelling type, with additional tests triggered if a dwelling fails. If a tested dwelling exceeds 8 m³/(h·m²), all dwellings of the same type must be tested and remediated. The airtightness programme must be submitted to and approved by Building Control before work starts.

Remediation & Retesting

If a dwelling fails the pressure test (result >8 m³/h/m²), the builder must identify and seal air leakage paths — typically at service penetrations, junctions between elements, window/door perimeters, and loft hatches — and retest until the target is achieved. Common air leakage paths include electrical socket boxes, soil pipe penetrations, and the wall/floor junction at the base of the cavity.

Good Airtightness Practice — On-Site Tips

Appoint an airtightness champion on site — someone responsible for maintaining the air barrier throughout construction
Use proprietary airtightness tape at all junctions, penetrations, and overlaps in the membrane or board air barrier
Pre-plaster pressure testing at first-fix stage allows remediation before plastering covers up leakage paths
All service penetrations through the air barrier (pipes, cables, ducts) must be sealed with airtight grommets or flexible sealant
Loft hatches must be draught-stripped and insulated — a common source of air leakage in completed dwellings

Thermal Bridging & Psi-Values

Thermal bridging occurs where heat flows more easily through certain parts of the building envelope than others — typically at junctions between elements (e.g. wall/floor, wall/roof, window reveals) and at structural elements that penetrate the insulation layer. Thermal bridging is quantified bylinear thermal transmittance (Ψ — Psi-values)measured in W/(m·K), and is included in SAP working as theHeat Transfer Coefficient (HTB).

Ψ-Value Working Options in SAP 10.2

Option: Default; Method: No working — SAP default y = 0.15; Typical y-factor: y = 0.15 W/m²K; SAP Impact: Worst outcome; Cost / Effort: Zero cost
Option: ACD; Method: DCLG Accredited Construction Details; Typical y-factor: y ≈ 0.08–0.10 W/m²K; SAP Impact: Good improvement; Cost / Effort: Free library
Option: Determined; Method: BS EN ISO 10211 FEA per junction; Typical y-factor: y ≈ 0.04–0.06 W/m²K; SAP Impact: Best outcome; Cost / Effort: Engineering fee
Option: Certified systems; Method: BBA / LABC / NHBC certified system Ψ-values; Typical y-factor: y ≈ 0.05–0.08 W/m²K; SAP Impact: Good–best; Cost / Effort: System purchase

Thermal Bridging — The Most Overlooked Part L Issue

Thermal bridging can add 20–50% to the determined heat loss of a well-insulated building if poorly managed. On many projects, using the default y-value of 0.15 makes Part L compliance very difficult or impossible despite achieving good U-values. Always appoint a thermal bridging specialist at design stage — using Accredited Construction Details or determined Ψ-values is strongly recommended on all new build projects.

Future Homes Standard — What’s Coming

TheFuture Homes Standard (FHS)is the UK government’s planned upgrade to Part L for new dwellings in England, intended to ensure all new homes are built to a near-zero carbon standard. The FHS was originally intended for implementation in 2025, but as of early 2026 the final FHS regulations have been delayed — the government is consulting on the detailed requirements.

Future Homes Standard — Status as of February 2026

The Future Homes Standard has been subject to significant delays. The government ran a consultation in 2023 and is expected to publish the final FHS Building Regulations in 2026/2027 for implementation thereafter. In the meantime, thePart L 2021 edition remains the current legal requirementfor all new homes in England. Always check GOV.UK for the latest FHS implementation timeline before designing for future compliance.

What the Future Homes Standard is Expected to Require

No New Gas Boilers

The FHS is expected to effectively ban new gas boilers as the primary heating system in new homes — homes would instead be heated by heat pumps, heat networks, or other low-carbon systems. This is the central policy objective of the FHS.

~75–80% CO₂ Reduction

The FHS is intended to deliver approximately 75–80% lower CO₂ emissions compared to a home built to the 2013 Part L standard — a major step change beyond the ~31% improvement already delivered by Part L 2021.

Tighter Fabric Standards

The FHS consultation proposed further tightening of U-values — with walls potentially at 0.15 W/m²K, floors at 0.11 W/m²K, and roofs at 0.09 W/m²K — alongside very low air permeability targets to ensure fabric-first performance.

Overheating & Part O

The FHS works in tandem withApproved Document O(overheating), which was introduced alongside Part L 2021 in June 2022. As homes become better insulated and airtight, overheating risk increases — Part O requires assessment and mitigation measures for all new dwellings.

Future Buildings Standard (Non-Domestic)

A parallel Future Buildings Standard (FBS) applies to new non-domestic buildings — also subject to consultation and delay. The FBS covers offices, retail, and public buildings with similar low-carbon heating and tighter fabric targets.

Implementation Timeline

The FHS is expected to be implemented through an amendment to the Building Regulations. A transition period will be provided after the final regulations are published. Design teams should plan now for heat pump-ready buildings to avoid the need for costly retrofits when FHS takes effect.

Existing Buildings — Approved Documents L1B & L2B

When carrying out work to existing buildings, Part L requires that the energy performance of the work itself — and in some cases of the building as a whole — meets minimum standards. The requirements are less onerous than for new buildings, but non-compliance is still a Building Regulations offence.

Key Triggers and Requirements — Existing Dwellings (L1B)

Extensions

All new building elements in extensions must achieve thelimiting U-values from Table 4 of AD L1B: walls 0.28 W/m²K, floors 0.25 W/m²K, roofs 0.20 W/m²K, windows 1.40 W/m²K. Where an extension cannot achieve these values due to existing building constraints, the SAP working for the whole dwelling must demonstrate no significant worsening of energy performance. Conservatories with ≥75% glazed roof are exempt if thermally separated from the main house.

Replacement Windows & Doors

Replacement windows must achieve at minimum aBFRC Window Energy Rating of Band C(equivalent to approximately U = 1.40 W/m²K, g = 0.35). Replacement doors must achieve U ≤ 1.40 W/m²K for solid doors, or the window standard if ≥50% glazed. This is notifiable work under Fenestration Self-Assessment Scheme (FENSA) or equivalent — installation by a non-registered installer requires a Building Regulations application.

Roof & Loft Insulation

When a roof is re-covered (more than 50% of the roof covering replaced), insulation must be upgraded if it is technically, functionally, and economically feasible to do so. Thecost-effectiveness testis key — if the simple payback period is under 15 years, the upgrade is considered economically feasible and should be carried out. The target is U ≤ 0.16 W/m²K for pitched roofs and 0.18 W/m²K for flat roofs.

Replacement Boilers & Heating Systems

Replacement boilers must have a minimum seasonal efficiency consistent with a SEDBUK rating of at least92% (ErP A-rated). All new or replacement heating systems must include adequate heating controls — minimum programmer, room thermostat, and thermostatic radiator valves (TRVs) on all radiators except the room with the room thermostat. Hot water cylinders must be factory-insulated to ≥35mm foam.

Consequential Improvements — L2B (Non-Domestic >1000m²)

For existing non-domestic buildings over 1000m² where major works are proposed (new heating systems, extensions, change of use),consequential improvementsare required under AD L2B. The building owner must spend up to 10% of the principal works value on improving the energy performance of the existing building — typically by upgrading insulation, heating controls, or lighting. This requirement only applies where the principal works exceed a cost threshold — check the current thresholds in AD L2B.

Scotland, Wales & Northern Ireland — Devolved Standards

Building energy efficiency standards are a devolved matter. Each UK nation has its own equivalent of Part L with different U-value targets, energy working methods, and implementation timelines. Never assume England’s Part L applies outside England.

Standard:Approved Document L (Part L), 2021 edition — in force from June 2022.

SAP 10.2 for dwellings; SBEM for non-domestic
Wall U: 0.18 W/m²K (notional)
Air permeability: max 8 m³/h/m²
Future Homes Standard pending 2026/27
Free download:gov.uk

Standard:Approved Document L (Wales) — Welsh Government edition, separate from England since 2014.

SAP 10.2 required; EPC on completion
Welsh government has its own implementation timeline
Generally comparable U-values to England
Part F ventilation and overheating also devolved
Download:gov.wales

Standard:Scottish Building Standards — Section 6 (Energy). Updated regularly by the Scottish Government.

Uses SAP 2012 (RdSAP for EPCs) — not SAP 10.2
Wall U-value requirement: 0.17 W/m²K (often stricter than England)
Scotland introduced near-zero energy buildings standard earlier than England
Building Warrant process (not Building Regs approval)
Download:gov.scot

Standard:Technical Booklet F1 (Conservation of Fuel and Power in Dwellings) — Northern Ireland Building Regulations.

Administered by district councils (not MHCLG)
SAP-based compliance for dwellings
Generally slightly behind England in target stringency
Technical Booklet F2 covers non-domestic buildings
Download:finance-ni.gov.uk

Part L — Legislative Timeline 2006–2026

Part L has been progressively tightened since 2006, with each revision delivering significant improvements in energy performance standards for new buildings. Understanding the timeline helps designers working on projects that may have been started under previous editions.

Use Our Free Concrete Guide for Part L-Compliant Slab Design

Ground floor slab thickness and insulation specification directly affects your Part L floor U-value. Use ourMobile Concrete Calculatorto estimate slab volumes — then verify your floor U-value with a BS EN ISO 13370 ground floor U-value working. OurBS Standards Guidecovers the full Part L and Eurocode 2 framework.

Part L Building Regulations — FAQ

Frequently asked questions about Part L, U-values, SAP, and energy efficiency requirements for UK construction in 2026.

New building elements:All walls, floors, and roofs forming part of the extension must achieve the limiting U-values in Table 4 of AD L1B (walls 0.28 W/m²K, floors 0.25 W/m²K, roofs 0.20 W/m²K)
Windows and glazed doors:Must achieve minimum BFRC Window Energy Rating Band C or U ≤ 1.40 W/m²K
Conservatories:Exempt from Part L U-value requirements if: the conservatory is at ground level; has ≥75% of its roof area and ≥50% of its wall area as translucent material; is thermally separated from the main house by walls, windows, and doors that meet the limiting standards; and has independent heating controls. If it lacks thermal separation, the conservatory is treated as an extension and full Part L applies
Loft conversions:The new roof slope elements, dormer cheeks, and any new floor over the existing first floor must achieve limiting U-values. Existing roof slope insulation does not need upgrading simply because a loft conversion is being carried out — but new or replacement elements must comply
Porches:Porches with a floor area ≤30m² are exempt from Part L, provided they are at ground level and thermally separated from the main house
Heating controls:Where a new extension is heated, adequate controls (room thermostat, programmer, TRVs) must be provided — this is often overlooked on small domestic extensions

New dwellings — always required:A full SAP 10.2 working is mandatory for every new dwelling in England. You need both a design-stage SAP (submitted with the Building Regulations application) and an as-built SAP (submitted on completion, used to generate the EPC)
Extensions to existing dwellings — usually not required:For a typical domestic extension where limiting U-values are achieved, a full SAP working is not normally required. The engineer or designer simply confirms the limiting U-values are met. However, if limiting U-values cannot be achieved for any element, a SAP working must be produced to demonstrate that the overall dwelling energy performance does not deteriorate significantly
Material change of use:Where a building or part of a building is converted to a dwelling (e.g. barn conversion, commercial-to-residential), a SAP working is required to demonstrate the converted dwelling meets the limiting standards in AD L1B
Non-domestic buildings:SAP is not used for non-domestic buildings — SBEM (Simplified Building Energy Model) or a Dynamic Simulation Model (DSM) is required instead under AD L2A/L2B
EPC for sale or let:An EPC is required when selling or letting any dwelling. EPCs are produced from RdSAP (Reduced data SAP) assessments carried out by an accredited domestic energy assessor — these are separate from full SAP compliance working

Standard cavity masonry (102mm brick + 100mm cavity + 100mm block):To achieve 0.18 W/m²K, approximately 100mm full-fill mineral wool or 85mm partial-fill PIR (e.g. Kingspan Kooltherm K108) in the cavity is required. Partial-fill leaves a residual air gap for moisture management
Timber frame:Achieving 0.18 W/m²K is more straightforward — 140mm studs with mineral wool between + external insulation board can achieve 0.15 W/m²K or better. Timber frame naturally suits fabric-first approaches
Solid wall constructions:For solid wall new builds (rare for residential, more common in some commercial), external or internal insulation of approximately 100–150mm PIR is required to achieve compliant U-values
Limiting vs notional:Remember — the limiting U-value for new dwelling walls is 0.26 W/m²K (the absolute worst case allowed). Achieving 0.26 W/m²K instead of 0.18 W/m²K may allow less insulation but will penalise the SAP working and may prevent achieving the Target Primary Energy Rate. In practice, 0.18–0.21 W/m²K is what most new build housebuilders target
Thermal bridging:Even with excellent U-values, poor junction detailing (thermal bridging) can undermine compliance — wall insulation and junction detailing must be considered together

Step 1 — Identify leakage paths:The tester will use a smoke pencil or tracer gas during the test to identify where air is leaking. Common failure points include: electrical back-boxes and socket outlets on external walls; soil pipe and service penetrations through floors and ceilings; the junction between ground floor slab and wall (DPC level); loft hatch perimeters; poorly fitted windows and door frames; extraction fan housings; and any unsealed gaps around steelwork passing through the air barrier
Step 2 — Remediate:All identified leakage paths must be sealed. Typical remediation includes: applying airtightness tape or foam to service penetrations; replacing poorly fitted foam-filled socket back-boxes with airtight alternatives; re-sealing window frame perimeters with flexible sealant; fitting draught-proofing to loft hatches; and applying expanding foam to gaps in the floor/wall junction
Step 3 — Retest:Once remediation is complete, a full retest to ATTMA TS1 is carried out. The retest result must be ≤8 m³/h/m² to achieve compliance
Impact on larger developments:On a development with an approved airtightness programme, if one dwelling fails its sample test, all other dwellings of the same type on the development must be tested individually — significantly increasing testing costs
SAP as-built update:The actual pressure test result must be entered into the as-built SAP working — a poor result (even if ≤8) directly worsens the EPC rating
Best practice:Carry out an interim pressure test mid-construction (e.g. at second fix before boarding) — this catches problems while they are still easily accessible and avoids costly post-completion remediation

Replacement gas boiler (L1B):Must be a condensing boiler with a minimum seasonal efficiency of 92% (ErP A-rated or equivalent SEDBUK Band A). Non-condensing boilers are not permitted for replacement installations in existing dwellings except in very limited circumstances (e.g. where flue position makes a condensing boiler genuinely impractical — this must be confirmed in writing by the installer)
Heating controls:A replacement boiler must be accompanied by adequate heating controls: minimum a programmer/timer, a room thermostat, and TRVs on all radiators except the room containing the room thermostat. If acceptable controls are already present, they do not all need to be replaced — but any missing control must be added
Heat pump installations:Heat pump installations are notifiable under Building Regulations (Part L and Part F for ventilation if MVHR is fitted). The heat pump must be sized and installed correctly per manufacturer’s guidance and MCS (Microgeneration Certification Scheme) standards if the homeowner wants to claim RHI/BUS grant funding
Gas Safe registration:All gas work must be carried out by a Gas Safe registered engineer — this is a separate legal requirement from Building Regulations. Gas Safe registration does not substitute for Building Regulations compliance notification
Self-certification schemes:Competent person schemes (e.g. HETAS for biomass, NAPIT for electrical heating) allow qualified installers to self-certify compliance without a separate Building Regulations application — the installer notifies the local authority on the homeowner’s behalf
Hot water cylinder:Replacement cylinders must be factory insulated to ≥35mm foam and have a thermostat capable of reaching 60°C for legionella protection

Typical EPC outcome for gas-heated new build:A new dwelling with a gas boiler meeting Part L 2021 fabric standards typically achieves EPC Band B (81–91 SAP points). It is generally not possible to achieve Band A (92+ SAP points) with a gas boiler under SAP 10.2
Typical EPC outcome for heat pump new build:A new dwelling with an air source heat pump meeting Part L 2021 fabric standards typically achieves EPC Band B to low Band A (88–96 SAP points) due to the very low CO₂ factor for electricity in SAP 10.2
Important caveat:The EPC band displayed to homebuyers is based on SAP points (a composite energy cost score) — not on primary energy or CO₂ emissions. A home can be Part L compliant without achieving Band A. EPC ratings are also sensitive to assumed fuel costs, which change annually
Mortgage lender requirements:Some lenders (and government green mortgage products) specify minimum EPC Band B or Band A for mortgage products. This is a commercial requirement separate from Building Regulations — but it increasingly drives developers to target Band A even where Part L only requires compliance with TPER/TFEE
Future Homes Standard:When the FHS is implemented, new homes built with heat pumps and enhanced fabric are expected to achieve EPC Band A as a typical outcome

Current legal requirement (2026):Part L 2021 — in force since June 2022. All new homes must comply with this standard. Gas boilers are still permitted as the primary heating system
Future Homes Standard:A planned amendment to the Building Regulations for England — expected to be consulted on and finalised in 2026/2027. Not yet law as of February 2026. Gas boilers are expected to be prohibited in new homes once FHS takes effect
CO₂ reduction:Part L 2021 delivers ~31% improvement over the 2013 standard. FHS is expected to deliver ~75–80% improvement over the 2013 standard
Primary heating:Part L 2021 — gas boilers permitted (and still the majority heating system in new homes). FHS — heat pumps or heat networks expected to be required as the primary heating system
Fabric standards:Part L 2021 notional wall U-value: 0.18 W/m²K. FHS consultation proposed: ~0.15 W/m²K — a further tightening requiring additional insulation
What designers should do now:Design new homes to be “heat pump ready” — ensure adequate floor insulation, low U-value fabric, high airtightness, and sufficient space for an air source heat pump unit. This minimises the cost of FHS compliance when it takes effect and future-proofs developments against changing regulations
Planning ahead:For large residential developments, the planning and design period may span the FHS transition. NHBC and the housebuilding industry are actively monitoring the FHS programme — check the NHBC, HBF, and GOV.UK websites for the latest implementation dates

Not mandated as a prescriptive requirement:There is no rule in Approved Document L1A that says “you must fit solar panels.” Part L is performance-based — any combination of measures that achieves the TPER and TFEE targets is acceptable
Often required in practice (gas-heated homes):For a new gas-heated home to achieve the Target Primary Energy Rate under SAP 10.2, the notional building includes a solar PV allowance. If the actual dwelling does not include PV but has a gas boiler, it is very difficult to achieve the TPER without PV — because the notional benchmark already includes it. Many housebuilders fit 1–2 kWp of PV as standard on Part L 2021 gas-heated homes specifically to pass SAP
Heat pump homes:For homes heated by an air source heat pump, PV is often not necessary to achieve TPER compliance — the low electricity carbon factor in SAP 10.2 means heat pump homes typically pass without PV, though PV still improves the EPC rating
Fabric first principle:The TFEE metric ensures fabric performance cannot be fully substituted by renewables — even if you fit large PV arrays, you still must achieve the fabric energy efficiency target. Good insulation and airtightness cannot be bypassed
Future Homes Standard:The FHS consultation indicated PV may be more formally incorporated into the notional specification. Heat pump-heated FHS homes may or may not require PV — to be confirmed when final regulations are published

Related UK Construction Guides & Resources

Free companion guides to use alongside this Part L summary for your UK construction project.

Professional Disclaimer — Always Use the Current Approved Document

This page is a summary guide for UK construction professionals, designers, and homeowners. It does not constitute professional building regulations advice. Always use the current edition of Approved Document L (freely available atgov.uk) and consult a qualified energy assessor for SAP working and compliance verification. Standards change — verify current requirements before commencing design.

For related building and design resources, you can also visit ArchitectsEmporium.com.