Building Regs Part L and Heat Pumps: SAP Calculations, EPC Impact and Fabric-First Requirements

Summary

Approved Document L is the Building Regulations document covering energy efficiency in buildings in England and Wales. For heat pump installers, there are two relevant scenarios: installing a heat pump in an existing dwelling (Part L1B) and specifying a heat pump in a new build (Part L1A). In both cases, understanding the SAP 10.2 methodology, the carbon factor for electricity, and the effect on EPC ratings is essential for advising customers and completing correct MCS/building control documentation.

Key Facts

• Approved Document L1A — new dwellings; requires compliance with a Target Primary Energy Rate (TPER) and a Target Emission Rate (TER); heat pumps are a highly effective route to compliance due to their low primary energy demand
• Approved Document L1B — existing dwellings (extensions, material changes of use, controlled fittings and services); replacing a heating system triggers requirements for heating controls and minimum boiler efficiency (or heat pump performance)
• SAP 10.2 — Standard Assessment Procedure version 10.2; the UK government's method for calculating the energy and environmental performance of dwellings; used for EPCs and Part L compliance; assigns a carbon factor to electricity of approximately 0.136 kg CO2/kWh (vs ~0.210 kg CO2/kWh for gas in SAP 10.2) — significantly lower than the old SAP 9.92 electricity carbon factor, which made heat pumps look worse on EPCs
• EPC (Energy Performance Certificate) — based on SAP; the letter rating (A–G) and SAP score; required when selling or renting a property; a heat pump installation typically moves a property up 1–2 EPC bands compared to a gas boiler (due to lower electricity carbon factor and higher SCOP)
• Fabric-first principle — Part L1B does not mandate fabric upgrades (wall insulation, double glazing) as a condition of heat pump installation; however, the installer must ensure the design heat loss calculation shows the heat pump is sized correctly for the as-built fabric — if the fabric is poor, the heat pump may be larger and the design flow temperature higher, potentially failing the MCS efficiency target
• Minimum efficiency standard — for a replacement heat pump (like a replacement boiler), the system must meet the minimum seasonal efficiency requirements; under current Part L1B guidance, a heat pump replacing a boiler should achieve SCOP ≥ 2.5 (space heating) at the design conditions for the property
• Heating controls — Part L1B requires time and temperature controls to be provided when a heating system is replaced; a basic room thermostat plus programmer satisfies this; weather compensation satisfies it more completely
• BUS eligibility and EPC — the Boiler Upgrade Scheme requires the property to have a valid EPC dated within the last 10 years; the EPC must not recommend loft or cavity wall insulation as improvements (if it does, those improvements must be completed before BUS is applied)
• New build Part L1A — under 2021 Part L1A (which came into force June 2022), new homes must meet tighter TPER targets; heat pumps substantially contribute to Part L compliance because SAP 10.2 credits high SCOP and the low CO2 factor of electricity; many new build developers now specify heat pumps as the primary heating system for Part L reasons as much as for sustainability
• Wales — Approved Document L applies in Wales with modifications; SAP 10.2 is also used; specific Wales technical guidance applies
• Scotland — Scottish Building Standards use SECS (Scottish Energy Certification Scheme) and Section 6 (Energy); different from England/Wales; Scottish Heat in Buildings Strategy drives higher heat pump adoption in new builds

Quick Reference Table: EPC Impact of Heat Pump vs Gas Boiler

EPC estimates based on SAP 10.2. Actual values depend on floor area, fabric specification, DHW use.

Detailed Guidance

SAP 10.2 and the Electricity Carbon Factor

The most significant change in SAP 10.2 (adopted 2021, mandatory 2022) for heat pump installations was the revision to the electricity carbon emission factor:

• SAP 9.92 (previous): electricity carbon factor ~0.519 kg CO2/kWh — made heat pumps look carbon-intensive on EPCs
• SAP 10.2: electricity carbon factor ~0.136 kg CO2/kWh — reflects the decarbonisation of the UK grid and is more representative of grid electricity today

This change dramatically improved the EPC ratings achievable with heat pumps. A property heated by an ASHP at SCOP 3.0 using SAP 10.2 electricity has a CO2 intensity of approximately 0.136 ÷ 3.0 = 0.045 kg CO2/kWh of heat — substantially lower than gas at ~0.21 kg CO2/kWh.

For customers asking "will a heat pump improve my EPC?" — yes, in most cases by 1–2 bands, primarily due to this electricity carbon factor improvement.

Part L1B: Existing Dwellings

When Part L1B applies to heat pump installations: Part L1B is triggered by "controlled services" work — replacing the primary heating system constitutes a controlled service. This means the installation of a heat pump (replacing a gas boiler) must comply with Part L1B requirements:

1. Heating system efficiency: The heat pump must be appropriately sized and expected to achieve the minimum SCOP
2. Heating controls: Time and temperature controls must be provided (if not already present); add a room thermostat and programmer/timer if the existing controls are absent or non-functional
3. Insulation of pipework: All new pipework must be insulated per the efficiency standards in Part L1B Table B1
4. Building notice or self-certification: The work must be notified to building control (or self-certified under a Competent Person Scheme — heat pump installation under MCS is typically self-certified via a relevant scheme)

Fabric upgrades are NOT required: Part L1B does not require the homeowner to upgrade the building fabric (walls, roof, floor, windows) as a condition of installing a heat pump. However:

• The MCS heat loss calculation must show the heat pump is correctly sized for the actual heat loss
• If the fabric is poor, the design heat load is high, the design flow temperature may be elevated, and the BUS grant may require an EPC check (which may flag insulation improvements as required)
• Customers should be advised that improving insulation will improve SCOP and reduce running costs

Part L1A: New Dwellings

Heat pumps in new builds: Since the 2021 revision to Part L1A (effective June 2022), the TPER (Target Primary Energy Rate) for new homes is significantly more demanding. The notional specification that must be equalled or beaten now effectively mandates high-performance heating systems. Heat pumps are one of the primary routes to compliance:

• ASHP at SCOP 3.8 with UFH and low-temperature emitters → Part L compliant in most cases
• Gas boiler (condensing, 90% efficiency) → typically fails the new TPER for most new build configurations
• Heat pump + solar PV → comfortable compliance with TPER and carbon targets

SAP calculation at design stage: For a new dwelling, an accredited energy assessor must produce a SAP 10.2 calculation at design stage (Design Stage SAP, for Building Regulations) and at completion (As-Built SAP, for the EPC). The heating system specification is a key input.

Future Homes Standard: The Future Homes Standard (anticipated 2025–2026 implementation) will further tighten new build requirements, effectively mandating heat pumps or hydrogen-ready heating in all new dwellings. Heat pump installers and new build developers should monitor the final regulations.

EPC and BUS Grant: Practical Requirements

BUS pre-conditions: The BUS grant (£7,500 for ASHP/GSHP) requires:

1. A valid EPC for the property (issued within the last 10 years)
2. The EPC must not currently recommend loft insulation or cavity wall insulation (if it does, these must be installed before applying for BUS)
3. The property must not currently have a heat pump as its primary heating system

Effect of heat pump on future EPCs: Once the heat pump is installed, a new EPC should be obtained to capture the improvement in SAP score. The improved EPC is:

• Evidence of the property's improved energy performance (useful for sales/rental)
• The basis for the BUS "treated" property records
• Required for future BUS renewal (if the scheme requires re-application)

Frequently Asked Questions

My customer's EPC recommends cavity wall insulation. Does this mean they can't get the BUS grant?

Not permanently — it means cavity wall insulation must be installed before they can claim BUS. Cavity wall insulation is typically an inexpensive improvement (often available at reduced cost via ECO4 scheme). Once installed, a new EPC must be issued showing the recommendation has been completed, and the customer can then apply for BUS.

Does installing a heat pump require building control notification?

Under MCS certification (an accredited Competent Person Scheme), the installation is self-certified and the installer notifies building control on behalf of the customer. The customer does not need to make a separate planning application or building notice. The MCS installer handles the notification.

Will a heat pump always improve an EPC?

In most UK properties running on gas, yes — the lower CO2 emission factor of electricity under SAP 10.2 means a heat pump produces less CO2 per kWh of heat, improving the environmental rating. However, the EPC also considers the running cost per kWh of heat (the energy cost element). At current electricity/gas price ratios (~4:1), a heat pump may increase the estimated heating cost element of the EPC even while reducing the CO2 element. The overall SAP score is a combination of both. In practice, the EPC band usually improves.

Regulations & Standards

• Approved Document L1A (2021 edition) — new dwellings; TPER/TFEE targets for Part L compliance

• Approved Document L1B (2021 edition) — existing dwellings; requirements for heating system replacement

• SAP 10.2 — Standard Assessment Procedure; energy and CO2 calculations for EPCs and Part L compliance

• Future Homes Standard — forthcoming regulations; will tighten new build requirements further

• MHCLG Approved Document L — gov.uk — full text of Part L1A and L1B

• SAP 10.2 specification — gov.uk — SAP methodology and carbon factors

• DESNZ BUS guidance — gov.uk — BUS EPC requirements

• bus grant scheme guide — BUS eligibility including EPC pre-conditions

• mcs 007 heat pump standard — MCS self-certification and building control notification

• heat pump sizing heat loss — heat loss calculation and its relationship to Part L compliance

• heat pump in older properties — fabric improvements and their Part L and EPC effects