FHS Compliance Pathways: HEM vs SAP 10.3 Routes Explained

The Future Homes Standard will ultimately offer two compliance routes: the Home Energy Model (HEM) via the ECaaS platform, and SAP 10.3 as an interim alternative. Both routes use the notional building approach, comparing the proposed dwelling against a reference building, but each route has its own notional building specification with different fabric values, calibrated so that both achieve the same overall FHS performance standard.

The Notional Building Approach

FHS compliance is based on comparing the proposed dwelling (the actual home being designed) against a notional building (a reference dwelling). This approach has been used in Part L for years, but the FHS notional building specification is significantly more demanding.

How It Works

1. The notional building is automatically generated with the same size, shape, and orientation as the proposed home, but with standardised specifications for fabric, heating, ventilation, and renewable energy
2. Both the proposed dwelling and the notional building are assessed using the same calculation methodology (HEM or SAP 10.3)
3. The calculation produces target rates from the notional building (TER, TPER, TFEE) and dwelling rates from the proposed home (DER, DPER, DFEE)
4. The proposed dwelling passes if its dwelling rates are equal to or better than (i.e. lower than or equal to) all the target rates

FHS Notional Building Specifications

AD Part L 2026 points each compliance route to its own notional building specification. The HEM route uses values from the dwelling notional buildings companion document; the SAP 10.3 route uses Appendix R of the SAP 10.3 specification. The fabric values differ because the two calculation methods model performance differently, but both are calibrated to achieve the same overall FHS performance standard.

* Under HEM, window and door U-values in the notional building are calculated based on the actual dimensions of each opening rather than using a standard value. The HEM values shown are from the government's consultation response (March 2026); the final dwelling notional buildings document has not yet been published on GOV.UK. SAP 10.3 values are from Appendix R of the SAP 10.3 specification.

For detailed discussion of each fabric element, see our Part L Changes page.

Compliance Metrics

The FHS uses three primary compliance metrics. The proposed dwelling must meet or beat all three targets simultaneously:

Target Emission Rate (TER) and Dwelling Emission Rate (DER)

These measure carbon dioxide emissions in kg CO₂/m²/year. The DER must be ≤ the TER. This is the headline metric that drives the 75% carbon reduction.

The FHS assessment uses forward-looking carbon emission factors (averaging expected grid carbon intensity from 2025 to 2029) rather than the historical factors used in previous versions of SAP. This properly credits the trajectory of grid decarbonisation and gives electric heating systems (heat pumps) a more favourable carbon factor than under older SAP versions.

Target Primary Energy Rate (TPER) and Dwelling Primary Energy Rate (DPER)

These measure primary energy consumption in kWh/m²/year. Primary energy accounts for the full energy chain, including extraction, generation, and distribution losses. The DPER must be ≤ the TPER.

The primary energy metric prevents excessive reliance on any single energy source and ensures overall energy efficiency, not just low carbon.

Target Fabric Energy Efficiency (TFEE) and Dwelling Fabric Energy Efficiency (DFEE)

The TFEE/DFEE acts as a backstop for fabric performance, measured in kWh/m²/year. It ensures that compliance is not achieved solely through services (a large heat pump and extensive PV) while neglecting the building envelope. The DFEE must be ≤ the TFEE.

Backstop (Limiting) Values

In addition to the three performance metrics, individual fabric elements have backstop U-values that cannot be exceeded regardless of trade-offs elsewhere:

These backstop values are the maximum permissible U-values from AD L1 Table 3.1. To meet the target fabric energy efficiency rate, most elements need to perform significantly better than these limits.

The Two Compliance Routes

Route 1: HEM via ECaaS

The HEM route is the government's preferred long-term compliance methodology. Assessments are performed through the ECaaS (Energy Calculation as a Service) platform, a centralised cloud API run by MHCLG.

How it works:

1. The assessor enters dwelling data into a software interface built by an approved software provider
2. The software submits the data to the ECaaS API, which runs the HEM calculation using the Rust implementation of HEM
3. ECaaS returns the results, including dwelling rates (DER, DPER, DFEE), comparison against the notional building, and a pass/fail determination
4. The software displays the results and generates the compliance documentation

Advantages of the HEM route:

• More accurate modelling: half-hourly timesteps capture real-world performance of heat pumps, PV, and batteries
• Consistent results: every assessment uses the identical calculation engine, eliminating inter-software discrepancies
• Better credit for good design: orientation, thermal mass, solar self-consumption, and smart controls are properly rewarded
• Future-proof: HEM will be the sole compliance methodology once the dual period ends

Considerations:

• Data requirements are higher: significantly more detailed input data than SAP, including specific product make/model data
• Calculation time is longer: approximately under 2 seconds per calculation via ECaaS, comparable to SAP
• Internet connection required: the calculation is performed server-side via the API
• New workflow: assessors need to learn the new data requirements and interface

Route 2: SAP 10.3

SAP 10.3 is an updated version of the Standard Assessment Procedure that has been retained as an interim compliance route during the transition to HEM.

Advantages of the SAP 10.3 route:

• Familiar workflow: assessors who know SAP can use familiar tools and processes
• Faster turnaround: SAP calculations are near-instantaneous
• Lower data requirements: fewer input fields than HEM
• Existing software: SAP software providers can update existing tools to SAP 10.3

Limitations:

• Less accurate modelling: monthly timesteps cannot capture the real-world performance of heat pumps, PV self-consumption, or battery storage as precisely
• May be less generous: for well-designed homes with good orientation, battery storage, and smart controls, HEM may give more favourable results
• Temporary: the SAP route will eventually be withdrawn; the industry must transition to HEM
• Software inconsistencies: the multiple third-party SAP engine model continues, with its historical inter-software variability

HEM vs SAP 10.3: Side-by-Side

Choosing Your Compliance Route

The choice between HEM and SAP 10.3 depends on the project, the assessor's experience, and the building design:

Consider the HEM Route When:

• The design features heat pumps, solar PV, and/or battery storage, HEM models these more accurately and may give more favourable results
• The building has good orientation and glazing strategy, where HEM's half-hourly solar modelling rewards good passive design
• You want consistent, auditable results, since ECaaS guarantees identical calculations regardless of software provider
• You are building at scale and want to invest in learning the HEM workflow for long-term benefit
• The design includes innovative technologies that benefit from half-hourly modelling (smart controls, demand-side response)

Consider the SAP 10.3 Route When:

• The assessor is experienced with SAP and the project timeline is tight
• The design is straightforward: a standard house type with heat pump and basic PV, where the more detailed HEM modelling may not provide significant advantage
• Specific product data for all heating and ventilation equipment is not yet available in the PCDB for HEM
• The project is early in the transition and the team is still building HEM expertise

HEM Data Requirements

One of the most significant practical differences is the volume and specificity of data that HEM requires. Testing during the HEM pilot showed that a standard house type takes approximately 1 hour 40 minutes to input in HEM versus around 20 minutes in SAP (excluding geometry and U-value calculations).

New data points required by HEM include:

• Site altitude and noise nuisance potential
• Detailed geometry, more precise than SAP requires
• Individual hot water outlet specifications including tap flow rates, shower types (mixer, electric, thermostatic)
• Detailed pipework with lengths and insulation for all hot water distribution
• Specific product data: make and model for all heating and ventilation equipment, referenced against the PCDB
• PV inverter specifications including efficiency curves
• Thermal bridge details: psi-values for each junction, or accredited construction detail references

Frequently Asked Questions

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