The Future Homes Standard offers two compliance routes during the transition period: SAP 10.3 (an updated version of the familiar SAP methodology) and HEM (the government's new half-hourly dynamic simulation). Both routes assess the dwelling against the same notional building specification, but the underlying calculations differ. This page compares the two routes to help assessors, architects, and developers choose the right approach for their projects.
Choose your compliance route
Answer a few questions to find out whether SAP 10.3 or the Home Energy Model is the right choice for your project.
Step 1: What is your project timeline?
Side-by-Side Comparison
| Aspect | SAP 10.3 | HEM |
|---|---|---|
| Calculation method | Monthly steady-state | Half-hourly dynamic simulation (17,520 timesteps/year) |
| Notional building source | SAP 10.3 Appendix R (same specification, e.g. walls 0.18 W/m²K) | Dwelling notional buildings document (same specification, e.g. walls 0.18 W/m²K) |
| Software delivery | Third-party SAP software providers | Centralised ECaaS API |
| Calculation speed | Near-instantaneous | Under 2 seconds per calculation (via ECaaS) |
| Assessment time (typical house) | Baseline | Reported at around 3x SAP by some consultation respondents |
| Heat pump modelling | Fixed seasonal efficiency (COP) | Dynamic COP at each timestep based on conditions |
| Solar PV | Annual generation estimate | Half-hourly generation + self-consumption modelling |
| Battery storage | Limited modelling | Full charge/discharge at each timestep |
| Thermal mass | Simplified parameter | Dynamic modelling at each timestep |
| Thermal bridges | Annual uplift factor | Calculated at each timestep |
| Carbon factors | Forward-looking (same as HEM, e.g. 0.086 kgCO₂/kWh for grid electricity) | Forward-looking (same as SAP 10.3, e.g. 0.086 kgCO₂/kWh for grid electricity) |
| Missing data handling | Standard SAP defaults | Standardised defaults; heating shortfall priced as direct electric |
| Assessor familiarity | High (existing workflow) | Low (new methodology and platform) |
| Long-term status | Interim (will eventually be withdrawn) | Primary (government's preferred methodology) |
SAP 10.3: The Familiar Route
SAP 10.3 is an updated version of SAP carrying the FHS notional dwelling specification (Appendix R), revised carbon emission factors, and updated primary energy factors. It retains SAP's monthly calculation approach and is delivered through existing third-party software providers.
Advantages
- Familiarity: assessors already know the SAP workflow, data entry process, and software
- Speed: calculations are near-instantaneous, allowing rapid iteration on designs
- Lower data requirements: SAP requires less detailed input data than HEM
- Established software: existing SAP software is mature, with well-understood interfaces and workflows
- No ECaaS dependency: assessments can run locally without relying on the ECaaS platform's availability
Limitations
- Less accurate technology modelling: heat pumps, solar PV, and battery storage are modelled less precisely than in HEM
- Monthly resolution: cannot capture within-day or within-month performance variations
- Self-consumption is a fixed factor: the share of solar PV electricity used on-site comes from a standard self-use factor rather than from modelled demand
- Interim status: will eventually be withdrawn; investing heavily in SAP 10.3 workflows is a short-term strategy
- Software inconsistency: different providers may produce marginally different results
HEM: The Future-Proof Route
HEM is the government's preferred long-term compliance methodology. It uses a half-hourly dynamic simulation based on BS EN ISO 52016-1:2017, delivered exclusively through the ECaaS platform.
Advantages
- More accurate modelling: half-hourly timesteps capture real-world performance, particularly for heat pumps, solar PV, and thermal mass
- Solar PV self-consumption: properly credits electricity used on-site, not just total generation
- Battery storage: full charge/discharge modelling at each timestep
- Dynamic heat pump COP: models efficiency based on actual source and sink temperatures at each timestep
- Consistency: every assessment uses the identical calculation engine via ECaaS
- Future-proof: HEM is the government's long-term methodology; early adoption builds capability
- Design flexibility: more accurate modelling may reveal compliance margin that SAP misses, enabling design optimisation
Challenges
- Longer assessment time: some respondents to the FHS consultation reported HEM assessments taking around three times as long as the equivalent SAP assessment
- More data required: detailed geometry, individual hot water outlets, specific product data, inverter specifications
- Calculation time: under 2 seconds per calculation via ECaaS, comparable to SAP
- Data-hungry: missing data falls back on standardised defaults instead of real product performance
- ECaaS dependency: requires internet connectivity and platform availability
- Learning curve: new methodology, new platform, new data requirements
Choosing the Right Route
The choice between SAP 10.3 and HEM depends on your circumstances. Here is practical guidance for different situations:
| Situation | Recommended Route | Reasoning |
|---|---|---|
| Standard house types with clear compliance margin | SAP 10.3 | Faster, familiar, no additional training needed |
| Complex designs with heat pumps + solar PV + battery | HEM | More accurate technology modelling may reveal compliance margin |
| Team not yet trained on HEM | SAP 10.3 (short-term) | Pragmatic while building HEM capability |
| Long-term capability building | HEM | Start building experience now; SAP 10.3 will be withdrawn |
| Marginal compliance designs | Try both | One route may pass where the other doesn't |
| High-volume production plots | SAP 10.3 initially, then HEM | SAP 10.3 for speed; transition to HEM as teams upskill |
Data Requirements Compared
HEM requires significantly more detailed input data than SAP 10.3. The additional data points include:
| Data Category | SAP 10.3 | HEM |
|---|---|---|
| Building geometry | Floor area, room count, orientation | Detailed 3D geometry with zone-level resolution |
| Hot water | Total demand from floor area | Individual outlet specifications (taps, showers, baths) |
| Pipework | Basic distribution assumptions | Detailed lengths and insulation for each run |
| Heating products | Product type and basic efficiency | Specific make/model with EN 14825 performance data |
| Ventilation | System type and basic performance | Specific product data, duct lengths, terminal details |
| Solar PV | Panel area, orientation, tilt | Panel specifications + inverter make/model/efficiency |
| Site data | Region and basic orientation | Noise nuisance potential, detailed shading |
The additional data requirements are the primary reason HEM assessments take longer. However, they also enable more accurate modelling, particularly for heating systems and renewable technologies. For detailed preparation guidance, see our Assessor Transition Guide.
Practical Tips for the Transition
- Run parallel assessments early on. Submit a few designs through both routes to understand how results compare for your typical house types
- Invest in data collection. Good data is essential for HEM. Establish processes to capture the additional data points from designers, manufacturers, and site teams
- Build product data libraries by collating make/model data for your standard heating, ventilation, and PV specifications
- Prepare for more detailed data entry. While HEM calculations run in under 2 seconds via ECaaS, the additional data input requirements mean each assessment takes longer overall
- Engage with ECaaS and familiarise your team with the platform's API and workflow before it becomes mandatory
- Use SAP 10.3 as a backstop. If an HEM assessment fails unexpectedly, SAP 10.3 provides an alternative route during dual running
Frequently Asked Questions
Should I use SAP 10.3 or HEM for FHS compliance?
It depends on your team and project. SAP 10.3 is pragmatic for teams not yet trained on HEM. It is familiar, faster, and uses existing software. HEM provides more accurate modelling and may reveal compliance margin or design flexibility. Both routes assess the dwelling against the same notional building specification.
Do SAP 10.3 and HEM give the same compliance result?
Not necessarily. The notional building specification is the same on both routes, but the calculation methods differ, so results can diverge. A dwelling that marginally passes on one route may marginally fail on the other, particularly for designs relying on heat pumps and solar PV where HEM's dynamic modelling is more accurate. Most well-designed FHS homes will pass on both routes.
When will SAP 10.3 stop being accepted for FHS compliance?
SAP 10.3 has a guaranteed minimum lifetime of 24 months from when the statutory instrument is laid, with six months' notice before withdrawal. HEM was due no earlier than three months after the March 2026 consultation response, but in June 2026 its launch was pushed back to the coming months. The exact withdrawal date depends on when the statutory instrument is laid.
Can I switch between SAP 10.3 and HEM during a project?
Each assessment uses one methodology, and the consultation response says either may be used during dual running. Because assessments are per dwelling, in practice teams expect to use SAP 10.3 for standard house types and HEM for complex designs, but the response does not set out per-plot mixing rules or when the choice must be made.
Is it harder to pass compliance with HEM than SAP 10.3?
Not inherently, but results may differ. The notional fabric is the same on both routes; the difference is in the modelling. HEM models heat pumps and solar PV more realistically (dynamic COP, self-consumption) than SAP 10.3's monthly averages. Well-designed FHS homes pass comfortably on both. Designs relying on generous SAP assumptions may find HEM less forgiving.
Related Pages
FHS Compliance Pathways
Full guide to the notional building approach and compliance metrics.
SAP vs HEM: Methodology Comparison
Broader comparison of SAP and HEM methodologies beyond FHS compliance.
For SAP Assessors
What the transition to HEM means for energy assessors.
Assessor Transition Guide
Detailed preparation guide for the move from SAP to HEM.