Home Energy Model: What’s Changed for the FHS

The Home Energy Model (HEM) is the government’s eventual replacement for SAP – but “eventual” is doing real work in that sentence. HEM’s own launch has slipped more than once, most recently on 8 June 2026, and it now sits on a genuinely separate timeline from the Future Homes Standard’s confirmed 24 March 2027 in-force date. For housebuilders and assessors, that distinction matters: SAP 10.3 is what you’re using for FHS compliance right now, and will be for some time after the Standard itself takes effect.

Updated August 2026: HEM’s launch was delayed again on 8 June 2026. SAP 10.3 remains the sole approved methodology for Future Homes Standard compliance, with HEM now expected “in the coming months.”

In this blog you’ll learn:

• What the Home Energy Model actually is and how its half-hourly modelling differs from SAP’s monthly calculation
• The current, confirmed status of HEM’s rollout as of August 2026 – separate from the FHS in-force date
• The new categories of data HEM requires, and why missing data carries more commercial risk under HEM than it ever did under SAP
• A worked example of a standard house type that passes comfortably under SAP 10 but fails outright under HEM, and what closed the gap

What is the Home Energy Model?

The Home Energy Model (HEM) is the calculation methodology intended to eventually replace SAP for both Building Regulations compliance and EPC generation. Where SAP works from monthly averages, HEM runs a half-hourly simulation – 17,520 timesteps a year – to build a genuinely dynamic picture of how a home performs, rather than a monthly snapshot.

HEM’s Current Status – and Why It’s Not Tied to the FHS Launch Date

It’s easy to assume HEM will simply be ready by the time the Future Homes Standard comes into force on 24 March 2027. That’s not how the two timelines actually relate.

The Future Homes Standard’s Approved Documents, published 24 March 2026, confirmed SAP 10.3 as the sole approved compliance methodology at launch – meaning HEM isn’t required, or expected, to be available on 24 March 2027 itself. HEM’s own approval runs on a different clock:

• HEM was expected to become available no earlier than three months after the 24 March 2026 Approved Documents – a window that opened around June 2026
• On 8 June 2026, the government delayed HEM’s launch during final internal assurance; as of this update, it’s expected “in the coming months,” with no fixed date confirmed
• Once HEM is approved, a minimum 24-month dual-running period begins, during which both SAP 10.3 and HEM are valid for FHS compliance
• SAP 10.3 will then be withdrawn, with the government committing to six months’ notice before that withdrawal date is set

In short: SAP 10.3 isn’t a short bridge to be used for a few months before FHS arrives – it’s the tool you’ll be using both before and for some time after the Standard itself comes into force, until HEM launches and the 24-month dual-running window closes.

HEM vs. SAP 10.3

Beyond the calculation frequency, the practical difference for anyone specifying or modelling a home is the sheer increase in the data HEM requires. Many of the new inputs need specialist evidence from across the project team, not just the assessor’s usual sources.

External factors and building fabric

• Dwelling altitude and potential for noise nuisance, which may require a formal noise impact assessment
• Detailed distant shading data from nearby buildings or trees, and the solar absorption coefficient of external finishes based on their colour and material
• Window g-values, frame factors and precise openable areas for overheating calculations – plus the presence and thermal resistivity of internal doors, blinds and curtains

Heating, hot water and ventilation

• For wet heating systems: the thermal mass of the entire distribution system, design flow temperature, flow rates and the specific Ecodesign control class (Class I to VIII)
• Flow rates for basin taps, kitchen taps and baths (not just showers, as under SAP), plus bath volume in litres
• Full pipework specifications for both primary and secondary runs, including internal and external diameters, insulation thickness and thermal conductivity

Renewables and energy storage

• Battery storage capacity, age, charge/discharge efficiency, location and maximum/minimum charge and discharge rates
• Solar PV panel dimensions, array ventilation strategy (roof-integrated versus on-roof) and detailed inverter specifications

Two further differences worth knowing about, beyond the data burden itself:

Regional, not UK-average, climate data. HEM drops SAP’s single UK-average weather assumption in favour of 16 localised regional climate datasets. A house type that clears compliance in Bristol can fail the same assessment in a colder or wetter region – so a “copy-paste” standard house type across a multi-region portfolio can no longer be assumed to pass everywhere it currently does.

Tighter notional air permeability. In BEE’s own testing against the HEM beta, the notional air permeability target drops further than under SAP 10.3 alone – from 4.0 m³/(h·m²) at 50 Pa under the interim SAP 10.3 notional building, to 3.0 m³/(h·m²) under HEM.

Is the Home Energy Model Actually More Accurate Than SAP?

It’s a fair question to ask of any tool still in beta: is HEM genuinely more accurate, or just more complicated? The government’s own Home Energy Model consultation sets out the validation work carried out so far, testing HEM against both established modelling tools and real, monitored homes.

On the modelling side, HEM was benchmarked against PHPP and ESP-r – two well-established building energy models – across a set of archetype dwellings, and showed good alignment with both on space heating demand and internal temperature. On the real-world side, HEM was tested against monitoring data from occupied and unoccupied homes, including two test houses from the International Energy Agency’s Annex 58 project, the Camden Passivhaus and three homes at the Marmalade Lane development in Cambridge. At Marmalade Lane specifically, once calibrated to each dwelling, HEM tracked the measured data consistently more closely than SAP 10.2 did over the same comparison.

None of this makes HEM a finished product – the government’s own documentation flags plenty of areas still under development – but it’s a reasonable answer to “why bother,” backed by primary evidence rather than just the promise of a more detailed methodology.

The Commercial Risk of Missing Data Under the Home Energy Model

In SAP, a missing data point usually meant a neutral default – a minor inconvenience. Under HEM’s ECaaS engine, missing inputs default to the worst-performing equipment plausible on the market. That can push a genuinely borderline design into an outright compliance failure, triggering a late-stage redesign that a slightly more organised data room would have avoided.

There’s a real time cost too: in our own testing, a standard house type that takes around 20 minutes to model in SAP 10.2 took roughly 1 hour 40 minutes in HEM – in the same order as the wider 5x increase in modelling time we’ve seen quoted elsewhere for HEM’s 200-plus structured data parameters. [EDITOR NOTE: both figures are our own internal testing, from slightly different points in HEM’s beta – worth a quick check with Ryan before republishing on which is the more current estimate, since ECaaS has moved from limited beta toward wider access since these were first recorded.] Combined with the calculation lag on HEM’s cloud engine itself – often 5–10 minutes per run – the assessor workload increases substantially, and fee structures are likely to follow.

A Worked Example: Same House Type, SAP Pass, HEM Fail

The clearest way to see what’s changed is to run one design through both methodologies. Testing a standard 120m² detached house type – a typical 2021 Part L specification, air permeability 4.0, an air source heat pump at 45°C flow temperature, dMEV ventilation, and a 2.7 kWp solar PV array – against both tools produced two very different verdicts:

• Under SAP 10: the design passed comfortably, with a Dwelling Emission Rate roughly 78% better than the Target Emission Rate.
• Under the HEM beta: the identical specification failed outright, missing the target by over 23%.

The gap came down to two things HEM captures that SAP doesn’t. First, HEM evaluates the heat pump’s efficiency dynamically against half-hourly regional weather data rather than a static seasonal average – so cold snaps show up directly as an efficiency penalty. Second, HEM uses updated, forward-looking carbon and primary energy factors for a decarbonising grid, which recalculates how grid electricity and on-site generation interact.

The fix, in this case, wasn’t more insulation – the fabric had already cleared its own checks. It was a modest uplift in solar PV, from 2.7 kWp to 4.5 kWp, which brought the design back into compliance. On a wider portfolio test of the same house type, the required uplift varied noticeably by orientation: a south-facing plot needed roughly 4.2 kWp of additional array to clear the target, while an identical house type facing north needed around 5.3 kWp – meaning orientation is no longer a cost-neutral decision once solar sizing is treated as a compliance requirement rather than an optional extra.

What’s Next?

HEM is still coming, but not on the same clock as the Future Homes Standard itself. The practical step now is the same either way: get your data room in order – shading, glazing specs, pipework, inverter details – so that whichever methodology you’re modelling against, missing information isn’t the thing that costs you a redesign.

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Future Homes Standard Modelling

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We’ve undertaken Future Homes Standard Modelling for a number of housebuilders already (Cotswold Homes, Woodstock Homes, Acorn Property Group) – if you’d like your house types modelled against SAP 10.3 now, with an eye on what HEM will eventually ask for, our team can talk you through the practical steps.