The Future Homes Standard is shifting compliance from basic carbon box-ticking to a performance-led specification approach driven by the Home Energy Model (HEM) and SAP 10.3 transitional logic. For developers, the key question is simple:
What do I need to build into my specification today, so I don’t get caught by redesign costs when Future Homes Standard enforcement hits?
This guide outlines exactly what your design specification should look like if you want planning approval, energy compliance and build cost certainty under the new regime, not just under outgoing SAP methods.
The Future Homes Standard Specification Standard You Should Be Designing to Now
If you want to avoid redesign and cost uplift when HEM becomes active, here is the minimum viable specification profile we recommend:
| Element | Specification Standard That Survives HEM Compliance |
|---|---|
| Heating System | Low-temperature heat pump system designed for 35–45°C flow. Emitters must be sized for low-flow operation, legacy 55–60°C radiator sizing will fail efficiency modelling. |
| Ventilation Strategy | MVHR or hybrid mechanical ventilation designed alongside airtightness. Future-proof values: 3 m³/h·m² @50Pa or better, with balanced supply/extract and defined free area logic. |
| Airtightness | Treat airtightness as a performance system, not a target. HEM models infiltration continuously, and uncontrolled leakage erodes performance each half-hour timestep. |
| PV and Battery Planning | PV cannot be a late bolt-on. The model evaluates on-site use vs export, so battery and inverter space allocation must be shown in early plans. |
| Plant Space Provision | Early design drawings must allow heat pump unit, hot water cylinder, buffer vessel, expansion vessel, PV inverter and battery pack space. Under ECaaS modelling, missing plant volumes cause design failures. |
| Overheating & Part O Strategy | Overheating is not a separate report, it feeds directly into energy balance. Solar gain, shading, ventilation purge profiles and glazing orientation must be planned in RIBA Stage 2–3, not left for post-submission adjustment. |
| Controls & Smart Load Strategy | HEM evaluates system behaviour at 30-minute intervals. Systems must allow modulation, setback control and flexible load response, not just simple on/off boiler profiles. |
Why These Specs Matter - The HEM Engine Logic
The Home Energy Model is not an evolution of SAP, it is a physics-based simulation engine structured as follows.
Within each zone, building elements are modelled using node-based thermal structures. Opaque elements like walls and floors are typically represented using multi-node profiles to capture heat storage and release through the fabric over time, while glazing uses a simplified node representation to track transparent heat gains and losses.
Unlike SAP, which resets thermal conditions monthly, HEM runs continuously at 30-minute intervals, carrying forward thermal mass effects from one timestep to the next. This means a poor overheating strategy or undersized emitter will not just affect a compliance number, it will lead to visible thermal drift in the model, making failures far harder to hide.
The engine then layers in dynamic system behaviour:
- Heating and hot water systems are simulated live, responding to setpoints and demand rather than applying a fixed efficiency.
- Heat pump performance is plotted against part-load curves, so oversized or poorly matched emitters trigger cycling penalties.
- PV generation is split into used vs exported energy, meaning a dwelling that exports most of its generation will score weaker than one designed around self-consumption with battery storage.
- Internal gains and solar gains are applied by timestep, using real weather position data and occupancy profiles, not static monthly coefficients.
This is why the Future Homes Standard cannot be treated as a compliance document exercise. It is a behavioural simulation, and only well-planned specifications survive that level of scrutiny.
SAP 10.3 - The Warning Shot Before HEM
Before HEM becomes enforced, SAP 10.3 acts as a transitional step. It tightens:
- CO₂ emission factors and grid carbon intensity assumptions
- Heat pump performance treatment using realistic seasonal curves
- PV and self-consumption credit logic
- Rejection of generic “default” specification values
- Data validation requirements before submission
At Energy Digest, we already model all new schemes to SAP 10.3-aligned assumptions, even if submission is under SAP 10.2. This ensures no future redesign shock when the FHS model becomes mandatory.
Design Once, Pass Once: The Energy Digest Future Homes Standard Framework
When we work with architects and developers at concept stage, our goal is not to scrape a pass under current SAP rules. Instead, we shape the specification so it is already aligned with Future Homes Standard performance logic. That means we size emitters for true low-temperature heat pump operation, not just theoretical COP figures; we optimise roof PV layout and battery integration early, treating generation as part of the energy balance rather than a late-stage carbon offset. We combine ventilation and overheating strategy into one continuous airflow and purge logic, ensuring that what satisfies Part O also supports heating and cooling balance under HEM simulation.
Rather than leaving plant decisions to compliance stage, we allocate physical space for heat pumps, cylinders, inverters and storage equipment within the architectural layout, so these systems do not become bolt-ons that trigger redesign later. Finally, our models are structured in an ECaaS-ready format, so once centralised API submission replaces desktop SAP, your design data transfers cleanly without rejection or rework.
Why Developers Appoint Energy Digest Early
Time and again, compliance issues do not arise because a heat pump or PV array was the wrong choice, they arise because the design team did not plan around how the Home Energy Model actually evaluates performance. By the time most assessors get involved, specifications are fixed, plant rooms are drawn, and airtightness targets have been assumed without considering half-hourly energy balance modelling. That is when redesign costs start accumulating.
Our approach prevents that entirely. We embed Future Homes Standard thinking at RIBA Stage 2–3, so your development is engineered to satisfy tomorrow’s assessment engine, not just today’s paperwork. If you want a dwelling that passes now and still passes once HEM becomes mandatory, the only sustainable route is to design for the Future Homes Standard from the outset, not retrofit SAP compliance afterwards, and that is exactly how Energy Digest works.
