FHS and HEM now have a clear path
The government has now published both the Future Homes and Buildings Standards response and the Home Energy Model: Future Homes Standard assessment response.
Together, the two responses give both the Future Homes Standard and the Home Energy Model a clear delivery path. FHS comes first, with SAP 10.3 as the launch method. HEM follows shortly after, with government making clear that SAP 10.3 will only be temporarily available while the sector transitions towards working solely with HEM.
That matters because the industry is not preparing for one change, but two. It will first need to deliver under a tougher new-build standard, then adjust again as HEM comes into use and SAP 10.3 is phased out.
The responses also make clear that although HEM and SAP use comparable notional buildings, they will not always return the same compliance result. They also show why HEM is a more detailed and more useful basis for understanding building performance in practice.
The consultation response arrives roughly two years after the consultation itself. Vulcan has used the period since consultation to prepare for HEM properly, and to make HEM workflows fast and easy to use, as we argued in Faster beats familiar.
That is why we think HEM should be adopted quickly once it is ready, and actively prepared for before then.
The short version
The Future Homes Standard will come into force on 24 March 2027 for non-higher-risk building work, subject to transitional provisions. Projects already sufficiently advanced in the Building Control process may still proceed under the previous Part L, provided the relevant documents are submitted by 24 March 2027 and construction begins by 24 March 2028, unless other transitional provisions apply.
For new dwellings, government has confirmed the more demanding consultation benchmark for the notional dwelling - Option 1 - but changed the way solar is handled. Instead of leaving PV only inside the notional benchmark, it has created a separate requirement for on-site renewable electricity generation. That reduces the scope to trade away solar purely through better performance elsewhere.
The FHS requirement will generally be met where solar panels cover 40% of a dwelling's ground floor area, while still allowing lower provision where reasonable. The response explicitly says it does not intend to force simplistic roofscapes, and says no panels may be appropriate where the available roof space cannot reasonably generate 720 kWh/year. Buildings with a storey 18m or more above ground containing dwellings are also exempt.
The main compliance metrics stay familiar: TER, TPER and FEE remain. Delivered energy is added only as a voluntary reporting metric, via the Building Regulations England Part L (BREL) report where HEM is used.
There are other changes: the response removes the terrace averaging approach, so each dwelling must meet TER, TPER and FEE in its own right. The Approved Documents now set out two routes for dwellings and buildings connected to new or existing heat networks. It also tightens expectations around commissioning, handover and homeowner information, including Regulation 40C and use of competent person schemes for heat pump and mechanical ventilation installations.
For methodology, the rollout is staged. SAP 10.3 will be the sole approved method first. HEM will follow no earlier than 3 months later. Once HEM is available, both methods will run in parallel for at least 24 months. The government expects to give six months' notice before SAP 10.3 stops being available for filing Building Control plans for FHS compliance.
Six points that matter
1. SAP 10.3 is transitional
The response makes clear that SAP 10.3 is transitional only. It says SAP 10.3 will be available at FHS launch, while HEM is expected to become an approved methodology no earlier than 3 months after publication of the response. It also explains why: government says the HEM ecosystem would not be completed and validated to a sufficient standard by FHS launch for it to be available in the lead-in period before the standard comes into force.
Even so, the Government response says HEM remains the methodology of choice for new dwellings, with SAP 10.3 only temporarily available during dual-running and HEM intended to become the sole method at the earliest opportunity after at least 24 months.
Vulcan take: a longer transition makes early HEM capability more valuable, not less. It increases the gap between firms that prepare early and those that do not. Firms that treat SAP 10.3 as the only thing worth preparing for are likely to end up doing the transition twice.
2. HEM and SAP results will diverge
One of the most revealing passages in the FHS response is buried in the explanation of the notional dwelling, the benchmark home used to set the compliance target. Government says HEM can create a heat pump optimally sized to the dwelling, whereas SAP cannot readily include that functionality. So the SAP notional heat pump is fixed at a coefficient of performance (COP) of 2.5 for all dwellings.
Vulcan take: anyone assuming SAP 10.3 results will translate cleanly into HEM later is taking a risk. The HEM response also says government reviewed HEM's air source heat pump modelling against prior testing and known performance trends. HEM should ultimately provide the more credible baseline for heat pump-led design, with efficiency emerging from the model rather than being baked in as a flat assumption.
3. Weather is standardised for consistency
The HEM response confirms a single standardised weather file for England during FHS compliance, explicitly to maintain consistency during simultaneous running with SAP 10.3. It also explicitly notes that this reduces the accuracy of estimated energy consumption and can affect the apparent optimal sizing of heating systems. Government says it will continue exploring location-dependent weather for future HEM uses.
Vulcan take: this is understandable for compliance, with a standard weather file useful for comparability, but it does compromise design insight. It provides a weaker basis for estimating bills, comfort and heat pump sizing. This means that in practice, a compliance run and a design run may need different weather assumptions.
4. Unmet demand is still a real difference
Government has softened the treatment of unmet demand - cases where the model says the heating system cannot fully meet the required load - after accepting that the consultation version was too punitive. HEM:FHS still calculates unmet demand explicitly and keeps it within the Part L compliance logic. SAP assumes the dwelling is fully heated and, where the specified system is not enough, fills the gap with assumed secondary electric heating rather than surfacing unmet demand as a separate result. Government has now reduced the HEM penalty so it is less likely to distort results or encourage oversized systems, but this remains a real methodological difference between HEM and SAP.
Vulcan take: Unmet demand is one of the places HEM shows its value. HEM makes system adequacy more visible, especially under peak conditions, rather than smoothing everything into an annual result. The consequence: borderline designs that look acceptable in SAP may be revealed to be insufficient in HEM.
5. The wrapper now matters almost as much as the engine
The HEM response answers criticism that the consultation version had overly complex inputs. Government has undertaken a comprehensive simplification exercise. Pipework lengths are inferred from building dimensions and tapping points. Detailed emitter parameters are replaced with representative products-database models. Some lower-impact inputs are defaulted, while other technical inputs are reduced to simpler multiple-choice selections such as solar absorption bands and thermal-mass categories.
The FHS wrapper also standardises the calculation period and timestep, internal gains, heating and cooling schedules, cold water temperatures, hot water draw-off assumptions, and post-processing factors such as emissions and primary energy.
Vulcan take: the wrapper now does much of the regulatory work: it decides what is fixed, inferred, simplified or exposed. That means it increasingly shapes not just the calculation, but the workflow, the evidence burden and the QA burden. The same response also says the wrapper supplements the core engine's reporting, while the core calculation still produces detailed half-hourly results files. That makes running the compliance wrapper and the core outputs side by side genuinely useful: one for a standardised FHS result, the other for deeper diagnosis, design insight and QA.
6. HEM is still expanding
First, government is putting more weight on validation. It says HEM is a valid building physics engine for the purposes of FHS, backed by comparison against ESPr for infiltration and ventilation and review of air source heat pump modelling against prior testing and known trends. It also says validation is ongoing, and that more real-world testing is needed. Open-sourcing the core engine and FHS wrapper makes that scrutiny and contribution easier.
Second, government is pointing toward a closer relationship between modelled and measured performance. The wider FHS response says it intends to publish a call for evidence on the energy performance gap and building performance evaluation, focused on cost-effective and non-intrusive post-construction or post-occupancy testing.
Third, government says work is ongoing on issues such as time-varying grid factors in Part L compliance. This points toward a more dynamic treatment of energy over time, even if the compliance method is not there yet.
Finally, the Part O section on overheating shows ongoing dissatisfaction with current methods. Some respondents pointed to HEM as an alternative integration route and called for better alignment with TM59, the standard dynamic overheating assessment method, and future changes to Approved Document O. Government has also indicated there will be a broader review of Part O. Given that HEM can output internal temperatures directly, it is an obvious candidate to support more integrated overheating assessment in future.
Vulcan take: HEM is not just replacing SAP as a compliance calculator. It is creating a platform for more granular outputs, better post-processing and eventually a tighter relationship between modelled and measured performance. That is one reason we built H3: if policy is moving toward performance-gap evidence and more serious validation, the industry will need practical ways to compare HEM outputs with measured reality.
Final thought
The government has confirmed that the industry is moving towards a more detailed and more demanding form of energy modelling, while trying to cushion the transition through staged rollout, wrapper simplifications and temporary coexistence with SAP 10.3.
That does not reduce the importance of HEM. It increases it. Firms that treat HEM as a side issue will still be catching up when SAP is finally gone.