Home Homeowners Architects & Developers JV Partnership Services Extensions & Renovations Materials PHPP Portfolio Calculators Blog Free Guides About Project Room Contact
← All articles

The Home Energy Model: what's actually replacing SAP, and when

By Paweł Okurowski Published 19 June 2026 ~6 min read

The UK is replacing SAP — the 30-year-old method for calculating a home's energy performance — with the Home Energy Model. But there is a gap between the headline and the reality. When the Future Homes Standard comes into force in March 2027, the Home Energy Model will not be the methodology used to demonstrate compliance. Here is what the published government documents actually say.

A newly built energy-efficient house at golden hour with rooftop solar PV panels and an air-source heat pump, brick and timber cladding, in the rural West Midlands
The shape of a Future Homes Standard new build — rooftop solar PV, an air-source heat pump and no gas boiler. The methodology that signs it off is the question this article answers.
Figure 1: The Future Homes Standard compliance-methodology timeline, 2025 to 2030 A timeline drawn from the March 2026 Future Homes and Buildings Standards package. The Future Homes Standard package was published on 24 March 2026. On 24 March 2027 the Future Homes Standard comes into force, with SAP 10.3 as the sole approved compliance methodology at launch. After launch the sequence is: SAP 10.3 only, then the Home Energy Model is approved as an alternative on a date still to be confirmed, then SAP 10.3 and HEM run in parallel for no less than 24 months, after which SAP 10.3 is withdrawn and HEM becomes the sole methodology. Separately, reformed HEM-based EPCs are expected from around 2027, replacing SAP and RdSAP, with the current rating retained until at least the end of 2029. What actually assesses your new home, and when The Future Homes Standard compliance methodology — from the March 2026 government package Future Homes Standard in force → 2025 2026 2027 2028 2029 2030 24 March 2026 FHS package published 24 March 2027 · FHS in force SAP 10.3 = sole methodology SAP 10.3 only at launch → HEM approved date to be confirmed → SAP + HEM dual-run ≥24 months, then SAP out On the EPC side: the "Home Energy Model: Energy Performance Certificates" consultation ran 21 Jan – 18 Mar 2026. Reformed HEM-based EPCs are expected from c.2027 (replacing SAP/RdSAP); the current rating is retained until at least end-2029. Dates from GOV.UK, MHCLG and DESNZ. Items marked "to be confirmed" or "expected" are indicative and may move.
Figure 1 — The compliance sequence, drawn from the March 2026 Future Homes & Buildings Standards package and related GOV.UK / MHCLG / DESNZ publications. Items marked "to be confirmed" or "expected" are indicative.

If you have read anything about UK building regulations in the last year, you will have seen the claim that "SAP is being replaced by the Home Energy Model". That is true — but the timing is widely misunderstood. A lot of people now believe that from March 2027, new homes will be assessed using the Home Energy Model. They will not. The government confirmed a different sequence in March 2026, and if you are planning a new build, an extension or a deep retrofit, the distinction matters.

SAP 10.3 is currently available as the sole approved methodology. — Department for Energy Security and Net Zero, GOV.UK Standard Assessment Procedure guidance (June 2026)

What SAP is, and why it's being replaced

The Standard Assessment Procedure (SAP) has been the UK government's method for assessing the energy performance of dwellings since 1993. It is the engine behind your EPC and behind Part L compliance for new homes. Its core limitation is structural: it works on monthly averages. It takes a year, splits it into twelve blocks, and calculates an average for each. That was a reasonable simplification in 1993. It is a poor fit for a home with a heat pump, solar panels, a battery and smart controls, where the value of the technology depends on what happens hour by hour — when the sun shines, when the grid is clean, when the home actually draws heat.

The Home Energy Model (HEM) is the government's replacement. Instead of twelve monthly averages, it runs a half-hourly simulation across 17,520 timesteps per year, built on the international standard BS EN ISO 52016. It models heat pumps, solar PV, batteries and ventilation dynamically, and its codebase is open source — published in October 2025 — so the industry can scrutinise it.

Figure 2: SAP's 12 monthly timesteps versus the Home Energy Model's 17,520 half-hourly timesteps Two panels comparing resolution. On the left, SAP 10.x is shown as twelve monthly bars forming a coarse seasonal heating curve — one value per month. On the right, the Home Energy Model is shown as a dense field of fine vertical lines with a jagged half-hourly demand trace over the top, representing 17,520 timesteps per year, which is 365 days times 48 half-hours. The same year is modelled at roughly 1,460 times the resolution. Same year, very different resolution How SAP and the Home Energy Model slice a year of a home's energy use SAP 10.x 12 monthly timesteps JFMAMJJASOND One averaged value per month Home Energy Model 17,520 half-hourly timesteps 365 days × 48 half-hours = 17,520 12 averages versus 17,520 simulated half-hours — the same year at roughly 1,460× the resolution.
Figure 2 — The difference in resolution: 12 monthly steps under SAP versus 17,520 half-hourly steps under HEM.

The part most people get wrong: SAP 10.3 is the methodology at launch

Here is where the headline and the reality diverge. On 24 March 2026, the Ministry of Housing, Communities and Local Government (MHCLG) published the final Approved Documents for the Future Homes Standard, alongside Building Circular 01/2026. The regulations come into force on 24 March 2027, with a 12-month transitional period for projects already in the pipeline.

But the Home Energy Model is not ready to be the compliance methodology at that point. In its own words, the government decided "in the final stages of internal assurance" to delay HEM's launch so that it goes live "as robust as possible". The consequence is straightforward:

  • At FHS launch (24 March 2027): SAP 10.3 — an updated version of SAP that BRE maintains for the government — is the sole approved methodology for demonstrating compliance.
  • HEM follows later: the government will only approve it as an alternative once it meets the full criteria of an approved methodology. Its stated floor was "no earlier than 3 months after" the March 2026 response — but it had still not gone live by mid-2026, so the real arrival date is later and not yet fixed.
  • Then a dual-running period: once HEM is approved, SAP 10.3 and HEM run in parallel for no less than 24 months before SAP 10.3 is withdrawn and HEM becomes the sole methodology.

So the practical answer to "which method assesses my new home in 2027?" is SAP 10.3, not HEM. The Home Energy Model is coming — but on a longer runway than the headlines suggest.

What the Future Homes Standard itself requires

The methodology is the measuring stick. The standard is what you have to hit. Approved Document L (now titled "Energy and Greenhouse Gas Emissions") sets the bar for new dwellings from March 2027. The headline points confirmed in the published documents:

  • A step-change in carbon — the government expects a typical new home built to the standard to produce around 75–80% less carbon than one built to the 2013 Part L baseline. That figure is an expected outcome, not a number you certify against; compliance is judged against a notional building.
  • Low-carbon heating — in practice, no new homes built around fossil-fuel boilers; the notional dwelling assumes a heat pump.
  • On-site renewable generation — a brand-new functional requirement (Part L3) obliging a system of on-site renewable electricity generation on new dwellings. The duty is technology-neutral, but in practice that means rooftop solar PV for most homes.

These standards are demanding, and they reward fabric quality and proper installation — exactly the things a monthly-average method struggled to credit and a half-hourly model captures. It is the same fabric-first logic behind a PHPP assessment, and the direction of travel is clear even while the methodology catches up.

And the EPC side: existing homes

HEM is not only about new builds. A separate consultation — "Home Energy Model: Energy Performance Certificates", which ran from 21 January to 18 March 2026 — sets out how HEM will be used to assess existing dwellings and produce reformed EPCs with four new headline metrics: fabric performance, heating system, smart readiness and energy cost. These HEM-based EPCs are set to replace the current SAP/RdSAP rating, with the familiar energy-efficiency rating shown alongside for comparison until at least the end of 2029. The launch timetable is still moving — the government's own target of late 2026 is, in its words, "ambitious", and industry now expects the new EPCs in the second half of 2027 — and a government response is expected later in 2026.

For landlords and owners of older housing stock, this is the bigger unknown. An EPC band can shift under a different methodology with no physical change to the building — which is precisely why a proper fabric assessment, rather than a desktop EPC, is worth having before you commit to a retrofit spec. It is the same trap we flagged in our 2026 MEES reset guide: the rating can move even when the building does not. (One date to keep straight: 1 October 2029 is a landlord grandfathering deadline in the rented-sector minimum-standard rules, not the day HEM "switches on" for EPCs.)

What this means if you're building or retrofitting now

A few practical takeaways, without the hype:

  • Don't design to a method that isn't live yet. If your project completes around or before 2027, SAP 10.3 is the compliance route. Specify to the standard, not to a model that arrives later.
  • Fabric-first still wins under either method. Both SAP 10.3 and HEM reward a good thermal envelope, airtightness and properly commissioned ventilation — the things a PHPP assessment is built to optimise. None of that is methodology-dependent.
  • Existing-home EPCs are the bigger unknown. If you own pre-1919 or solid-wall stock, how HEM treats it won't be fully clear until the band boundaries are confirmed. Plan around fabric performance, not a single letter grade.
  • Check GOV.UK for the live position. Several dates here (HEM approval, EPC reform) are described by the government as indicative and may move — the Standard Assessment Procedure guidance is the page to watch.

Where APMBuild fits

APMBuild Ltd is a Herefordshire-based construction company specialising in energy-efficient building, deep retrofit and direct European materials supply, working across Herefordshire, Worcestershire, Shropshire, Powys, Monmouthshire, the Cardiff area, the West Midlands and Wales. If you are weighing up a new build or a retrofit against the changing standards, a fabric-first PHPP pre-assessment is the right place to start — it gives you U-value targets and a performance picture that hold up whichever methodology your compliance route eventually uses (here's how PHPP modelling works). Get in touch to talk it through.

FAQ

Home Energy Model & SAP — frequently asked questions

Will new homes be assessed using the Home Energy Model from 2027?

Not at launch. When the Future Homes Standard comes into force on 24 March 2027, SAP 10.3 is the sole approved methodology for demonstrating compliance. The Home Energy Model will be approved as an alternative later, once it meets the full criteria — a date the government has not yet fixed. When it does arrive, SAP 10.3 and HEM run in parallel for no less than 24 months before SAP 10.3 is withdrawn and HEM becomes the sole methodology.

What is the difference between SAP and the Home Energy Model?

SAP works on twelve monthly averages. The Home Energy Model runs a half-hourly simulation across 17,520 timesteps per year, built on the standard BS EN ISO 52016, and models heat pumps, solar PV, batteries and ventilation dynamically rather than as a monthly average.

When does the Home Energy Model replace SAP for EPCs?

Reformed, HEM-based EPCs are set to replace the current SAP/RdSAP rating. The government's original target was late 2026, which it called "ambitious"; industry now expects launch in the second half of 2027, with the familiar energy-efficiency rating shown alongside for comparison until at least the end of 2029. The metrics and band boundaries are still being finalised, with a government response expected later in 2026 — so the timing is indicative and may move.

What does the Future Homes Standard require?

The government expects a home built to the standard to produce around 75–80% less carbon than one built to the 2013 Part L baseline (an expected outcome, judged against a notional building rather than a number you certify against). It also means low-carbon heating in practice — no new homes built around fossil-fuel boilers — and a new functional requirement (Part L3) for on-site renewable electricity generation, which in practice means rooftop solar PV for most homes.

Which methodology should I design my 2027 project to?

SAP 10.3. If your project completes around or before 2027, SAP 10.3 is the compliance route — specify to the standard, not to a model that arrives later. Fabric-first design wins under either methodology, because both reward a good thermal envelope, airtightness and properly commissioned ventilation.