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Part L 2026: The SAP to HEM Timeline

Read time: 3 minutes
July 21, 2026

SAP has calculated the energy performance of new homes in England since 1994. That is changing. Much of the industry has been calling the next update to Part L the Future Homes Standard, or FHS. We are going to use the term Part L 2026 instead, since that is the name that has stuck for the version of the regulations published on 24 March 2026, in the same way the current rules are known as Part L 2021.

Timeline of key dates for the Part L 2026 and SAP to HEM transitionA vertical timeline showing five milestones: Approved Documents published on 24 March 2026; Part L 2026 coming into force on 24 March 2027 with SAP 10.3 as the sole method; HEM becoming available at least three months later, date not yet confirmed; the Part L 2021 transitional period ending on 24 March 2028; and SAP being withdrawn 24 months after HEM launches, date not yet confirmed. A dashed line marks today, July 2026, between the first two milestones.Today, July 202624 March 2026Approved Documents L and Fpublished24 March 2027Part L 2026 comes into forceSAP 10.3 is the sole method3+ months later (tbc)HEM becomes availablealongside SAP 10.324 March 2028Part L 2021 transitionalperiod ends+24 months after HEM (tbc)Dual running endsSAP withdrawn, HEM onlyConfirmed dateDate not yet confirmed

Part L 2026 Key dates

24 March 2026
Approved Documents L 2026 was published.

24 March 2027
Part L 2026 comes into force for standard new dwellings, subject to transitional provisions. SAP 10.3 is the sole approved compliance methodology at this point. HEM is not yet available, even though this is the commencement date for the regulations themselves.

24 March 2028
Twelve months later, the standard transitional period ends. After this date, every new dwelling must comply with Part L 2026, whichever methodology is used to demonstrate it.

At least three months after 24 March 2027
HEM becomes available as an alternative compliance route alongside SAP 10.3. This was the original plan. As of the most recent government update, in June 2026, this element has already slipped, and no new date has been confirmed. Treat this as the earliest realistic point HEM appears, not a guarantee.

Twenty four months after HEM becomes available
This is the minimum length of the dual running period, during which a project can be assessed under either SAP 10.3 or HEM.

After dual running ends
SAP 10.3 is expected to be withdrawn for new sites, at which point HEM becomes the sole compliance route. Government has said it will give six months' notice ahead of this. The calendar date depends on when HEM actually launches, so we cannot give a fixed date for this step yet.

How this Part L 2026 timeline has already changed

It is worth being honest about how much this has moved, because it tells you something about how to plan around it.

  • At consultation stage, HEM was widely expected to be the primary or sole methodology from the point Part L 2026 came into force. That did not happen.
  • The government's March 2026 consultation response confirmed instead that SAP 10.3, updated with the new Part L 2026 targets, would be the sole methodology at launch, with HEM to follow at least three months later.
  • On 8 June 2026, the government delayed HEM's launch again, this time with no replacement date given, citing further internal assurance work to make sure the model is robust before it goes live.

None of this changes where the transition is heading. It does mean that any date given for HEM's actual availability, including the ones in this guide, should be treated as the earliest likely point rather than a fixed appointment. We will update this article as the government confirms more, and you should check gov.uk directly before a decision depends on a specific date.

What actually changes once HEM is live

HEM calculates energy use half hourly rather than monthly. It works through 17,520 timesteps across a year, using local weather data, instead of smoothing everything into twelve seasonal averages.

Comparison of SAP monthly calculation against HEM half hourly calculationTwo panels over the same year. The left panel shows SAP as twelve blocky monthly bars forming a seasonal heating curve, labelled 12 data points a year. The right panel shows HEM as a dense, continuously fluctuating half hourly line over the same seasonal shape, labelled 17,520 data points a year. A caption below reads: same building, same year, different level of detail.SAP: monthly averagesHEM: half-hourly simulationJanDecJanDec12 data points a year17,520 data points a yearSame building, same year. Different level of detail.

It is built on the international standard BS EN ISO 52016-1, and delivered through a government cloud platform called ECaaS (Energy Calculation as a Service), rather than through separate desktop software from different providers. Every assessment runs on the same engine and produces the same result from the same inputs. We cover ECaaS in more detail in a separate article, so we will not repeat that here.

How ECaaS delivers HEM through one shared calculation engineFour software provider boxes at the top, each with an arrow converging down into a single central box labelled ECaaS cloud engine, built on ISO 52016-1. A single arrow leads from that engine down to a box labelled same result every time, same inputs same output. A caption notes that previously each provider used its own separate engine.Provider AProvider BProvider CProvider DECaaS cloud engineBuilt on ISO 52016-1Same result, every timeSame inputs, same outputPreviously, each provider used its own separate engine.

What to do now

  1. Work out which side of 24 March 2027 your project falls on, and whether it can rely on the Part L 2021 transitional arrangements. If your building notice or full plans will not go in before that date, plan for Part L 2026 from the outset.
  2. Brief your assessor early. If a project might straddle the point where SAP is eventually withdrawn for new sites, build that uncertainty into the planning timeline now rather than at submission.
  3. Ask your energy consultant which methodology your Energy Statement or Part L submission currently relies on, and how it would need to change under HEM. A vague answer is a sign to ask harder questions.
  4. Tighten up data collection on live projects. If HEM's defaults really are more punitive than SAP's, the projects with the cleanest specification data will move through compliance with the least friction.
  5. Keep an eye on the EPC changes, even though they are a later phase. Replacing a single A to G rating with separate metrics will affect valuation and sales conversations as well as compliance, so it is worth understanding ahead of time.
  6. Treat any date you read for HEM's launch or full adoption as provisional. Check gov.uk directly before a decision depends on it.

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