By Tom Handford MRICS
If you work with new homes, existing housing stock or modelled energy performance, you are going to hear a lot more about the Home Energy Model, or HEM. It is the government calculation methodology that will replace the Standard Assessment Procedure (SAP), and it will change how homes are assessed for both Building Regulations compliance and Energy Performance Certificates (EPCs). Here is what is changing, when, and what we think you should be doing about it.
The short version
HEM was announced in December 2023 and is described by government as a new name for a new methodology. SAP currently does two main jobs: showing that new homes comply with Part L of the Building Regulations, and producing EPCs for all homes. HEM is meant to take over both, and to sit behind the wider policies that rely on SAP outputs, such as minimum standards for rented homes and retrofit funding schemes.
It covers dwellings only. Non-domestic buildings will continue to be assessed using SBEM, under a separate programme linked to Approved Document L2 and the Future Buildings Standard.
How HEM will be delivered
The way HEM is delivered is a big change in itself, quite apart from the maths behind it.
It is open source. The HEM codebase is published openly, so you can see changes to the model before they are formally adopted. SAP, by contrast, has historically been a document that software developers built products around.
The engine is controlled by government. The core calculation is hosted by the Ministry of Housing, Communities and Local Government (MHCLG) through Energy Calculation as a Service (ECaaS). Only energy assessor accreditation schemes can access it, and they build the applications that assessors actually use. Software providers no longer implement the engine separately.
Each use case has its own wrapper. The engine is used through a wrapper that sets out the inputs and outputs for a particular purpose. There is a Future Homes Standard wrapper for new build compliance, and an EPC wrapper is being developed for certificates. For existing homes, the wrapper takes over the job that Reduced Data SAP (RdSAP) does today.
The product database is being rebuilt. Government plans to rebuild the Product Characteristics Database from underlying test data, adding new classes of product and keeping performance data separate from the core engine.
One thing still to be settled is cost. Government has said there will be fees for using and maintaining ECaaS, and the arrangements are still under discussion between MHCLG and the accreditation schemes.
How HEM differs from SAP
The headline change is time. SAP works on a monthly time step, whereas HEM simulates a home every half hour across the modelled year. That is the biggest technical shift, and it makes most of the other improvements possible.
Different standards underneath. SAP is based on the BRE Domestic Energy Model (BREDEM). HEM is based on BS EN ISO 52016-1:2017, with infiltration and ventilation calculated to BS EN 16798-7:2017.
Better treatment of modern technology. Half hourly simulation allows heat pumps, heat batteries, electrical storage, smart tariffs and load shifting appliances to be modelled realistically, along with the way variable supply and demand interact. A monthly calculation cannot capture this well, which is where SAP has struggled.
A more modular structure. Each HEM module has a fixed set of inputs and outputs. SAP has been extended repeatedly over three decades, and government acknowledges that this has left it lacking structural clarity.
More flexible data entry for existing homes. RdSAP is all or nothing, using a fixed reduced data set. The HEM proposals let an assessor enter as much detail as is available, module by module, with defaults filling the gaps. The aim is to support both detailed assessments and quicker, cheaper ones.
The important point is that HEM results will not simply carry on from SAP results. They will differ, and there is no simple way to convert from one to the other.
Where things stand: the timeline
December 2023. HEM announced, with two consultations: HEM as the replacement for SAP, and HEM Future Homes Standard assessment. Both closed on 27 March 2024.
May 2025. Government confirmed a dual methodology approach, with both SAP 10.3 and HEM available for Future Homes Standard compliance during a phased transition.
October 2025. The Department for Energy Security and Net Zero (DESNZ) published its response to the HEM consultation, confirming delivery through ECaaS, the move to BS EN ISO 52016-1:2017, increased time resolution and the change to BS EN 16798-7:2017 for infiltration and ventilation.
January 2026. Alongside the Warm Homes Plan, DESNZ consulted on using HEM to generate EPCs for existing homes, replacing SAP and RdSAP. The consultation closed on 18 March 2026.
February 2026. MHCLG confirmed that SAP 10.3 would initially be the only methodology available for demonstrating Future Homes Standard compliance.
24 March 2026. MHCLG published the Future Homes Standard and Approved Document L 2026. Approved Document L2 2026 followed on 27 March, ahead of Future Buildings Standard implementation in 2027.
24 March 2026 (expectation). At publication, MHCLG indicated that HEM approval was expected no earlier than three months later, which pointed to late June 2026 at the earliest.
8 June 2026. Government announced that it had delayed the launch of HEM after final internal assurance, to make sure it is as robust as possible when it goes live. HEM therefore did not launch alongside the Future Homes Standard, and SAP 10.3 remains the sole approved methodology. Government said HEM will be introduced in the coming months, with no firm date given.
24 March 2027. The Future Homes Standard takes effect. Homes subject to a building regulations application after this date must be built to it.
24 March 2028. Transitional deadline. Homes subject to an application before 24 March 2027 must have commenced construction before this date to remain under the previous version of Part L.
October 2030. Domestic private rented properties are required to meet EPC band C, assessed under whichever methodology is in force at that point.
As far as we can see, there has been no confirmation of a launch date since the June announcement, and SAP 10.3 remains the sole approved methodology for Future Homes Standard compliance. The dates on which HEM becomes available and then mandatory are still unconfirmed, so please check both before building them into a programme or contract.
What about EPCs?
HEM does not change the EPC on its own. EPC reform is running alongside it, and the January 2026 consultation set out how the two would come together for existing homes. The proposals include:
Four headline metrics in place of a single rating: fabric performance, heating system performance, smart readiness and energy cost.
Each metric scored from 1 to 100 and grouped into A to G bands, with energy cost proposed to be shown in pounds as well as a band.
No home heated solely by fossil fuel being able to reach band C or better on the heating metric, with low carbon systems scoring higher.
The existing legacy EPC score continuing to appear on certificates until at least the end of 2029, to support the transition.
A new name for the modular version of HEM used for existing dwellings, with options consulted on including HEMEX, HEMIE, MdHEM and RdHEM.
Government also asked how band thresholds should be set, particularly for band C under minimum energy efficiency standards. For landlords, this is where the design of the metrics stops being a technical question and becomes a commercial one.
What this means for you
If you are a housebuilder or design team, expect two methodologies to be in circulation during the transition, giving different results for the same dwelling. Compliance strategies, notional dwelling comparisons and specification decisions made under SAP 10.3 should not be assumed to carry across to HEM. If a project spans the transition, be clear about which methodology applies and when.
If you work with low carbon and smart technologies, this could be good news. HEM is designed to represent heat pumps, storage, smart tariffs and load shifting properly, so specifications that were penalised or poorly represented under a monthly calculation may perform differently, in some cases better. Re-test rather than assume.
If you are a landlord or asset owner, the metrics that decide compliance with minimum standards are being redefined at the same time as the methodology behind them. A property close to a threshold today may land differently under the new metrics, particularly if it relies on fossil fuel heating. Improvement programmes running towards the October 2030 deadline should allow for that.
If you are an assessor or work with an accreditation scheme, the engine will be provided centrally and accessed through scheme software connected to ECaaS. Software, training and quality assurance all need rebuilding around it, and modular data entry changes what is collected on site.
If you rely on modelled outputs, anything that quotes a SAP figure, including planning submissions, funding applications, sales material and contractual performance targets, should be reviewed for how it will read once the methodology changes.
Things to keep in mind
HEM and SAP results are not directly comparable, so do not apply a conversion factor between them.
Government describes validation of HEM as an ongoing process, supported by inter model comparison and real world case studies, rather than as complete.
Run times may be longer than SAP because of the half hourly simulation, which matters for iterative design work.
The cost of using ECaaS has not been confirmed.
Timetables differ across the UK. The Future Homes Standard applies in England, confirmed changes to Welsh Building Regulations take effect from March 2027, and Scotland and Northern Ireland have separate arrangements.
HEM does not address overheating, which continues to be assessed under Approved Document O, or embodied carbon.
What to do now
Establish which methodology each live project will be assessed under, and record the building regulations application date against the 24 March 2027 and 24 March 2028 milestones.
Re-test typical house types and specifications once HEM is available, rather than assuming SAP 10.3 conclusions still hold.
Review contracts, planning conditions and marketing claims that reference SAP outputs or EPC bands.
If you manage a portfolio, model the effect of the proposed metrics on properties near a band threshold, paying particular attention to fossil fuel heated stock.
Confirm that your assessors and consultants will be working on scheme software connected to ECaaS, and allow time for training.
In summary
HEM is a fundamental change rather than an update to SAP. It replaces a monthly calculation with a half hourly simulation, moves the engine under government control, and arrives alongside a reworked EPC. Its results will not match SAP, and key dates, including when it becomes available and mandatory and what it will cost to use, are still to be confirmed. That uncertainty is a reason to prepare now, not to wait. Know which methodology each of your projects will fall under, re-test your key specifications once HEM is available, and review any contracts or claims that depend on a SAP figure. Those who plan ahead will be best placed when the transition arrives.
