Home Energy Model

HEM is the new, dynamic Home Energy Model that will replace SAP as the UK’s official methodology for assessing domestic energy performance. HEM works at a time interval of 30 minutes with more input data and will model building performance far more accurately than SAP.

SAP is the current Standard Assessment Procedure used for Part L compliance and to identify the EPC of a new building or extension. SAP uses monthly calculations and is based on assumptions about occupancy, heating patterns, hot water usage and external temperature.

SAP & HEM Key Differences

Home Energy Model (HEM)

HEM is the next‑generation methodology that will replace SAP for assessing domestic energy performance and underpin the Future Homes Standard. 2025. Designers developed HEM as an open, modular, physics-based model that can support multiple policies—including Part L, FHS, and EPC reform—as well as future low-carbon technologies.

Standard Assessment Procedure (SAP)

SAP is the UK government’s current methodology for rating the energy performance of dwellings and checking compliance with Building Regulations Part L, as well as generating EPCs.
SAP uses standardised monthly calculations to estimate energy use, costs and emissions based on typical occupancy and fixed building services.

Key Technical Differences

Time step: SAP uses monthly time steps, while HEM runs half‑hourly simulations, giving a high‑resolution picture of demand and operation.
Data/detail: SAP works with a relatively limited dataset; HEM requires more detailed geometry, fabric, system and occupancy inputs (e.g. rooms, appliances, shading), increasing modelling time and accuracy.
Technology Modelling: SAP has limited capability for smart tariffs, storage and controls; HEM explicitly models smart technologies, time‑of‑use tariffs and flexible operation.

Regulatory & Practical Differences

Regulatory role: SAP is currently used for Building Regulations Part L compliance and EPCs; HEM will take over these roles as it rolls out with the Future Homes Standard from 2025 onward.
Transition: Government will run SAP and HEM in parallel for a limited period to allow industry to adapt before HEM becomes the sole methodology.
Workflow impact: A typical dwelling that might take minutes to model in SAP can take several times longer in HEM, so assessor time and assessment costs are expected to increase..

Home Energy Model

Reports

This is still under consultation but it is very likely that two reports will be required as is currently the case with SAP:

Design Stage Compliance Report (Notional vs. Proposed): A direct equivalent of the SAP Design Stage Report. It will be generated by the HEM software and will demonstrate how the proposed dwelling’s design meets (or exceeds) the required performance targets.
As-Built Compliance Report (Notional vs. As-Built): The equivalent of the SAP As-Built Report, generated once construction is complete and actual site data (e.g., airtightness test results, commissioning certificates for heating systems, final product U-values) replaces any assumed or design-stage values. This report will confirm that the dwelling, as built, still meets the required performance targets.
Scotland, Wales & NI: It is very probable that that they will adopt the HEM with their own national wrappers that will include the appropriate variations for their particular circumstances.

Summary of HEM & SAP Concepts

The core HEM terminology overlaps with SAP (fabric, systems, gains, losses etc.), but introduces a more granular, simulation‑style set of terms around time‑steps, zoning, controls and system operation.

We have created a comparative list to guide you on the radical changes in building that are bound to follow.

Download our guide on SAP and HEM classifications.

Download

Home Energy Model will Differentiate Between New Builds & Extensions

UK Building regulations

Part L

The Home Energy Model is primarily being developed to underpin the Future Homes Standard for new dwellings but will also be applied to extensions, material alterations, and change of use to existing dwellings.

Separate Regulatory Requirements (Part L): In the UK Building Regulations, Part L (Conservation of Fuel and Power) has historically had different requirements for new dwellings versus work to existing dwellings.

While the numbering and structure will evolve with the Future Homes Standard, the fundamental principle of having different performance targets and compliance pathways for new builds versus extensions will remain. This is because it is generally much harder and more expensive to achieve the same levels of fabric performance in an extension to an existing building as it is in a brand-new, purpose-designed structure.

Approved Document L1A historically applied to new dwellings.
Approved Document L1B historically applied to existing dwellings, including extensions and material alterations.

Extensions

Compliance Pathways

For extensions, the compliance methodology typically involves ensuring the new elements of the extension (walls, roof, floor, windows, doors) meet specific limiting U-values and design criteria.

There are often different ways to demonstrate compliance for extensions:

Elemental approach: Meeting the U-value limits for each individual new element.
Whole-dwelling approach: If an extension features a lot of glazing (exceeding certain limits, e.g., 25% of the floor area), a whole-dwelling calculation (using SAP, and eventually HEM) may be required. This demonstrates that the combined existing dwelling + proposed extension is no worse than the existing dwelling + a “notional” extension built to minimum standards. This provides design flexibility.

HEM’s adaptability

“Wrappers”

The Home Energy Model is being designed with a modular structure, including “Core Calculations” and “Wrapper Components.” This allows the core simulation engine to be applied to different policy uses. It’s expected that there will be different “wrappers” or methodologies within HEM to cater specifically for:

New Dwellings (Future Homes Standard compliance): Very stringent DER, DFEE, and DPER targets.
Existing Dwellings (Extensions, Alterations, Conversions): Potentially less stringent targets or different compliance pathways compared to new builds, reflecting the practicalities of working with existing structures.
Even under the current Part L (2021 update, implemented June 2022), the limiting U-values for elements in extensions are generally slightly less stringent than for new builds. For example, a new wall in an extension might have a limiting U-value of 0.18 W/(m²K), whereas a new wall in a brand new dwelling might need to achieve 0.11-0.15 W/(m²K) under future standards. HEM will embody these different target values depending on whether it’s assessing a new build or an extension.

Benefits of ICF with Mandatory HEM Use

ICF Overcomes ALL Disadvantages

ICF construction inherently addresses the very challenges that low-mass timber frame and externally-insulated cavity walls face with dynamic thermal modeling and heat pump synergy.

In essence, ICF construction directly provides the “fabric first” benefits that the Home Energy Model is designed to accurately measure and that heat pumps thrive on:

Significant internal thermal mass for buffering temperatures and load shifting.
Exceptional airtightness to minimize uncontrolled heat loss/gain.
Continuous insulation to achieve very low U-values and eliminate thermal bridges.

Therefore, as the Home Energy Model becomes the standard for compliance, ICF dwellings are very well-positioned to demonstrate superior energy performance. Achieving excellent DER, DFEE, DPER ratings and provide an optimal environment for highly efficient heat pump operation. Especially compared to their timber frame or conventional cavity wall counterparts.

Find out more about the sustainability of ICF, Timber Frame and Cavity Wall construction techniques:

Sustainability

The SAP process does not identify the dramatic reduction in energy costs and additional year-round comfort provided by an ICF home, but the Home Energy Model will.

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