The Home Energy Model and its wrappers

1 From SAP to HEM, EPC, MEES and FHS
2 What are assessment wrappers?
3 Key elements of FHS
4 Background
5 Related articles on Designing buildings

[edit] From SAP to HEM, EPC, MEES and FHS

The Home Energy Model, or HEM, will be introduced in the UK to replace the Standard Assessment Procedure (SAP), which is currently used to demonstrate that new homes comply with Part L of the Building Regulations and to produce EPC's. The reduced version of SAP (RdSAP) is the current methodology for calculating EPCs for existing buildings.

Under the Minimum Energy Efficiency Standards (MEES) since April 1, 2020, all private rented properties (both domestic and commercial) must have an EPC rating of at least 'E', with plans for this to be raised the minimum standard to 'C' by 2030, with interim targets starting earlier. Currently the methodology to calculate EPCs uses RdSAP, however this will change over the coming years towards an option to use either HEM or RdSAP, and eventually just HEM.

The introduction of HEM gives an opportunity to introduce flexibility around the inputs assessors make within EPC assessments, depending on the data and tools available to them. To balance this against the need to have a consistent and efficient assessment process in the interests of clarity for consumers, and time and cost effectiveness for assessors a consultation was launched; The Home Energy Model:Energy Performance Certificates. HEM methodology for assessing existing dwellings and producing new EPC Metrics.

The Future Homes Standard (FHS) is the forthcoming update to Part L (Conservation of Fuel and Power) of the Building Regulations for England, representing the most significant change to new-build standards in over a decade. The standard requires new homes to produce 75–80% less carbon emissions than homes built to the 2013 baseline, a substantial step beyond the 2021 Part L uplift which only required a 31% reduction and compliance will be demonstrated using the Home Energy Model (HEM) or SAP 10.3 during a transitional period.

It is expected that the final regulations will be laid before Parliament during 2026, and FHS is expected to come into force later in 2026 with a transitional period for the implementation is expected to be 12 months ending in 2027.

[edit] What are assessment wrappers?

Where SAP was effectively the calculated or estimated potential energy consumption on a monthly basis, the HEM has the potential to calculate or estimate this on a half-hourly basis. As such, in order to model the energy buildings might use at this level, more detailed data points or inputs are likely to be required, making the model potentially more complex to use.

To manage the broad range of data points or inputs which the new HEM will be capable to interpreting, it is likely that the model will have the possibility to use a number of assumptions rather than inputting individual data for each project. These assumptions may range in relation to the purpose of the assessment, the occupancy patterns, localised weather patterns, links to other building standards, white goods, air tightness, ventilation, and so on. They may also expand with time. The model is intended to be flexible enough to account for and allow for adjustments based on specific buildings while being simple enough to be workable.

The current government consultation The Home Energy Model:Energy Performance Certificates. HEM methodology for assessing existing dwellings and producing new EPC Metrics looks at aspects of these changes. The consultation looks at a ‘modular’ approach, where the assessor enters as much data as they have access to in each dwelling. The modular structure of the HEM core engine permits this approach, where data for a given module can be provided in multiple ways without interfering with the rest of the calculation. In this new approach, the “all-or-nothing” of RdSAP would be replaced by a flexible series of input simplifications. These aspects are being covered by what are termed as wrappers.

The Home Energy Model: Future Homes Standard assessment is a methodology designed to assess compliance with the 2025 Future Homes Standard (“FHS”). The Home Energy Model and FHS assessment wrapper together make up the Home Energy Model: FHS assessment. see An evaluation of the Home Energy Model and Future Homes Standard assessment wrapper. The HEM:EPC wrapper is effectively the same approoach for the production of EPCs on which views are being sought in the consultation described above.

[edit] Key elements of FHS

The FHS sets carbon targets deliberately at a level that fossil fuel heating systems cannot meet, meaning that in practice air source or ground source heat pumps will become the primary heating technology for new homes, with heat networks using low-carbon sources also acceptable. Hybrid boilers, hydrogen-ready boilers, and gas boilers will not comply, though existing homes remain unaffected. Alongside this, solar PV panels will become a functional requirement under Building Regulations, with developers required to achieve 40% solar coverage of a building's floor area where feasible, helping new homes generate and self-consume renewable electricity and reducing energy bills by an expected 40–50% compared to conventionally built properties.

FHS also introduces significantly improved fabric performance targets through a whole-building approach, with the notional dwelling specification requiring external walls at 0.18 W/m²K, roofs at 0.11 W/m²K, and windows at 1.2 W/m²K, all considerably better than current 2021 standards. The airtightness target of 3 m³/(h·m²) at 50 Pa is particularly demanding, less than half the current regulatory maximum, and will typically require mechanical ventilation with heat recovery (MVHR) to maintain indoor air quality. Given that highly insulated, airtight homes can be prone to summer overheating, the FHS works in conjunction with Part O of the Building Regulations, with HEM's half-hourly simulation well suited to modelling overheating risk and tracking internal temperatures throughout the year.

[edit] Background

In 2021, the government appointed a consortium of experts led by the BRE to carry out a multi-year project to develop a replacement for SAP, which was first published in 1993. SAP was introduced into the Building Regulations to assess the energy performance of dwellings and has been updated periodically in 1998, 2001, 2005, 2009, 2012, and 2022. The accuracy of EPC's has received much criticism over the years see Are Energy Performance Certificates Accurate? HEM was first consulted on in 2019 with further technical detail set out in a 2023 consultation, the government responded in early 2026 with regulations anticipated to come into force by late 2026.

The replacement of SAP and the introduction of HEM is a significant change that will impact a wide variety of areas concerning the estimation, demonstration of compliance, and, to some extent, the assessment of energy use in buildings. Given the importance and timeframe of development, the government appointed a consortium led by Etude to quality-assure the work undertaken. QA processes aim to ensure that issues with the current SAP methodologies are identified and clearly defined, evidence for suggested improvements is thorough, validation processes are robust, the decision-making process is clear, robust, and documented, and project governance is working effectively with the government, key user groups, and technical experts associated with key decisions. Below are some of the various publications related to the various inter-related themes.

Open consultation: Home Energy Model (HEM) methodology for assessing existing dwellings and producing new Energy Performance Certificates metrics (The Home Energy Model: EPC assessment consultation) Published 21 January 2026 Consultation document part of a series of consultations and documents published by the government relating to Energy Performance Certificates (EPC) and the Home Energy Model (HEM). Seeks views on the parts of the new Home Energy Model methodology which are specific to Energy Performance Certificates.

Closed consultation: The Home Energy Model consultation (2023) and government response (2025). Home Energy Model: replacement for the Standard Assessment Procedure (SAP) Explains the overhaul to the SAP methodology and sought views on the approach taken by its replacement, the Home Energy Model. The government response detailed key decisions made on HEM core methodology following review of consultee feedback. Proposed changes in the Future Homes Standard, to ensure new homes to have mandatory solar were announced in June 2025.

The Reforms to the Energy Performance of Buildings regime consultation outcome (2024) partial response 2025. This consultation sought views on reforms to EPCs, including principles for new metrics to be used in generating EPC ratings. In particular, the Technical Annex to Chapter 2 of the consultation sets out the issues arising within metric design, which the present proposals aim to address. The EPB consultation will be of interest to those who want to understand the proposed changes to EPCs, which drive the metric design proposals for HEM:EPC.

Home Energy Model: Future Homes Standard assessment consultation (2023) and Home Energy Model: Future Homes Standard assessment accompanying technical documentation The government previously consulted on a wrapper for HEM to support assessments determining compliance with Building Regulations Part L (the Future Homes Standard). This consultation is accompanied by several technical documents which go into further detail on the assumptions and validation exercises that have been carried out. Although the EPC wrapper may differ in some respects and contain updates to some assumptions, these documents remain the best way to understand how the EPC assessment will approach input standardisation, where this is common to both new and existing dwellings. These technical documents will be of interest to those who want to understand the justifications and evidence base behind the standardisation assumptions used in the model (relating to energy demand, weather, fuels etc).

The Home Energy Model reference code. Full Python source code for the Home Energy Model and the Home Energy Model: EPC assessment has been published in a series of Git repositories. The government has not published a user interface for HEM:EPC to accompany this consultation. The reference code will be of interest to those who want to understand how the model has been implemented in code, and those wishing to fully clarify their understanding of the new methodology. It will also be of interest to any potential contributors to the Home Energy Model.