HEM vs. SAP: What’s Changing, When It’s Changing, and What It Will Cost

Energy regulation in the UK is entering a period of transition for new homes. The long-established Standard Assessment Procedure (SAP), which assesses residential properties and produces Energy Performance Certificates (EPCs), is planned to be phased out and replaced by the Home Energy Model (HEM), a new methodology that the government intends to provide a more accurate representation of a home’s energy performance.

For developers, designers, and energy consultants, this shift is not just technical. It affects EPCs, compliance with Building Regulations Part L, cost planning, software selection, and the calculation of energy use, carbon emissions, and energy efficiency ratings.

Key points:

• SAP is being replaced by HEM under the Future Homes Standard.
• HEM introduces a new calculation engine and more realistic modelling of energy consumption, heating systems, and new technologies like heat pumps and battery storage.
• The transition is commonly discussed as taking place across 2026–2027 before HEM becomes the default, although the exact timing and length of the transition period have not yet been confirmed.
• These changes should improve accuracy, but they will also affect project costs, timelines, and compliance strategy.

If you’re planning a residential project or need to demonstrate compliance under evolving regulations, contact Falcon Energy to support your SAP assessments today and prepare for HEM.

What is SAP, and why has it been so important?

The Standard Assessment Procedure (SAP) has been the UK’s primary methodology for assessing a home’s energy performance for decades. SAP calculations underpin:

• Energy Performance Certificates (EPCs)
• Compliance with Building Regulations Part L
• Energy efficiency ratings for residential properties
• Comparisons of energy usage and energy consumption

SAP uses a monthly calculation approach to estimate a building’s energy use based on standardised assumptions. The current methodology evaluates heating systems, hot water demand, ventilation, fabric performance, and energy supply using fixed occupancy and behaviour patterns.

This approach has provided consistency and simplicity, making SAP assessments widely applicable across different use cases. However, it also has limitations, particularly when applied to modern homes using new technologies and low-carbon systems.

Why the government intends to replace SAP

The government aims to reduce carbon emissions, regulate energy demand more effectively, and deliver homes that perform as designed. SAP’s current methodology struggles to reflect:

• Energy flexibility and smart technologies
• Heat pumps replacing gas boilers
• Battery storage and on-site generation
• More complex heating systems
• Realistic patterns of energy use

As homes become more efficient and technologies more advanced, SAP’s monthly, static approach no longer offers a sufficiently accurate representation of a building’s energy performance as we look ahead. This gap is one of the critical reasons the government intends to replace SAP with HEM.

For more information about modern and sustainable energy technologies, see our energy consultancy services, where we can advise on the best options for your development or property.

What is HEM (Home Energy Model)?

The Home Energy Model (HEM) is the new UK methodology being developed to replace SAP for assessing a home’s energy performance, energy consumption, and overall energy efficiency rating. It’s being introduced as a key part of the Future Homes Standard, a government-led update to Building Regulations (including Part L) intended to ensure new residential properties are built to significantly higher energy-efficiency standards, produce lower carbon emissions, and be ready for future technologies.

In practical terms, the Future Homes Standard raises the bar on what “good” looks like for a new home’s energy use. It’s designed to support the government’s aims to reduce emissions from homes by encouraging low-carbon heating (such as heat pumps) and reducing reliance on gas boilers, while ensuring homes can demonstrate compliance through a methodology that reflects how buildings perform in real life.

Unlike SAP, which is largely based on averaged assumptions over longer periods, HEM uses dynamic simulation and higher time resolution (often hourly or sub-hourly) to provide a more accurate representation of a home’s energy use. That enables:

• More detailed modelling of energy demand across different times of day
• Better use of weather data to reflect changing external conditions
• Improved representation of how occupants drive energy usage
• More realistic modelling of heating systems, hot water, and electricity demand
• Stronger assessment of modern and emerging solutions (including energy flexibility and battery storage)

Overall, HEM is designed to deliver more accurate energy assessments, helping projects better predict real-world performance, and making it clearer when demonstrating compliance under the Future Homes Standard.

HEM vs. SAP key differences

Category	SAP	HEM	Why it matters
Calculation methodology	Monthly average approach.	Dynamic calculation engine using finer time intervals.	Better captures peaks in demand and day-to-day performance (especially for heat pumps).
Accuracy and realistic modelling	Simplified assumptions over longer periods.	More realistic modelling of energy use patterns and system interactions.	Improves confidence when demonstrating compliance and comparing options.
Technology readiness	Not built around the widespread use of newer systems.	Designed to model modern low-carbon and flexible technologies.	Better reflects heat pumps, battery storage, and smarter controls.
Data and time resolution	Broader time periods and simpler data inputs.	Higher-resolution weather data and shorter calculation steps.	Provides more detail on when energy is used, not just how much.
Practical takeaway	Good for consistent, standardised estimates.	Closer to real-world performance in how homes operate.	Compliance and design decisions rely more on time-based evidence vs broad averages.

Calculation methodology

SAP calculations rely on a monthly average approach. HEM calculations use a dynamic calculation engine that assesses energy use at much finer time intervals.

This shift allows HEM to capture peaks in demand, variations in weather, and the performance of systems like heat pumps more realistically.

Accuracy and realistic modelling

HEM offers a more accurate representation of:

• A home’s energy performance
• Energy consumption patterns
• The impact of new technologies
• Interaction between systems

This is critical for demonstrating compliance and meeting government aims for low-carbon homes.

Technology readiness

SAP was not designed for widespread deployment of:

• Heat pumps
• Battery storage
• Energy flexibility solutions
• Smart controls

HEM has been developed specifically to model these technologies effectively.

Data and time resolution

HEM uses higher-resolution weather data and shorter calculation time steps. This enables:

• More detail in energy usage modelling
• Improved understanding of demand
• Better assessment of energy supply and storage

In simple terms, SAP gives an averaged estimate of energy use, while HEM is designed to model how a home actually uses energy day-to-day. That means decisions about compliance and design will rely more on detailed, hour-by-hour-style performance results, especially for modern systems like heat pumps, rather than broad monthly averages.

Impact on Energy Performance Certificates (EPCs)

Although EPCs will continue to be the main document for communicating a home’s energy rating and energy efficiency rating, the underlying methodology used to generate them will change. HEM-based assessments are expected to:

• Improve the reliability of energy ratings
• Better reflect a home’s actual energy use
• Support more meaningful energy efficiency improvements

For homeowners, this should make EPC recommendations feel more relevant. For developers and lenders, it means EPC data may become a stronger indicator of a property’s likely running costs and the overall building’s energy performance.

When will HEM replace SAP?

Whilst only indicative, HEM is intended to be introduced alongside the Future Homes Standard, with SAP remaining in use for a transition period. Some industry briefings suggest this transition could be 2026–2027, but the length of the dual-running period and the point at which HEM becomes the default are still subject to final confirmation.

If you’re starting a new build project now, SAP is still the most likely route in the near term, but it’s sensible to plan for HEM if your project timeline could extend into future compliance dates under the Future Homes Standard and Part L updates.

Cost implications – what will it cost?

In the short term, when applicable, booking HEM testing (and any associated modelling) is likely to cost more than a typical SAP assessment, mainly because it requires new tooling, additional inputs, and more analyst time.

Short-term costs

In the early stages, HEM may increase costs due to:

• New software providers entering the market
• Training requirements
• More detailed modelling and data inputs
• Longer assessment times during the formative stage

Long-term value

Over time, HEM is expected to reduce risk by:

• Minimising performance gaps
• Improving design decisions earlier in projects
• Enabling better compliance outcomes
• Supporting cost-effective energy efficiency measures

Overall, expect HEM to cost more than SAP at first, as it involves more detailed modelling, new tools, and processes. Over time, as the approach becomes standard and quicker to deliver, pricing should stabilise, and the return is that you’re less likely to face late-stage redesigns, compliance delays, or unexpected costs because performance has been modelled more realistically from the start.

How to prepare for the transition from SAP to HEM

To prepare for HEM vs SAP changes, projects should:

• Assess current compliance strategies
• Review heating system choices
• Consider future-ready technologies early
• Engage energy specialists with experience in both methodologies

Early planning is critical to reduce the risk of late redesigns, programme delays, and avoidable cost increases when the methodology changes.

Summary

The planned move from SAP to HEM represents a fundamental shift in how the energy performance of new homes is assessed in the UK. By introducing dynamic modelling, higher time resolution, and better treatment of modern technologies, HEM is designed to close the gap between predicted and real-world energy use, supporting the Future Homes Standard and the government’s wider carbon reduction goals.

While the intended transition will bring changes to compliance processes, costs, and project planning, it also creates an opportunity to make better-informed design decisions earlier and reduce the risk of late-stage issues.

Understanding how HEM vs. SAP differs, when changes are expected, and how to prepare is critical for anyone involved in residential development, and working with experienced specialists like Falcon Energy can help ensure projects remain compliant, cost-effective, and future-ready.

Contact Falcon Energy today about SAP and HEM services

Contact us today to discuss SAP assessments and prepare confidently for the transition to HEM. Our energy specialists support projects from early design through compliance, helping optimise energy performance and control costs.

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