SAP and Part L Volume 1: What Affects Your EPC Rating?

SAP calculations play a central role in demonstrating compliance with Part L Volume 1 for new dwellings, but achieving compliance doesn’t automatically mean you’ll get the EPC rating you expect. Fabric performance, heating, ventilation, air tightness, renewables, and even specification changes during construction can all affect the final result.

In Episode 8 of the Stroma Insights Podcast, we take a practical look at how SAP, Part L, and EPC ratings work together, what developers need to consider throughout design and construction, and how early assessment can help prevent unexpected compliance or EPC issues at completion. Listen to the full episode below or read on for practical guidance from our building performance specialists.

Episode 8: Understanding Part L Vol 1 Compliance – What Specifically Affects the EPC

What Is EPC and Why Does It Matter?

An Energy Performance Certificate (EPC) provides an A-G rating indicating the predicted cost efficiency of a dwelling based on standard assumptions about how it will be occupied and used. This means the figures won’t replicate an individual household’s actual energy bills, but they offer a consistent basis for comparing the energy performance of different homes.

For developers, the rating is increasingly important because buyers can use it to evaluate a property’s expected energy performance before deciding to purchase. A higher EPC can make a property more attractive to buyers concerned about running costs, while some housebuilders, lenders, and funding arrangements may also set minimum EPC targets.

What Is the Difference Between SAP and EPC?

SAP stands for “Standard Assessment Procedure” and is the methodology currently used to assess the compliance of new dwellings for Building Regulation Part L.

SAP calculations for Part L Volume 1 produce several outputs that help determine whether the proposed dwelling satisfies the relevant energy-efficiency requirements.

The Energy Performance Certificate (EPC) is generated using information from that SAP assessment, but its rating has a different purpose.

The EPC provides a cost-based measure of regulated energy use, while Part L compliance considers several performance measures across the dwelling.

Why Can SAP and EPC Results Differ?

Building Regulations Part L assesses several aspects of a dwelling’s energy performance, including carbon emissions, primary energy use, and the performance of the building fabric.

The EPC rating uses the same underlying investigation but focuses on the predicted cost of regulated energy use, so the two don’t always reward the same specification in the same way.

Heating gives us a useful example:

An efficient heat pump can decrease the carbon emissions from heating because it uses electricity instead of burning gas within the home. This can benefit the Part L Volume 1 calculation, where reducing carbon emissions is an important part of compliance.

However, the EPC also accounts for the cost of the energy used within the home. So, if electricity has a higher measured cost per unit than gas, an electrically heated home can have reduced carbon emissions while still receiving a lower EPC rating than expected.

This is why a specification can perform well for Part L compliance without automatically achieving the EPC band a developer is targeting.

A High EPC Rating Doesn’t Guarantee Part L Compliance

It’s possible for a dwelling to achieve a strong EPC rating and still fall short of one of the requirements assessed through Part L Volume 1. Equally, a specification that complies with Building Regulations won’t automatically deliver an EPC A or B.

If you have a set EPC target for your development, tell your SAP assessor at the beginning of the project. They can review the design against both your regulatory obligations and the performance you want the completed dwelling to achieve.

What Affects Your SAP Calculation and EPC Rating?

Modern SAP assessments place significant emphasis on how the complete dwelling performs. You have less scope to compensate for a weak part of the specification with one improvement elsewhere, so you need to think about each element as part of the wider design.

Key areas include:

• Wall, roof, and floor performance
• Windows and doors
• Thermal bridging
• Air tightness
• Ventilation
• Heating and hot water systems
• Solar PV and other renewable technologies

Fabric Performance and Thermal Bridging

U-values indicate how readily heat passes through elements such as walls, floors, roofs and windows, so the building fabric specification has a direct influence on the SAP calculation. Installation must also reflect that specification: insulation with gaps or poor fitting won’t perform as the design assumes.

Thermal bridging considers heat loss at junctions within the fabric, including around windows and where walls meet floors or roofs. These can contribute significantly to the overall calculation, which is why changing a wall build-up or insulation product can have effects beyond its headline U-value.

Air Tightness and Ventilation

Minimising unwanted air leakage can improve energy performance, but the proposed air tightness level must also support required ventilation levels. Intermittent extract fans, continuous mechanical ventilation, or mechanical ventilation with heat recovery will each have different design considerations.

An unexpectedly low air permeability result can therefore create its own ventilation constraints. This means that the SAP, air tightness, and Part F strategy should be looked at together, so the finished home can achieve the intended energy performance without compromising suitable ventilation.

Heating and Hot Water Systems

The exact heating design can affect both SAP compliance and the EPC rating. For heat pumps, SAP uses recognised product performance data, meaning the model selected and its combination with equipment – such as the hot water cylinder – can change the calculated result.

If the specified heat pump changes during procurement, ask your SAP assessor to check the replacement before installation. A product that appears equivalent may have different recognised performance data, which can influence the final SAP result and move the EPC into another rating band.

Solar PV and EPC Ratings

Solar PV can make an important contribution to energy performance, particularly as new-build standards place greater emphasis on renewable electricity. For an EPC, however, the calculation needs to recognise how the generated electricity benefits the individual dwelling being assessed.

This can call for additional thought on apartment developments (flats), where roof area is shared across several properties and PV may supply communal services. If a particular EPC rating forms part of the brief, the proposed renewable energy strategy should be checked during design.

Carry out SAP Calculations at Design Stage

The best opportunity to resolve SAP calculation and Part L Volume 1 issues comes before construction begins. A detailed design-stage assessment allows your assessor to model the proposed dwelling, identify any gaps, and test potential changes while the specification is still flexible.

For a robust SAP calculation, prepare as much confirmed information as possible, including:

• Detailed plans, sections, and elevations
• Wall, floor, and roof specifications
• Insulation products and thicknesses
• Window and door performance
• Thermal bridging details
• Heating and hot water systems
• Ventilation strategy
• Air tightness target
• Proposed renewable technologies

Some assumptions may still be necessary during design, particularly where products haven’t been selected. Keeping these to a minimum gives you a more reliable indication of both Part L compliance and the EPC rating you can expect at completion.

Check Specification Changes Before Installation

It’s natural for construction projects to evolve. Products may become unavailable or costly, and alternative specifications sometimes make more practical sense. These changes don’t necessarily create a problem, but wherever possible they should be checked against the SAP calculation before they’re incorporated into the build.

For example, changing an insulation product, window specification, heat pump, or other component can affect several parts of the energy efficiency calculation. Send the proposed replacement to your SAP assessor first, and they can confirm whether the dwelling still meets the required performance before the change becomes difficult to reverse.

Collect Part L Photographic Evidence During the Build

Part L Volume 1 requires photographic evidence to demonstrate that relevant construction details match the assessed specification. Some of the necessary elements will be concealed as the project progresses, so evidence must be captured at the appropriate construction stage.

Depending on the project, this may include photographs of:

• Insulation and building fabric details
• Relevant junctions within the construction
• Primary pipework insulation
• Heating system equipment and data plates
• Other elements required by the Part L photographic record

Photographs taken for building control, warranty providers, or other purposes won’t necessarily cover the information needed for SAP. At Stroma, we can give access to our app, which helps project teams capture, organise, and submit the relevant evidence by plot throughout construction.

Keep the Design and As-Built Assessment Aligned

If the design-stage SAP is based on a developed specification and the dwelling is constructed accordingly, the final evaluation should largely confirm the result already expected. At the as-built stage, information such as the final air permeability result, photographic evidence, and confirmed installed specification can be incorporated.

Significant differences become much harder to resolve if they’re first discovered at completion. Keeping your SAP assessor updated throughout the build process gives you the opportunity to review changes as they happen and maintain a clear picture of both Part L compliance and the expected EPC rating.

Completion Without Unnecessary Delays

The final details also need to be accurate before the EPC can be completed and lodged.

New developments are often managed using plot numbers during construction, so the Stroma team will need the confirmed postal addresses and a clear schedule showing which address corresponds with each assessed plot.

Providing this information alongside the final specification, air test results, and photographic evidence helps the completion process run smoothly. It also ensures the EPC records the correct property details for the homeowner who receives it.

In Summary

SAP calculations, Part L Volume 1, and EPC ratings are closely connected, but they don’t measure exactly the same thing. If your project has both a regulatory requirement and a target EPC rating, each needs to be addressed from the beginning.

The practical steps are straightforward:

1. Get a detailed design-stage SAP assessment before construction
2. Tell your SAP assessor about any specific EPC target
3. Consider fabric, services, ventilation, and renewables together
4. Check building specification changes before installation
5. Capture Part L photographic evidence throughout construction
6. Provide accurate as-built information and final property addresses
7. Keep your SAP assessor updated throughout the project

The more thorough the information is at design stage, the fewer unknowns you carry into construction and completion.

Talk to Stroma About SAP Calculations and Part L Volume 1

Stroma Built Environment supports residential developers, designers, and construction teams with SAP calculations, energy performance, and Part L compliance throughout the development process. We can assess your proposed specification, identify potential issues, and explain how design changes may affect both compliance and the final EPC rating.

If you’re planning a residential development, targeting a particular EPC rating, or need advice on an existing SAP assessment, speak to our team as early as possible. We’ll be happy to help you understand what information is required and what your current specification means for the rest of the project.

FAQs

What is the difference between SAP and EPC?

SAP is the assessment methodology used to calculate the energy performance of new dwellings and demonstrate relevant Part L requirements. An EPC is one of the outputs produced using the SAP assessment and delivers an A-G rating based on the predicted cost efficiency of regulated energy use.

Can a property pass Part L and still have a lower EPC rating?

Yes. Part L Volume 1 measures several performance requirements, including carbon emissions, primary energy, and fabric performance, while the EPC uses a cost-based rating. A dwelling can therefore comply with Building Regulations without achieving the EPC band originally expected.

When are SAP calculations required?

A design-stage SAP calculation should be carried out before construction begins so the proposed dwelling can be reviewed against Part L Volume 1. The calculation is then updated at completion using the confirmed as-built specification, air permeability result, and other required evidence.

What information is needed for a SAP calculation?

Your assessor will typically need detailed drawings and information covering the building fabric, windows and doors, thermal bridging, heating, hot water, ventilation, air tightness, and renewable technologies. Providing a developed specification minimises the number of assumptions required in the calculation.

Can I change the specification after the design-stage SAP calculation?

Yes, specifications can change during construction, but you should tell your SAP assessor before installing the replacement where possible. They can update the calculation and check whether the change affects Part L compliance or your expected EPC rating.

Why does Part L require photographic evidence?

Photographic evidence helps demonstrate that key parts of the completed dwelling have been constructed in line with the assessed specification. Because some details become concealed during construction, the relevant photographs need to be collected as work progresses.

How does EPC rating affect new build value?

A higher EPC rating can make a new home more attractive to buyers by indicating lower predicted energy running costs. It may also affect eligibility for certain green mortgages, funding arrangements, or developer requirements, although the EPC rating alone doesn’t determine a property’s market value.