Is a SAP Assessment Required for an Extension?

If you’re planning a new build residential dwelling or conversion, Building Regulations require that a SAP assessment be submitted during the planning stages, but are SAP assessments required for extensions?In this article, we go over everything you need to know about SAP assessments and extensions. 

So, is a SAP assessment required for an extension? Not all extensions will require a SAP assessment, provided that glazing accounts for no more than 25% of the extension. However, many modern homeowners that plan on building extensions favour light and airy rooms which will exceed this 25%, thus requiring a SAP assessment. 

Read on to learn more about when SAP extensions are required for extensions.

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Do I Need a SAP Calculation for My Extension?

SAP assessments are not always required for extensions, depending on the level of glazing that is included in the extension, and if the plans meet Building Regulations. 


However, there is a nuance with this; part L1B of the Building Regulations offers a more flexible approach to SAP assessments for extensions, but two other reports must be considered.

The Notional Report

A SAP calculation is made on the existing house with an extension that has been built to the exact specification of the Building Regulations, including exact U values (thermal insulation), and glazing capped at 25%.

The Proposed Report

A SAP calculation is then produced for the house and extension as they will actually be built. The Proposed Report may include upgrades to elements within the existing house, or overcompensation for other elements in the extension.

What This Means

With these two reports, it must be proven that the CO2 levels from the house and the extension that will actually be built must not exceed the CO2 levels detailed in The Notional Report. This ensures that, even with compensations made for additional glazing, the extension is energy efficient.


If you’re more interested in SAP assessments for new builds or conversions, view our recent blogs for more information. We detail everything you need to know to be able to get a good SAP score, or improve your EPC rating. 

Why Do I Need a SAP Calculation for an Extension?

SAP calculations are required in order to determine the energy efficiency of the extension. We carry out the two SAP calculations outlined above to demonstrate that the carbon emissions for the proposed extensions are equal to, or lower than, those of the Notional Report, which would only have 25% glazing.

How Do I Pass a SAP Assessment?

To pass a SAP assessment for an extension, you must prove that the carbon emissions of the house and extension will be lower than that of The Notional Report which has glazing capped at 25%. However, we do find that the extra solar gain through the increase in glazing can reduce carbon emissions. 

To compensate for this, there are a number of things which will help;

  • Use of double glazing
  • Insulation
  • Low energy lighting

Who Can Carry Out SAP Calculations?

Only accredited SAP assessors that are registered with a certified body are able to carry out SAP assessments. RS Energy are qualified ABBE On Construction Energy Assessors and are accredited by the Building Research Establishment (BRE). 

As such, we are experts on energy efficiency and compliance and are able to provide advice that helps you achieve compliance efficiently and cost-effectively.

How Long Do SAP Calculations Take?

How long SAP calculations take depends on the timeline of the project but, with extensions, you’ll likely also need to account for two SAP calculations. Each SAP calculation can take anywhere from 7-14 days to complete, but book your assessment in plenty of time so that this doesn’t setback your building timeline.

SAP Calculations with RS Energy

RS Energy provides a fully supported SAP Calculations service for Architects, Developers, and Builders in England and across the UK. As accredited On Construction Domestic Energy Assessors (OCDEA), we’ll efficiently take care of your SAP calculations from design to build and EPC lodging. 

We work on new build domestic dwellings, conversions, and extensions, as well as providing energy statements, and water calculations. Get in touch with us today to learn more.