When are SAP Calculations for Extensions Required?

SAP calculations are used to determine whether a new build or conversion meets the requirements set out in the Part L building requirements document. Sometimes SAP calculations are also needed for extensions. In this blog you will learn what conditions mean your extension would require a SAP calculation.


So when are SAP calculations for extensions required? SAP calculations are required for extensions if the proposed area of new glazing exceeds 25% of the total extension area, in order to comply with Building Regulations.


Read on to find out what is needed for SAP calculations for extensions and how your result can be improved.

When Would You Need an SAP Calculation for an Extension?

SAP Calculations for extensions are related to the amount of glazing your extension will have. If your extension is open to the main home building and forms part of the thermal envelope, you will likely require a SAP calculation to determine the amount of heat loss. This also ensures that your extension would meet Part L1b Building Requirements. Upon receiving an acceptable SAP calculation result, you will receive a written report along with the SAP worksheets..

The maximum allowable glazing for an extension is 25%, and exceeding this threshold would require a SAP calculation. As extensions are often built to allow more light in the living area, it is likely that your extension will require a SAP calculation. The purpose of the calculation is to demonstrate that the carbon emissions for the existing house and proposed extension are equal to or lower than the existing house plus a notional extension, which would only have the 25% glazing allowance.. During the design stage, you will need to produce a full set of scale plans for the extension.

From June 2022, SAP 10 will go live, alongside an updated version of the Part L Building Requirements document. Make sure you keep up to date with the regulations and plan the materials you intend to use to minimise heat loss. The aim is to make the windows and glazing as air tight as possible with proper fitting and sealing, while still allowing for appropriate ventilation in the extension.

What Does a SAP Calculation for Extensions Involve?

During a SAP Assessment, an assessor will carry out a SAP calculation, to compare the proposed extension to a notional extension of the same size and orientation. The aim of this comparison is to demonstrate that the carbon emissions for the proposed extensions are equal or lower to those of the notional extension, which would only have the 25% glazing. The information required by the SAP assessor to carry out the assessment includes:

  • An accurate set of construction drawings, including a site plan
  • Specifications of the building method
  • The type and thickness of the insulation that will be used for the extension
  • The approximate age of the existing house
  • Details of windows and doors to be installed including the glazing to be used
  • Any heating systems that will be used in the extension
  • The type of lighting to be used
  • Details of the ventilation system 
  • Details of any renewable energy systems to be installed 

After the completion of the SAP calculations you will receive a report alongside the SAP worksheets which can be forwarded to your Building Control Officer. This will show that your new extension complies with Part L1b Building Regulations.

For more information on SAP calculations for extensions read our recent blog or contact a member of the RS Energy team and we will advise you from beginning to end.

How Can SAP Calculations for Extensions be Improved?

Planning ahead and ensuring high quality workmanship is essential in achieving a good SAP calculation result for your extension. Invest in good materials from the start, insulate the walls and ensure any openings are properly sealed to reduce energy waste. Changing your mind on the material or construction methods once building has commenced can be a costly mistake that could result in major delays to project completion.

To meet Part L1b Building Regulations new elements should meet the following u-values:

  • Floor – 0.22W/m2k
  • Walls – 0.28W/m2k
  • Roof (flat and insulated at rafters) – 0.18W/m2k
  • Roof (insulated at joists) – 0.16W/m2k
  • Windows – 1.6W/m2k whole frame u-value
  • Doors including glazed doors – 1.8W/m2k whole frame u-value

SAP Calculations for Extensions at RS Energy

In order to help you achieve compliance, we will carry out two SAP calculations, to compare the existing house + the proposed extension to a notional extension of the same size and orientation. RS Energy’s expert team provides SAP calculations for new builds, conversions and extensions.

Contact a member of our team today to find out whether you need an SAP calculation for your extension, and how we can help you meet the requirements.

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