SAP Calculations for Extensions

SAP Calculations for Extensions

If your proposed extension contains large areas of glazing you will likely need a SAP calculation/ Heat loss assessment to ensure your proposal meets energy efficiency requirements.

Elmhurst Energy Services can complete SAP calculations for extensions to ensure your project complies with Building Regulations/ Standards.

When are SAP Calculations for Extensions required?

Part L1b of the Building Regulations (and regional variations) states that the total area of new glazing should not "exceed the sum of 25% of the total floor area of the extension plus the area of openings that no longer exist as a result of the extension works" unless 'compensating measures' are used elsewhere in the plans. If the sum is greater than this, a calculation will need to be justified to Building Control or a Verifier (Scotland). 

It is likely that extensions will exceed this amount as most individuals prefer to let more light in and maintain the appearance of open space with large glazed doors and windows all round the extension. 

Elmhurst Energy Services and SAP Calculations for Extensions

A full SAP Calculations can be used to demonstrate that "reasonable provision" has been made in order to ensure Building Regulation/Standards compliance, regardless of the amount of glazing included. This method also offers much greater design flexibility than other compliance methods. Our expert consultants have over 20 years of experience in this area and can guide you through all the options for what needs to be done.  


What we do to ensure compliance:

1. We use SAP to calculate the CO2 emissions for your existing dwelling along with your proposed extension.

2. We then calculate the CO2 emissions for the existing dwelling this time including a 'notional extension'. A notional extension is an extension that meets the minimum standards stated within the approved document/technical booklet.

3. Compliance is achieved, provided the 'proposed' CO2 emissions are no greater than the 'notional' CO2 emissions.

U-Value Calculations and Extensions 

An alternative but more stringent approach to compliance would be to undertake a U-Value Calculation. This would be used to demonstrate that new thermal elements introduced with the proposed extension, better the Area Weighted U-Values/ Standards shown in The Government's Approved Documents/ Technical Handbooks (See U-Value Calculations for further information).

Call us now on

01455 883 259

 and discuss your project with us, there’s no obligation and we’re happy to advise you.

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Related Links

SAP Calculations for Conversions > SAP Calculations for New Builds >

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