At EPC & SAP Part L, we're here to support your extension project every step of the way, from design to completion. Our goal is to ensure that all assessments are carried out to achieve compliance while offering cost-effective and practical solutions to ensure the seamless delivery of your project.

When do you need extension SAP Calculations?

Building Regulations Part L1b stipulates that all extensions with a glazing area exceeding 25% in comparison to the extension floor area must have extension SAP Calculations or heat loss calculations conducted.

For example:

If your extension measures 20m2, you have an 'allowable' glazing area of 5m2 (25% of 20). If your extension doesn't exceed this figure, heat loss calculations or extension SAP Calculations are unnecessary for Building Regulations compliance. However, if your extension surpasses the 25% threshold, SAP calculations or heat loss calculations will be required to demonstrate compliance.

Percentage calculations, points to consider:

When determining if extension SAP calculations are necessary, consider the following:

- All openings are accounted for in the calculations, including glazed doors, windows, and rooflights. Ensure these areas are included.
- If you're covering or removing any windows/doors during the extension process, these areas can be added to the 25%. This adjustment can be advantageous if you're just over the 25% mark. For instance, if your 20m2 extension previously had french doors covering 2.5m2 of glazing on the external wall, which will be removed to accommodate the extension entrance, this 2.5m2 of removed glazing can be added to the 25%, increasing your 'allowable' glazing area from 5m2 to 7.5m2.

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Learn more about our SAP calculations service, including the process and inclusions, by visiting our website.