What are Excessive Glazing Calculations
Modern extensions often feature large windows, rooflights and sliding doors. However, under Building Regulations Part L, glazing in an extension is normally limited to:
Up to 25% of the extension’s floor area
(plus any existing openings that are removed)
Because glazing loses more heat than insulated walls, Building Control requires extra checks when this limit is exceeded.
When Are Excessive Glazing Calculations Needed?
You will need Excessive Glazing Calculations if your extension includes:
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Large areas of glazing or bifold doors
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Full-height windows
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Roof lanterns or multiple rooflights
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Any design where glazing exceeds 25% of the floor area
If the limit is exceeded, Building Control will ask for a formal calculation to show your extension still meets energy-efficiency requirements.
How Compliance Can Be Demonstrated
1. Area-Weighted Heat Loss Calculation
This compares a “standard compliant” extension with your proposed design.
If the heat loss in your design is equal to or lower than the standard, the extension can be approved.
2. Full SAP Calculation (if required)
If heat loss is too high, a full SAP assessment may be needed.
This can include reviewing insulation, windows, heating systems, and sometimes recommending small upgrades to the existing property to offset the additional glazing.
Information Needed for the Calculation
To complete the assessment, the following are required:
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Scaled construction drawings
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Existing and proposed floor plans
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Existing and proposed elevations
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Site plan with orientation
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Sections and technical details
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Details of walls, floors, roofs and insulation
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Window and door specifications (sizes and U-values)
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Heating system details
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BGA Architects can produce all necessary drawings and specifications, and coordinate with the energy consultant on your behalf.
Why These Calculations Matter
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Ensures your extension complies with Part L
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Avoids project delays at Building Control stage
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Allows you to retain large glazing features without redesign
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Helps manage costs and prevent late-stage changes