What is Excess Glazing?
When building a domestic extension in the UK, Building Regulations require you to limit heat loss through glazing. If your design includes more glazing than the standard allowance, it’s classed as excess glazing — and you’ll need to take extra steps to show the extension still meets energy efficiency requirements.
For most extensions, the maximum permitted glazing area is 25% of the new floor area, plus the area of any existing external windows or doors that are being removed (for example, when knocking through to connect the extension to the main house).
Measures to Mitigate Failing Assessments
If your proposed building design exceeds the carbon emission or energy performance limits, there are several approved ways to achieve Part L compliance. These include upgrading insulation to external walls, roofs, and floors; installing a high-efficiency boiler or low-carbon heating system; and incorporating renewable technologies such as solar PV panels or wastewater heat recovery systems.
Benefits of Compliance
Excessive glazing without proper energy efficiency measures can lead to higher heating and cooling costs, uncomfortable overheating in summer, and increased heat loss in winter. Poorly designed glazing can also result in non-compliance with building regulations, potentially delaying planning permission or building control approvals. Optimising window-to-wall ratios and incorporating high-performance glazing ensures better energy efficiency, improved thermal comfort, and compliance with Part L and SAP requirements.
By ensuring your extension meets the necessary standards, you’ll create a home that’s not only compliant, but also energy-efficient, comfortable, and cost-effective to run.