Helping highly glazed extensions achieve Part L compliance
At Barlings KWA, we provide professional excess glazing calculations for house extensions and glazed building alterations across Lincolnshire and the UK.
If your extension includes large bifold doors, roof lanterns, sliding doors, or extensive glazing, Building Control may require a calculation to demonstrate compliance with Building Regulations Part L.
We help homeowners, builders, architects, and developers achieve compliance quickly and cost-effectively.
What Are Excess Glazing Calculations?
Building Regulations Part L generally limits glazing in domestic extensions to:
25% of the extension floor area
plus
the area of any existing windows or doors being removed
If your proposed glazing exceeds this allowance, an excess glazing calculation is usually required to show that the extension still performs thermally as well as a compliant design.
This is common with:
- bifold doors
- sliding patio doors
- roof lanterns
- large fixed glazed screens
- corner glazing
- glazed rear extensions
Do I Need One?
You may need an excess glazing calculation if your extension includes:
- full-width bifold doors
- large rooflights or lanterns
- floor-to-ceiling glazing
- glass corner details
- modern “glass box” extensions
A typical example:
A 20m² extension would normally allow:
5m² glazing (25%)
If you are removing an existing 3m² patio door, your total allowance becomes:
8m² total glazing
Anything above this may require a calculation.
How We Demonstrate Compliance
At Barlings KWA, we carry out detailed calculations to compare your proposed extension against the notional Part L compliant standard.
This may include:
- area weighted U-value calculations
- SAP extension calculations
- thermal performance comparisons
- glazing ratio checks
- insulation upgrade recommendations
Where necessary, we advise on improvements such as:
- better wall insulation
- improved roof U-values
- upgraded glazing specifications
- lower U-value frames and glass
- reduced rooflight area
The aim is to help retain your preferred design while achieving approval.
Common Projects We Assess
We regularly assess:
- rear extensions with bifolds
- glazed kitchen extensions
- orangery-style extensions
- lantern roof extensions
- side return extensions
- wrap-around extensions
- highly glazed garden rooms
Request a Quote
Planning a highly glazed extension?
Contact Barlings KWA today for a fast quotation and expert advice.
Email:william@barlingskwa.co.uk
Call: 01522 797344