Light-filled extensions, practical advice
Keep your design moving.
Large sliding doors, bifolds and rooflights can transform an extension. If the proposed openings exceed the standard allowance, we can assess the design and explain the options.
William and Kerry help you work through the calculations and specification choices, with clear supporting information for Building Control.
Try our free excess glazing calculator →
Discuss your project →
What you receive
Understand the design and the way forward.
Review your proposals
We review the extension drawings, opening areas and available specification to establish the assessment needed.
Compare thermal performance
An area-weighted U-value calculation or SAP comparison may be appropriate, depending on the project.
Explain practical options
Where improvements are needed, we discuss fabric and glazing specifications and provide the agreed calculations and reporting.
Feedback from an extension client
“Kerry and William were both friendly and efficient and I was very promptly provided with calculations that satisfied the requirements of the Building Regulations.”Terrence Wren · Extension calculations client
Common reasons to get in touch
Planning more glass?
Bifolds and sliding doors
Wide openings connecting your extension to the garden.
Rooflights and lanterns
Additional daylight above a kitchen, dining room or living space.
Highly glazed designs
Corner glazing, side returns and contemporary glazed extensions.
Start with a simple check
Over the allowance doesn’t mean out of options.
The opening area is a starting point. A more detailed thermal comparison may show how your design can meet the relevant requirements.
Check your glazing area
Use our free calculator for an initial indication based on the measurements you enter.
Open the free calculator →Review the specification
Wall and roof insulation, window and door U-values and rooflight areas can affect the result. We explain which options are relevant to your design.
Ask us to review your extension →A straightforward process
From extension drawings to clear calculations.
Send your drawings
Share existing and proposed drawings, the project address and any construction or glazing specification.
Review the options
We confirm the assessment scope, carry out the calculations and discuss any improvements needed.
Receive your report
We provide the agreed calculations and supporting information for your project team and Building Control.
A real project · Lincolnshire
Excess glazing assessment for a Lincolnshire extension
A proposed 37.09 m² extension included 24.44 m² of new windows, glazed doors and rooflights. This exceeded the 9.27 m² glazing allowance used in the project’s reference calculation.
Barlings KWA prepared two SAP assessments: one modelling the proposed design and another modelling a reference version with less glazing. We also calculated the thermal performance of the extension’s walls, floor and roof.
The proposed specification combined improved floor and roof insulation with better-performing windows and rooflights. The calculations showed that the design, including its additional glazing, performed better than the reference version for carbon emissions, primary energy and fabric energy efficiency.
Our report concluded that the assessed design demonstrated compliance with Part L, providing supporting calculations for Building Control while retaining the proposed glazing area.
Planning an extension with large windows or rooflights? We can assess your drawings and explain what specification is needed to support your design.
Ask about your extension →Useful answers before you enquire
Your excess glazing questions.
Does a highly glazed extension automatically fail?
No. Exceeding the standard allowance may mean that an additional calculation is needed. We can assess the design and explain the available options.
Can I use the free calculator instead of a report?
The calculator provides an initial check. It does not replace a project-specific assessment or the evidence requested by Building Control.
What information should I send?
Send the existing and proposed drawings, project address and available details of the walls, floor, roof, windows and doors. We’ll confirm anything else needed.
Can you help me retain my preferred design?
That is our aim. We review the thermal performance and explain practical specification options. The result depends on the design and assessment.
When should I arrange the calculations?
Before finalising the specification or ordering glazing where possible, while there is still scope to consider changes.
Which Building Regulations guidance applies?
The applicable edition depends on the project and its regulatory arrangements. We can discuss this with your designer or Building Control. View the official Approved Document L guidance.
Your project, your quotation
Let’s look at your extension.
Typical cost of excess glazing calculations
£216–£288 including VAT (£180–£240 plus VAT at 20%). This is a guide price; we’ll confirm the final quotation after reviewing your drawings and project requirements.
Tell William and Kerry what you are planning and whether Building Control has requested calculations. We’ll confirm the information needed and provide a quotation.
Useful details: site address · existing and proposed drawings · glazing specification · construction details · deadline