Free Extension Excess Glazing Calculator
Free Part L screening tool
Extension Excess Glazing Calculator
Check whether the total area of windows, external glazed doors, roof windows and rooflights in a proposed domestic extension exceeds the standard opening-area allowance.
Maximum opening area = 25% of the new extension floor area + the area of existing openings removed or covered by the extension.
Need an excess glazing assessment?
Barlings KWA can provide area-weighted U-value calculations, SAP comparisons and supporting information for Building Control.
Request an assessmentThis calculator provides an initial indication only. It does not confirm full compliance with Part L or assess elemental U-values, thermal bridging, building services or other Building Regulations requirements. The applicable Approved Document edition and any transitional provisions should be confirmed for the project.
What does the excess glazing calculator check?
For a domestic extension, the standard opening-area allowance is calculated as 25% of the new extension floor area, plus the area of any existing windows and doors that will no longer exist or will no longer be exposed because of the extension. The opening area includes windows, external doors, roof windows and rooflights.
This calculator checks only the opening-area allowance. It does not assess U-values or confirm full Building Regulations compliance.
What happens if the opening area is too large?
Exceeding the standard allowance does not necessarily mean that the extension cannot comply. It may be possible to demonstrate compliance through an area-weighted U-value calculation or a SAP comparison that takes account of improvements elsewhere in the design.
Frequently asked questions
What counts as an opening?
Include windows, fully glazed external doors, roof windows and rooflights within the proposed extension.
Do bifold and sliding doors count?
Yes, where the bifold or sliding doors are fully glazed. Include the complete area of the glazed door opening.
Can existing windows and doors be added to the allowance?
Existing openings that are removed or will no longer be exposed because of the extension can be entered in the relevant field. Their area is added to the standard allowance.
Does exceeding the allowance mean the extension will fail?
Not necessarily. An area-weighted U-value calculation or SAP comparison may provide an alternative route to demonstrating compliance.
Does this calculator confirm Building Regulations compliance?
No. It provides an initial check of the opening-area allowance only. U-values, thermal bridging, building services and other Building Regulations requirements must also be considered.
Planning a home extension?
Keep your whole project organised
The ClearHomeProjects Complete Home Extension Pack includes a 44-page printable planner, Excel project toolkit and worked example to help you plan budgets, approvals, quotations, tasks, orders and construction progress.
View the Home Extension PlannerClearHomeProjects digital download – England Edition