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Extension Glazing Calculator

This free calculator is a reference method glazing area check for extensions in England. It checks your proposed windows, doors and rooflights against the opening allowance in Approved Document L, Volume 1, paragraph 10.7(d).

The allowance is 25% of your extension's internal floor area, plus any existing windows and doors covered or removed because of the extension. See our 25% glazing rule explained with a worked example.

Check my glazing

Already over the allowance? Try our free Area Weighted U-value calculator to check whether your insulation and glazing balance the extra heat loss.

Your measurements

Extension floor area

Use internal measurements in metres, or enter the total floor area in m².

Floor area: Enter length and width

Existing windows and doors covered by the extension

Only add windows and doors in the existing house that will be removed or will no longer face outside because of the extension. Approved Document L, Volume 1, paragraph 10.7(d)(ii) allows their area to be added to the opening allowance. If there are none, leave this section empty.

New windows, doors and rooflights

Add every opening in the extension, including roof windows and solid external doors. Measure the whole opening, including the frame, in metres. For rooflights, use the roof opening size.

You can see your result without entering contact details.

We save the measurements and result to help us understand extension designs.

This calculator checks the reference method opening allowance for extensions in England using the measurements you enter. It does not assess U-values or other compliance requirements of Part L. A pass does not confirm full Part L compliance or building control approval.

Results depend on accurate measurements. Check the figures and specification with your designer or assessor before ordering materials or starting work.

This is not a replacement for a full SAP calculation or area weighted U-value check.