Extension openings calculator

Part L · England & Wales

Extension openings calculator

Estimate the basic permitted area of windows, external doors, roof windows and rooflights in a domestic extension.

Project details

This calculator is for extensions to existing dwellings.

Jurisdiction

Extension internal floor area (m²)

Calculate floor area from length and width

Length (m)

Width (m)

Existing openings being lost

Add existing windows or doors that will no longer exist or be exposed because of the extension. Leave empty if none.

Type	Width (mm)	Height (mm)	Qty	Area	Remove

Proposed extension openings

Measure the overall width and height of each complete unit, including its frame.

Type	Width (mm)	Height (mm)	Qty	Area	Remove

Your calculation

Enter the extension floor area to begin

25% base allowance—
Lost-openings allowance—
Total permitted openings—
Total proposed openings—
Allowance remaining—

0% of the calculated allowance used

The basic allowance is 25% of the extension floor area plus eligible existing openings lost because of the extension.

This is an initial design check, not confirmation of Building Regulations compliance. If the proposed area exceeds the basic allowance, alternative calculations or compensating measures may still be available. Either by using an Area Weighted U-value Calculation (if the extension hasn't been built yet) or by using Sap Calculations and the whole house method. You can Get A Quote For Sap Calculations.