Extension Glazing Calculator - Reference Method

Planning an extension with bi-folds, a roof lantern, or lots of glass? Part L limits glazing to 25% of floor area (plus any windows you're covering over). Use this calculator to check if your design fits within that allowance. If you don't pass this reference method check, the same design could pass with an area weighted u-value check or a whole house SAP Calculation for your extension.

As described in Part L, section 10.7, this is a reference method check only. It doesn't replace a formal SAP calculation where one is required. This tool is in beta, please check for mistakes. If you find an error, please let us know at [email protected].

Extension Floor Area

The internal floor area of the proposed extension.

Openings Covered by Extension

Windows or doors in the existing house that will be covered or removed by the extension. Part L allows 25% of floor area plus the area of any openings being covered.

Proposed Glazing

Windows, doors and rooflights you're planning to install in the extension.

Thermal Performance (U-Values)

Confirm each element meets the Part L limiting U-value:

Can't tick all these boxes? You might still pass using the area-weighted method. If some elements perform better than the limit, they can make up for others that don't.

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Glazing Area Check

Pending
Floor area 0 m²
25% allowance 0 m²
Covered openings + 0 m²
Permitted glazing 0 m²
Proposed glazing 0 m²

U-Value Check

Pending

Confirm all elements meet Part L limits above.

Overall Result

Complete the form to see your result

When This Calculator Won't Be Enough

This works for most simple rear and side extensions. There are some limitations:

Not sure? Send us your drawings and we'll tell you what you need. No obligation.

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