Building Regulations L1B (2006 edition)
Heat Loss Calculator
(applicable only to domestic extensions)

How to use the Thermal Calculator

1. From the drawing, calculate the areas of each element and enter in the appropriate box below.
2. Enter the U-Value for the appropriate element (use manufacturers data).
3. Enter the required U-Value for the roof construction.

4. An area of allowable openings is automatically calculated (25% of total floor area), but you should also enter the area of any windows or doors which, as a result of the extension works, no longer exist or are no longer exposed.
5. Read result at bottom of spreadsheet.