Area Weighted U-value Calculator
Compare the heat loss through a proposed domestic extension with a notional extension of the same size and shape.
For domestic extensions in England under Approved Document L, Volume 1, effective from 15 June 2022.
1. Extension details
Optional project details are used only on screen and are not transmitted or stored.
2. Actual extension elements
Enter net exposed area and proposed U-value. Leave elements that are not present blank.
| Element | Net area (m²) | U-value (W/m²K) | Heat loss (W/K) |
|---|---|---|---|
| Total | 0.00 m² | 0.00 W/K |
Every element with an area must also have a positive U-value.
3. Notional opening allowance
The standard allowance is 25% of extension floor area, plus openings removed or no longer exposed because of the extension.
The proposed opening area must be greater than zero and the notional openings must fit within the wall and roof envelope.
Compare actual and notional extension calculations
| Element | Area (m²) | U-value | Heat loss |
|---|
| Element | Area (m²) | U-value | Heat loss |
|---|
What this calculator does and how to use it
What is an area-weighted U-value?
A U-value measures how readily heat passes through a building element. Lower values indicate less heat loss. An area-weighted U-value combines all exposed elements, weighting each U-value by its area.
The calculator multiplies each element area by its U-value, adds the heat losses, then divides by the total exposed area.
How to use it
- Select the standard that applies to the project.
- Enter the net exposed area and proposed U-value for every element used.
- Enter the extension floor area and any openings removed or covered by the work.
- Review both compliance checks and open the breakdown for the detailed comparison.