Extension calculator
Area weighted U-value method (Approved Document L, Volume 1, 2021, paragraph 10.9). This tool is for guidance only.
This simplified area weighted method is provided free of charge to support industry stakeholders. Where a project does not pass here, a full whole-dwelling extension assessment can provide a more precise route and may still demonstrate compliance.
Project details
Person completing this calculation
Actual extension fabric (inputs)
Enter net areas and U-values. Leave unused rows blank. Heat loss is area × U-value.
| Element | Net area (m²) | U-value (W/m²K) | Heat loss (W/K) |
|---|---|---|---|
|
External walls
|
0 | ||
|
Roof, insulation at rafters
|
0 | ||
|
Roof, flat roof
|
0 | ||
|
Roof, insulation at joists
|
0 | ||
|
Ground floor
|
0 | ||
| Openings | |||
|
Windows
|
0 | ||
|
Roof windows
|
0 | ||
|
Roof lights
|
0 | ||
|
Doors
|
0 | ||
| Totals | 0 | - | 0 |
Openings allowance (inputs)
Opening area by orientation (optional)
Optional breakdown to help you show how the allowance is distributed. Doors are not included in this breakdown.
| Orientation / type | Proposed openings (m²) | Share | Notional openings (m²) |
|---|---|---|---|
| North | 0% | 0 | |
| North East | 0% | 0 | |
| East | 0% | 0 | |
| South East | 0% | 0 | |
| South | 0% | 0 | |
| South West | 0% | 0 | |
| West | 0% | 0 | |
| North West | 0% | 0 | |
| Roof windows | 0% | 0 | |
| Roof lights | 0% | 0 | |
| Total | 0 | 100% | 0 |
Disclaimer
This tool is provided for convenience. Briary has not inspected your project, has not verified inputs, and was not involved in the design. Outputs depend entirely on what you enter.