Free building fabric tool
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 the entered design sits outside the comparison limits, a full whole-dwelling extension assessment can provide a more precise route and may still demonstrate compliance.
Your entries stay on this device and are used to create your saved input file or PDF summary. Briary records only privacy-minimised, pseudonymous tool events such as a view, result or download; no project or contact fields are sent. Clear the calculator after use on a shared device.
Project details
Person completing this calculation
Actual extension fabric (inputs)
Enter net opaque areas and opening areas, plus their 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 |
Roof opening locations
If you enter roof windows or rooflights, identify the roof construction that contains each type. This lets the comparison keep every gross roof plane the same size while opening and opaque areas are redistributed. If one opening type spans more than one roof construction, this simplified calculator is not suitable; use a project-specific assessment.
Confirm every applicable exposed wall, roof, floor, window, roof window, rooflight and door is represented. A missing row can make the comparison look better than the real design.
Openings allowance (inputs)
Opening area by orientation (optional)
Optional breakdown to help show how the window and roof-opening allowance is distributed. Doors are excluded from this table and its notional total.
| 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. The screen and PDF summary are not a SAP assessment, a building-control submission or evidence of compliance.