Extension calculator

Have the extension drawings, a schedule of exposed elements and evidence for the proposed U-values ready. Identify which roofs contain roof windows or rooflights. This is a design comparison, not a replacement for a project-specific Building Regulations assessment. Check the method and edition stated on this page are appropriate to your project.

In Setup, enter the extension floor area and the area of existing openings removed because of the extension. These are allowance inputs, not the net areas of the proposed windows. Select the roof construction containing each roof-opening type you use. Leave an unused roof-opening type unassigned. If one type spans different roof constructions, obtain a project-specific assessment instead of forcing the geometry to fit.

In Fabric, work down the table. Enter net opaque wall and roof areas: subtract the windows, doors and roof openings in those elements first. Enter the openings separately in their own rows. Use square metres for area and W/m²K for U-value. A row you use needs both a positive area and a positive U-value; leave both cells blank for an element that is not present. Do not enter a zero U-value as a placeholder.

In Review, confirm completeness only after checking every applicable wall, roof, floor and opening. The completeness answer is your confirmation, not something the calculator can infer from the drawings. The next-action panel lists missing inputs and geometry problems. Choose Show me to open the relevant step and highlight the field, correct it, then repeat with the next item.

The fabric backstop check and the area-weighted comparison are separate. Improving one element does not automatically resolve a backstop failure elsewhere. The notional comparison preserves the entered gross envelope geometry. A geometry warning means you should recheck areas, allowances and roof assignments, not adjust figures simply to produce a favourable outcome. Detailed results show the element checks.

Missing or invalid inputs, an unconfirmed schedule, or inconsistent geometry stop the PDF report, and so does a missing name or company in Project details: the report records who carried out the calculation. The PDF button remains usable and explains the restriction, highlights every outstanding field and takes you to it. A complete calculation that is outside the entered limits can still be reported; that does not turn it into a compliant result. Save inputs remains available while you gather missing information.

Use Project details for the reference, address, client, assessor/contact details and notes. These describe the report and are separate from the design inputs. The name and company of the person completing the calculation are needed before a PDF can be produced; everything else there is optional. The PDF is named after the project and the calculation date. Save inputs downloads a .briaryext file; Open inputs restores that file. A PDF is an output record, not an editable input file. Check the restored details and assumptions before reusing a previous calculation.

Check units, net versus gross areas, duplicated openings and unused rows with leftover U-values. If PDF generation fails after the inputs are complete, save your inputs, retry, and email toolsuite@briary.co.uk with the tool name, browser, exact message and a screenshot. Do not send confidential project information unless it is needed. Read the tool privacy notice before saving or downloading a report.