Floor area
Draw rectangles, circles or triangular areas in metres. Ground-floor and first-floor areas use separate editable rates per square metre.
FREE UK PLANNING TOOL
Plan a house extension and understand the likely cost
SolvBuildPlan is free browser-based house extension planning software. Sketch measured floor areas, compare ground and first-floor construction costs, add drainage and structural beams, calculate VAT and check the result against your budget.
Open the extension planner →What the extension planner can estimate
Drainage and water
Draw drainage routes at any angle or enter a manual allowance. Add the expected length of new water pipework.
RSJ support beams
Draw an RSJ beam between its endpoints. Its measured length is multiplied by an editable supply-and-installation rate.
Roof, VAT and budget
Match the roof to the ground-floor footprint or enter it separately, then see subtotal, VAT, Grand Total and remaining budget.
Indicative installed rates
| Work | Straightforward | Typical allowance | Difficult |
|---|---|---|---|
| Drainage | £150/m | £200/m | £350/m |
| RSJ beam | £250/m | £450/m | £750/m |
Important: These are editable early-stage allowances, not quotations. Ground conditions, beam weight, access, temporary works, waste removal, professional fees and regional labour prices can materially change the final cost.
How to use SolvBuildPlan
- Create and name a plan page.
- Choose a shape, enter a name, then click and drag on the metre grid.
- Set each floor area’s exact width, depth and height.
- Draw drainage pipes and RSJ beams between their endpoints.
- Edit the rates, VAT percentage and total project budget.
- Review or print the itemised estimate and confirm it with local professionals.
Frequently asked questions
Is this professional architectural or structural-design software?
No. SolvBuildPlan helps homeowners explore size and budget at an early stage. A qualified architect, architectural technologist, drainage specialist and structural engineer should confirm the buildable design.
Does it work on mobile devices?
Yes. The controls stack on smaller screens, the measured canvas supports touch interaction, and the cost panel remains available beneath the plan.
Can I make more than one option?
Yes. Create named plan tabs or duplicate an existing page to compare extension layouts.
Start a free extension plan →