Extension rough-cost estimator
Get a very rough single-storey extension budget from the floor area and a cost-per-square-metre rate, before getting real quotes.
Result
Base budget
Total budget with contingency
How to use this tool
- Measure the planned extension's floor area.
- Enter your own build rate per m² if available. The prefilled £2,200/m² is an adjustable illustration for rough planning, not a quote or guaranteed current price.
- Set a contingency percentage.
- Read the base and contingency-adjusted budgets.
Assumptions
- The prefilled £2,200/m² is a changeable example; it is not a price offered to the user or a fixed rate for extensions. Although the linked FMB guide discusses cost ranges, local quotes for a matching scope are a better basis.
- The calculation includes only the rate entered multiplied by floor area, then contingency. Planning/building-control charges, professional fees, VAT, surveys, utility changes and accommodation are excluded unless included in your rate.
Sources & reference data
Technical details: what the fields mean
- Extension floor area
areaM2 - Floor area of the planned extension.
- Cost per m² (check local quotes)
costPerM2Gbp - Your assumed build cost per square metre. Scope, ground conditions, region and specification affect the rate; use a comparable local quote where possible.
- Contingency
contingencyPercent - Extra budget for unexpected issues, common in groundworks and foundations.
Methodology
Base budget = area × cost per m². Total = base budget × (1 + contingency%).
Worked example
25m² single-storey extension at £2,200/m², 15% contingency
Inputs: Extension floor area: 25 m², Cost per m² (check local quotes): £2200/m², Contingency: 15%
Result: £55,000 base, £63,250 with contingency
Practical guidance
- Compare quotes against the same design and scope, including groundworks, specification and finish; otherwise the per-m² rates may cover different work.
- Before treating the estimate as a project total, check whether VAT, professional fees, approvals, surveys, utility changes and accommodation are in the entered rate; the calculator does not add them separately.
Common mistakes
- Using a single-storey rate for a two-storey extension, which typically costs more per m² of ground-floor footprint.
- Comparing rates without checking whether they include the same work or costs such as VAT and professional fees.
FAQs
Does this estimate include fees and VAT?
No. It multiplies your entered build rate by floor area and adds contingency. Include fees or VAT in your rate if appropriate, or budget for them separately.
Last reviewed: 2026-10-11
A very rough starting-point estimate, not a quote. This is not financial or investment advice.
This tool provides an indicative estimate, based on the assumptions stated above.
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