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 calculator
- Measure the planned extension's floor area.
- Enter a cost per m² — the default is a rough mid-range figure for a standard single-storey extension.
- Set a contingency percentage.
- Read the base and contingency-adjusted budgets.
Assumptions
- The default £2,200/m² is a rough illustrative figure for a standard single-storey extension — actual costs vary hugely by region, foundation type, roof design and finish level.
- Does not include planning permission/building control fees, architect fees, or VAT unless included in your own rate.
Technical details: what the fields mean
- Extension floor area
areaM2 - Floor area of the planned extension.
- Cost per m² (check local quotes)
costPerM2Gbp - Cost per square metre — extensions are typically pricier per m² than internal renovation due to foundations, roofing and external walls. Varies hugely by region and spec.
- 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: areaM2=25, costPerM2Gbp=2200, contingencyPercent=15
Result: £55,000 base, £63,250 with contingency
Practical guidance
- Get quotes from at least 2–3 builders — extension costs vary widely based on foundation conditions and design complexity.
- Two-storey extensions and extensions requiring complex groundworks typically cost more per m² than a simple single-storey box extension.
Common mistakes
- Using a single-storey rate for a two-storey extension, which typically costs more per m² of ground-floor footprint.
- Forgetting planning/building-control fees and professional fees in the overall project budget.
FAQs
How much does a 25m² extension cost?
At a rough £2,200/m² with 15% contingency, about £63,250 — get real local quotes, as costs vary hugely by design and region.