Extension Cost Calculator
Estimate the build cost of an extension from your own rate.
Estimated total cost
£55,000.00
- Build cost
- £55,000.00
- Fees and extras
- £0.00
The default rate is illustrative only, not a current market quotation — enter a rate from your own quote.
v1.0 · Reviewed September 2026
How this is worked out
The build cost is floor area × the cost per square metre you enter, and any fees and extras are added to give the total.
Worked example
- Floor area 25 m²
- Cost per m² £2,200
- Fees and extras £0
25 × 2,200 = 55,000
£55,000.00
Assumptions and limits
- The default rate is only a starting figure for the arithmetic. It is not a market average, so replace it with a rate from your own quotes.
- VAT, planning and building control fees, kitchens, bathrooms and landscaping are only included if you add them under fees and extras.