Cost per square metre estimates.
The cost per square metre method is the fastest way to produce an order-of-magnitude estimate. It uses historical data from similar projects to produce a rate per square metre of floor area.
Who this guide is for
Clients wanting early feasibility figures, builders preparing quick budget indications, and project managers who need a benchmark before detailed design is complete.
How to use the method
Identify the gross internal floor area in square metres. Select an appropriate rate based on project type, quality specification, and location. Adjust for site conditions, access, and regional labour cost variations. Multiply and add an allowance for external works and contingencies.
Miniature example
A 75 m² single-storey extension at £1,800/m² — mid-range specification with standard kitchen and bathroom fit-out. 75 × £1,800 = £135,000. Add 10% for external works (£13,500) and 5% contingency (£6,750). Total order-of-magnitude: £155,250.
Common mistakes
- Using a national average rate without adjusting for London or South-East premiums
- Applying the same rate to extensions, new builds, and refurbishments interchangeably
- Treating a square metre estimate as a reliable price rather than a starting point
Related guides and tools
Compare with approximate quantities and benchmark estimating. Use the Budget Estimate generator for square metre based cost planning.