Extension cost calculator
Get an instant ballpark estimate for your house extension based on size, region, and finish level. This is a guide, not a quote - every job is different.
Typical single-storey rear extension is 15-25m². A double-storey extension only needs its ground-floor footprint here.
Estimated cost
Based on m², -storey, finish.
This range covers build costs only - it doesn't include architect fees, planning application fees, structural engineer sign-off, or building control, which typically add another £3,000-£8,000.
Free to post. Vetted builders respond directly to you.
Before you post
How much deposit should you pay a builder?
A reasonable deposit protects the builder's costs. An unreasonable one is a warning sign.
How to check if a builder is insured (and why it matters)
Public liability insurance protects you, not just them. Here's what to ask for and how to verify it.
Questions to ask a builder before you hire them
A short list of practical questions that separate a good hire from a stressful one.
How this is calculated
We start from a typical UK base rate of around £1,800/m² for a standard-finish, single-storey extension outside the South East, then adjust for your finish level (high-spec fittings and materials add roughly a third), region (London and the South East cost more per m² due to labour and material costs), and whether you're adding a full storey above (this roughly adds two-thirds to the total, not double, since the roof and foundations are shared). Real quotes will vary based on ground conditions, party wall agreements, glazing choices, and structural complexity.
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