House extension cost calculator — Wales 2026
Estimate single, double or wraparound extension costs in Wales by size, fit-out tier and ground conditions.
Estimate only. Figures shown are 2026 market estimates compiled from published merchant retail prices and public listings. Actual cost depends on your property, scope, location and supplier. Always request three written quotes from accredited Welsh tradespeople before committing.
Estimated cost range
£45,000 – £60,000
Includes building work, basic services, plastering and fit-out. Excludes architect fees, planning fees, structural engineer.
Indicative ranges only — based on UK published per-m² extension cost data including FMB and HomeOwners Alliance benchmarks for 2025. Actual cost depends heavily on site conditions, planning, and finish level. Always get a structural engineer's report and 3 written quotes from FMB or TrustMark builders.
Get matched with FMB or TrustMark builders in Wales
Extensions need building regulations approval and may need planning permission. Use a builder registered with FMB or TrustMark for accountability.
Find a builder near me →