Single storey extension costs in the UK for 2026 by size and spec: 20 m² from £30k, 30 m² £45–60k, 40 m² £60–80k. What changes your final price. This guide is written by PG Property Repairs and Renovations, an FMB and TrustMark accredited building company based in Worthing, using real costs and timescales from projects completed across Worthing, Lancing, Shoreham, Brighton and the wider West Sussex area.