Costs

How Much Does a Single-Storey Extension Cost in 2026?

A realistic breakdown of what a single-storey extension costs in Dorset — from foundations to finishing touches, including the costs nobody tells you about.

8 min read1 February 2026

The short answer

A single-storey rear extension in Dorset typically costs between £1,800 and £2,800 per square metre for the build alone. A 4m x 6m (24m²) extension — the most common size — comes in at £45,000 to £67,000 including VAT. But the build cost is only part of the picture.

What the build cost includes

Foundations, walls, roof, windows, doors, plastering, electrics, plumbing, and basic decoration. It does not include the kitchen, flooring, or landscaping — those are on top.

The costs nobody tells you about

Party wall agreements (£700–£1,500 per neighbour), building control fees (£400–£800), skip hire (£300–£500 per skip), temporary kitchen setup, storage, and the cost of living elsewhere if the work is disruptive. These hidden costs can add £3,000–£8,000 to your total.

Professional fees

Design and planning fees vary wildly. A traditional architect charges 8–15% of the build cost (£3,600–£10,000). We charge project-specific pricing, starting with the Layout Ideas Pack, done remotely or in person — the planning submission is priced after that. You know what the pack costs before you start.

How to budget realistically

Take the build cost estimate, add 15% for contingency, add professional fees, and add £5,000 for the hidden costs above. For a typical 24m² extension in Dorset, budget £55,000–£80,000 all-in.

Written by the PlanBuild team

9 years designing extensions and renovations in Poole, Dorset.