A 20m² extension works out at £24,000-£34,000 for the shell, or £40,000-£56,000 for a standard finished living space, before VAT. Those are two alternative budgets for the same footprint. The shell calculation uses the June 2026 rate of £1,200-£1,700 per square metre. The standard room uses the guide’s specific 20m² project example, whose table is dated May 2024. See Checkatrade’s extension cost guide.

If your project attracts standard-rate VAT, the figures become £28,800-£40,800 for the shell and £48,000-£67,200 for the finished room. These examples allow you to compare stages before getting a price for your own property. Check the VAT guidance for building work.

What would you actually walk into?

The shell example covers foundations, walls and roof. Checkatrade’s scope leaves glazing and internal fit-out to follow. If you plan to phase the work, ask who will close the openings, secure the house and protect the unfinished build until the next trade arrives.

The standard-room budget can include internal walls, utilities, glazing, doors and flooring. Checkatrade says decoration is usually extra. A kitchen extension needs a second list covering cabinetry, worktops, appliances and their connections. A large opening through the existing rear wall may also need its own structural design and price. Mark each item on the same drawing before you compare quotations.

Three questions for your builder

Where will rainwater and existing drains run? How will materials reach the back garden? What floor, lighting and decoration will be finished at handover? For a terraced house, a hand-carry route through the hall can affect the price even when the drawings show the same floor area as a detached home.

Ask for separate figures for professional fees, approvals, enabling work and the built extension, with the VAT clearly shown. A drainage sketch and photographs of the access will help us price your proposal in Sevenoaks, Tonbridge or Tunbridge Wells. Read our drainage guide or explore BOL extensions.