House Extension Cost UK 2026

A house extension is one of the most significant investments you can make in your property. In most parts of the UK, extending is cheaper per square metre than buying a larger house once you factor in stamp duty, moving costs and estate agent fees. But extension costs vary enormously based on size, specification and location.

Cost per square metre

The most useful way to think about extension costs is per square metre. In 2026, typical rates across the UK are £1,400 to £2,500 per square metre for a single storey extension. Double storey extensions are more cost-effective at £1,200 to £2,000 per square metre because the foundations and roof are shared across two floors.

London and the South East sit at the top of these ranges, with some central London projects exceeding £3,000 per square metre. The Midlands, North of England, Wales and Scotland are generally 20-30% cheaper.

What is included

A standard extension quote should cover foundations, structural work, walls, roof, windows and doors, insulation, plastering, electrics, heating, basic decoration, and building regulations sign-off. It usually does not include kitchen or bathroom fitting, flooring finishes, or landscaping to repair the garden afterwards.

Planning permission

Many single storey rear extensions fall under permitted development. The key limits are: no more than 3 metres depth for attached houses (or 4 metres for detached) under standard rules, or up to 6 metres (8 metres detached) under the Larger Home Extensions scheme with prior approval. Double storey extensions almost always require full planning permission.

Hidden costs to budget for

Architectural drawings cost £1,500 to £4,000. Structural engineer calculations cost £500 to £1,500. Planning application fees are £258 in England. Building regulations fees are £400 to £1,000. Party wall agreements, if you share a wall with a neighbour, cost £700 to £1,500 per neighbour. These professional fees typically add £4,000 to £8,000 to the total project cost.