The complete guide to house extensions

Builders price extensions per square metre of new floor area, then add fixed costs for site setup, drainage, structural work and finishes. In 2026 the national range for a straightforward single storey extension is roughly £1,750–£2,900 per m² excluding VAT, before you fit a kitchen. London and the South East sit 15–30% above that.

The number that matters is not the rate — it is what the rate includes. A £1,800/m² quote that excludes glazing, kitchen, flooring and making good is more expensive than a £2,400/m² quote that includes all four. This is the single most common reason homeowners believe they were 'quoted low and charged high'.

Many single storey rear extensions fall within permitted development: up to 4m deep on a detached house, 3m on a semi or terrace, or 8m and 6m respectively under the larger home extension prior approval process. Side extensions, two storey work, anything forward of the principal elevation, flats, listed buildings and conservation areas all sit outside it.

Even where you do not need planning permission, apply for a Lawful Development Certificate. It costs a fraction of a planning application and is the document a buyer's solicitor will ask for. Building regulations approval is separate and always required.

• Permitted development rights are sometimes removed by an Article 4 direction or a planning condition on newer estates — check before you design.
• Building over or within 3m of a public sewer needs a build-over agreement from your water company.
• Party Wall Act notices are needed for excavation within 3m of a neighbour's structure, and for work on a shared wall.

Design and approvals take longer than construction on most projects. Allow 4–8 weeks for design, 8 weeks for a planning decision, 3–6 weeks for structural design and building control, and 6–10 weeks for party wall awards where neighbours are involved. These overlap, but not entirely.

On site, a single storey extension is typically 10–14 weeks, a double storey 16–24 weeks. Add contingency for weather in winter and for the two things that always take longer: foundations and glazing lead times.

Almost every extension overrun traces back to one of five decisions made before work started.

• Signing a quote based on sketches rather than a specification and drawings. Different builders then price different buildings.
• Accepting provisional sums for kitchen, glazing and flooring. These are placeholders, and they are always lower than what you will choose.
• No contract. For anything over £25,000 use a JCT Home Owner contract or similar — it fixes payment stages, variations and completion.
• Paying too much up front. Staged payments against completed work protect you; a 40% deposit does not.
• Changing your mind after first fix. Variations after the walls are up cost multiples of what they would have cost on paper.

Give every builder identical drawings, a written specification and the same provisional sums. Ask each to price the same document and to state clearly what is excluded. Then compare exclusions before you compare prices.

If one quote is 25% below the others, it is almost never better value — it is a different scope. Find the missing items. That is what an independent quotation review is for.

• What is excluded from this price, in writing?
• What foundation depth have you allowed, and what happens if we hit clay, roots or a drain?
• Which provisional sums are in the quote, and what allowance do they carry?
• Who pays for the structural engineer, building control and party wall surveyor?
• What are the payment stages, and what must be complete before each one?
• How many weeks on site, and what is your liability if the programme slips?
• Is scaffolding included, and for how many weeks?
• What warranty applies to workmanship, and is any insurance-backed guarantee provided?

• A quotation that fits on one page with no exclusions listed.
• A large deposit requested before any materials are ordered.
• No structural engineer involved where beams are clearly needed.
• Reluctance to provide public liability insurance details or previous client references.
• Verbal assurances that 'the kitchen is included' with nothing written down.
• Prices quoted without stating whether VAT applies.