House extension stages, costs and timings
A single storey house extension runs through seven stages, from setting up the site to snagging, over 12 to 16 weeks on site. A 30 m² extension costs about £60,000 to £99,000 to build, before VAT. Give each stage a price, and release the money for it once you have checked the work.
Updated 9 November 2026
What are the stages of a house extension?
Each stage, its usual share of the cost, what that is on a £60,000 job, and how long it takes on site. Stages often overlap.
| Stage | Share | On £60,000 | Time on site |
|---|---|---|---|
| 1. Site set-up | 5% | £3,000 | 1 week |
| 2. Groundworks, foundations and drainage | 18% | £10,800 | 2 to 3 weeks |
| 3. Walls, floor slab and structural opening | 20% | £12,000 | 2 to 3 weeks |
| 4. Roof, glazing and weathertight shell | 17% | £10,200 | 2 to 3 weeks |
| 5. First fix, insulation and plastering | 15% | £9,000 | 2 to 3 weeks |
| 6. Second fix, fit-out and decoration | 20% | £12,000 | 3 to 4 weeks |
| 7. Outside, snagging and handover | 5% | £3,000 | 1 to 2 weeks |
What should you check before you approve each stage?
Approve a stage when it is done and you have seen these. Ask for photos of anything that will be covered up.
1. Site set-up
Done when: Scaffold, fencing and skips in, services found, and the extension set out on the ground.
- A signed contract, dated drawings and an itemised specification.
- A party wall award or written neighbour consent, where the law needs one.
- A Lawful Development Certificate or a planning decision. A builder’s opinion is not proof.
2. Groundworks, foundations and drainage
Done when: Foundations dug, inspected and poured, drainage laid, ready for the floor.
- Building control’s dated inspection of the open trench, before the concrete went in.
- The extra cost of deeper foundations, if any, at the rate agreed before digging.
- Dated photos of the damp proof membrane laid and lapped, before the slab.
3. Walls, floor slab and structural opening
Done when: Walls up, slab poured, and the opening into the house formed on a steel beam.
- The structural engineer’s calculations for the steel that was actually fitted.
- Photos of the beam’s bearings and the open cavity, before they are covered.
- A measured check against the drawings, before any glazing is ordered.
4. Roof, glazing and weathertight shell
Done when: Roof on, windows and doors in, and the inside stays dry in rain.
- A hose test, or a look inside after a full day of rain.
- Photos of every flashing and upstand where the new roof meets a wall.
- The FENSA or CERTASS certificate for the windows.
5. First fix, insulation and plastering
Done when: Cables, pipes and insulation in, screed down, and the walls plastered.
- Socket, switch and light positions marked with you before anything was boarded.
- Photos of all the insulation, with the product label in shot.
- A dated pressure test for any underfloor heating, before the screed.
6. Second fix, fit-out and decoration
Done when: Sockets, radiators, doors and floors fitted, the kitchen or bathroom in, and decorated.
- Everything works: lights, sockets, heating and any appliances.
- The finish matches the standard agreed in writing, or the sample area you photographed.
7. Outside, snagging and handover
Done when: Garden made good, scaffold down, snag list closed and certificates handed over.
- The Building Regulations completion certificate in your hands.
- A handover pack: electrical, gas, glazing and structural certificates, and guarantees in your name.
- A dated, photographed snag list with every item closed.
How much does a house extension cost?
| Type | Typical cost |
|---|---|
| Single storey | £1,800 to £3,300 per m² |
| Single storey, about 30 m² | £60,000 to £99,000 |
| Double storey, about 60 m² | £108,000 to £180,000 |
| Side return | £2,800 to £3,800 per m² |
| Shell only, no finishes | £1,200 to £1,700 per m² |
Build cost before VAT. A new kitchen, professional fees and decorating usually cost extra. Small extensions cost more per m², and London and the South East can cost 25 to 50% more.
These ranges come from:
- Checkatrade, “How much does a house extension cost?” (opens in a new tab), read November 2026.
- Homebuilding & Renovating, “How much does an extension cost in 2026?” (opens in a new tab), read November 2026.
- Federation of Master Builders, “Single-storey extension costs: a practical guide for homeowners” (opens in a new tab), read November 2026.
How long does a house extension take?
About 12 to 16 weeks on site, or 14 to 20 weeks with the usual delays. Design, planning and building control approval add 10 to 20 weeks before work starts.
Do you need planning permission for an extension?
Often not. Many single storey rear extensions are permitted development. Get a Lawful Development Certificate to prove it, or a planning decision where permitted development does not apply.
How should you pay for a house extension?
Agree a price for each stage before work starts, and release it once you have checked the work.
Keep the first stage to about 5%, so little is at risk before anything is built. Keep 5% back from the last stage until the snag list is closed and the completion certificate is in your hands. Without it, the extension can block a sale.
With Renopay, the money for each stage is held securely until you approve the work.
Read next: How to structure the payments for a home extension · How much deposit should you pay a builder?