A well-designed extension typically delivers 8–15% uplift in property value plus the lifestyle benefit of more space and better daylight. UK extension prices in 2026 range from about £40,000 for a small single-storey rear in the South East to £150,000+ for a wrap-around in central London with sliding glass. This guide breaks down the four main extension types, what drives the build cost, and what to expect from the planning and party-wall process.
| Project scope | From | To | Notes |
|---|---|---|---|
| Small single-storey rear (≤15 m²) | £40,000 | £65,000 | Standard finish, brick + render |
| Medium rear (15–30 m²) | £55,000 | £90,000 | Most common type |
| Large rear (30–45 m²) | £75,000 | £120,000 | Kitchen-diner + sliding doors |
| Side return (Victorian terrace) | £30,000 | £70,000 | Tight to neighbours |
| Wrap-around (rear + side) | £80,000 | £150,000 | Steel beams, party-wall awards |
| Two-storey rear | £60,000 | £120,000 | Adds bedroom + bathroom |
Indicative ranges for London & South-East England, 2026. A site visit firms the figure to ±5%.
A bog-standard double-glazed PVC patio door is £1,500; an aluminium 4m sliding three-pane is £6–8k; a 6m bi-fold or pocket door system is £10–15k. Glazing alone can swing the budget by £15,000.
Clay subsoil with nearby trees (London plane, oak) drives foundations to 1.5–2.5m. Each additional metre of trench is roughly £600–£900 in dig, concrete and rebar.
Opening up between old and new typically needs 1–3 steels. Each opening is £2,000–£4,500 including structural calcs, beam supply, padstones and fire protection.
A new kitchen in the extension adds the kitchen budget on top of the build cost. A mid-range fitted kitchen is £8–15k in units, £2–5k in worktops, and £4–8k in appliances. Allow £20–30k total kitchen budget on a mid-spec project.
A side-return extension in a terrace where every brick has to be carried through the house adds 10–15% to labour and skip costs.
Single-storey rear extensions up to 6 metres (terraced/semi) or 8 metres (detached) are covered by Permitted Development under the Neighbour Consultation Scheme — no full planning, but you must apply for Prior Approval. Two-storey extensions, conservation areas, listed buildings and flats almost always need full planning. A Lawful Development Certificate is worth obtaining even where PD applies.
You need a Party Wall Notice (and usually an Award) for any excavation within 3 metres of a shared wall or 6 metres if foundations go deeper than the neighbour's. Allow 2 months and £900–£2,000 per neighbour in surveyor fees — not in the build cost.
A typical medium rear extension runs 14–18 weeks on site: 2 weeks for foundations, 4 weeks for the shell (walls and roof), 4 weeks for first-fix and insulation, 3 weeks for plaster and second-fix, 2 weeks for decoration and snagging. Add 4–8 weeks for drawings, structural calcs and Building Control before site starts.
In 2026, a typical 15–30 m² single-storey rear extension on a London terrace is £55,000–£90,000. Smaller projects start around £40,000, and 30+ m² extensions with high-end glazing can reach £120,000.
Yes — well-executed kitchen extensions in London and the South East typically return 8–15% uplift on market value. A £70,000 spend on a £600,000 home usually adds £50k–£90k in value.
A "kitchen extension" is usually a rear extension that creates a kitchen-diner. Add the build cost (£55–90k for a medium rear) to the new kitchen itself (£20–40k mid-range). Total £80–130k for the typical London project.
For Permitted Development extensions, a structural engineer and a draftsperson are enough. For full planning applications, conservation areas, or unusual designs, you do need a registered architect — typically £3,000–£6,000 for a medium extension.
A side-return fills in the alley alongside a Victorian terrace — typically adding 6–10 m². A wrap-around does the side-return + a full rear extension, often adding 25–40 m² and reorganising the whole ground floor. Wrap-arounds cost roughly 2× a side-return.
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