Average Cost of a Semi-Detached Extension in 2026 (UK)
A semi-detached house extension costs roughly £2,000–£2,900 per m² in 2026. That puts a typical 20m² single-storey rear or side extension at around £42,000–£58,000, while a full two-storey extension runs £70,000–£120,000+. Because you share a wall and foundations with the house next door, a Party Wall Act agreement is almost always part of the job. The final figure turns on the extension type, glazing, kitchen or bathroom fit-out and your region — here's the full 2026 breakdown.
Semi-detached extension cost by size — 2026
Single-storey figures are for a standard, finished specification. Source: BestBuilders 2026 quote data · 519 UK towns
Semi-detached extension cost by size (2026)
| Size | Typical cost (standard finish) | With premium glazing & fit-out |
|---|---|---|
| 12m² side infill | £26,000–£38,000 | £40,000–£48,000 |
| 15m² rear | £30,000–£44,000 | £46,000–£55,000 |
| 20m² (typical rear/side) | £42,000–£58,000 | £60,000–£72,000 |
| 25m² (large single-storey) | £52,000–£72,000 | £76,000–£90,000 |
| Two-storey (~40m² over two floors) | £70,000–£120,000 | £125,000–£160,000 |
Ranges assume a mid-range specification, standard foundations and a straightforward party wall agreement. London and the South-East typically sit 20–30% above these figures; parts of the North and Midlands 5–10% below.
Cost by extension type and region
The extension type you choose on a semi-detached house is usually dictated by where the space is — out the back, into the side passage, or up over the existing footprint. Here's how the common options compare per m² before regional uplift.
| Type | Cost per m² (2026) | Best for |
|---|---|---|
| Single-storey rear | £2,000–£2,700 | Kitchen-diner or extra living space — the most common semi extension |
| Single-storey side infill | £2,100–£2,900 | Widening a narrow galley kitchen into the side passage |
| Two-storey (side or rear) | £1,900–£2,500 | Adding a bedroom and bathroom above — cheapest per m² |
| Wraparound (side + rear) | £2,200–£3,000 | The biggest transformation; usually needs full planning |
Two-storey work is the cheapest per square metre because you pay once for foundations and roof but gain two floors. Regionally, expect London and the South-East at the top of every range, with the North-East, Yorkshire and much of the Midlands 5–10% below the national figure. Use our extension cost calculator or the double-storey extension calculator for a size- and region-specific estimate.
Where the money goes
| Element | Share of budget | Notes |
|---|---|---|
| Foundations & groundworks | 15–20% | Rises with poor ground, nearby trees or a shared drain to divert |
| Structure (walls, steel, roof) | 30–35% | Includes any steel beam for a knock-through to the existing house |
| Glazing (doors, rooflights) | 15–25% | Bi-folds, sliding doors and lanterns are the biggest single variable |
| Internal fit-out | 20–30% | A new kitchen or bathroom pushes the top of the range |
| Fees, regs & party wall | 5–12% | Design, structural engineer, Building Control and the party wall surveyor |
What's in the price — and what isn't
A builder's quote for the shell and fit-out usually covers groundworks, structure, roof, plastering, first and second fix, and decoration. What often sits outside the headline figure is where budgets slip:
| Item | Typical cost (2026) | Included as standard? |
|---|---|---|
| Kitchen units & appliances | +£8,000–£25,000 | Rarely — priced separately |
| Bi-fold or sliding doors (3m) | +£2,500–£5,500 | Often an upgrade line |
| Roof lantern (1.5m x 1m) | +£1,500–£3,000 | Upgrade |
| Structural steel & knock-through | +£1,500–£4,000 | Sometimes included — always confirm |
| Party wall surveyor | +£700–£2,000 | No — see party wall surveyor costs |
| Architect / technician drawings | £1,200–£3,500 | No — see architect costs |
| Skip hire & waste removal | +£400–£1,200 | Usually included — confirm |
| Removing an existing garage or lean-to | +£800–£3,000 | No, if the site needs clearing first |
Always ask whether the quote is for the shell only or a fully finished room, and whether the kitchen, flooring and decoration are inside or outside the figure. That single question explains most of the gap between two quotes.
How long a semi-detached extension takes
- Weeks −6 to 0 — survey, drawings, structural calculations, planning check, party wall notice served and Building Regs submission.
- Weeks 1–3 — groundworks: footings dug and poured, any shared drain diverted, slab laid.
- Weeks 4–8 — walls up, steels in, roof on, weathertight.
- Weeks 9–12 — glazing in, first-fix electrics and plumbing, insulation, plasterboard.
- Weeks 12–18 — plastering, floors, second fix, decoration, snagging and Building Control sign-off.
A single-storey rear or side extension is typically 12–18 weeks on site; a two-storey extension runs 16–24 weeks. Start the party wall process early — serving notice about two months before work protects the programme.
How much value it adds
A well-built extension on a semi-detached home usually adds value, but rarely more than it costs in cash terms — the real return is the extra usable space. As a rough guide, a kitchen-diner rear extension can add 5–12% to a home's value, and an extra double bedroom with a bathroom from a two-storey extension often adds 10–15%, subject to the local ceiling price for the street. Overspending past what the best house on the road sells for is the most common way to lose money. Check recent sold prices for extended semis nearby before you commit to a premium specification.
Planning permission, party wall and building regulations
Planning permission. Many single-storey rear extensions on a semi fall under permitted development — up to 6m deep under the prior-approval Larger Home Extension scheme, or 3m without it, within height limits. A single-storey side extension can be permitted development only if it's no more than half the width of the original house and under 4m high; anything wider, or any two-storey side extension, normally needs a full application. A wraparound almost always does. Confirm before you design — see planning guides for a side extension, rear extension and two-storey extension.
Party Wall Act. Because a semi shares a wall and often foundations with the attached property, you'll almost certainly need to serve a party wall notice on your neighbour and, if they dissent, appoint a surveyor. Budget £700–£2,000 and start about two months ahead.
Building regulations. Every extension must meet Building Regs for structure, insulation, fire safety, drainage and ventilation, regardless of planning. Your builder works to approved drawings and a Building Control inspector signs off the key stages — keep the completion certificate for when you sell.
Choosing the builder: your checklist
- Ask for two recent semi-detached extensions you can visit or phone the owners about — not just general building work.
- Ask to see public liability insurance and check it's current and adequate for the contract value.
- Check whether they belong to a trade body such as the FMB, and verify membership on the FMB register yourself.
- Get a written stage-payment schedule — never more than 10–15% up front — and a fixed programme.
- Agree who serves the party wall notice and who pays the surveyor.
- Get three itemised quotes on identical drawings so the numbers are genuinely comparable.
Where the budget goes wrong
- Glazing chosen late — bi-folds and a lantern can add £8,000 to a £45,000 job. Price them at design stage.
- Shared drain discovered on site — a manhole or sewer in the footprint costs £800–£2,500 to divert or build over. A pre-build drainage check removes the surprise.
- Party wall left too late — serving notice after work starts can stop the job for weeks.
- Kitchen budget forgotten — the build price rarely includes the kitchen going in it.
- No contingency — hold 10–15% back for what the ground reveals.
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