How Much Does a Semi-Detached Extension Cost in 2026? (UK)
A single-storey rear extension on a UK semi-detached home typically costs £40,000–£75,000 in 2026 (around £2,000–£3,000 per m²), while a full double-storey extension runs £75,000–£130,000. With one shared party wall and usually side access down the unattached flank, a semi sits neatly between a terrace and a detached home on price.
What does a semi-detached extension cost in 2026?
- Single-storey rear extension: £40,000–£75,000 (roughly £2,000–£3,000 per m²)
- Side return / side infill: £45,000–£70,000
- Double-storey (two-storey) rear or side extension: £75,000–£130,000
- Wraparound (L-shaped) extension: £70,000–£120,000
- Per-m² guide: £2,000–£2,750 standard finish, £2,750–£3,500+ high spec
A semi-detached house shares one party wall with the neighbour but almost always has clear side access down the unattached side. That access makes deliveries, muck-away and scaffolding far easier than on a mid-terrace, so semi extensions usually come in below the equivalent terraced job — yet above a detached one only because the shared wall triggers a Party Wall award. The single biggest swing factor is spec: a standard plastered shell sits near the bottom of each range, while a designer kitchen, structural glazing and bi-fold doors can add £8,000–£20,000+ on top.
Semi-detached extension cost by type (2026)
These are typical all-in build costs for a semi-detached home in 2026, covering foundations, structure, roof, glazing, plastering and building control sign-off. Greater London and the South East commonly run 15–30% higher.
Figures include foundations, structure, roof, glazing, plaster and building control. They exclude VAT-rated luxury finishes, furniture, and any unusual ground works. Always get 3 itemised quotes before budgeting.
What drives the price of a semi-detached extension?
The shared party wall
A semi has one wall in common with the neighbour, so you'll almost always need a Party Wall award before building near it. A surveyor's fee of roughly £800–£2,000 (sometimes paid for both parties) is normal — it's the main thing that nudges a semi above a fully detached build.
Side access
Most semis have an open side passage, letting diggers, materials and skips reach the rear without going through the house. That keeps labour and muck-away costs well below a mid-terrace, where everything is carried through the property by hand.
Planning route
A single-storey rear extension often falls under Permitted Development within the size and volume limits, saving time and fees. A two-storey or side extension usually needs full planning permission, adding design, drawings and council fees to the budget.
Ground & spec level
Poor ground, trees or drains can force deeper or piled foundations, adding thousands. After that, finish is king: a basic shell sits at the bottom of each range, while structural glazing, bi-folds and a high-end kitchen push you toward £3,500+ per m².
Common Questions
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