Right then — planning a house extension round Yeovil, Sherborne, Weymouth or further into Dorset and the West Country, and trying to pin down some proper numbers? You’re not alone. Online calculators love throwing about national averages that don’t mean a lot once you’re dealing with local ground, local labour and local planners.
We’ve put together a straight 2026 guide based on what jobs are actually costing in these parts. No fluff — just the figures and the things that quietly push the price up, so you can budget properly and avoid the nasty surprises.
The quick answer
For a typical single-storey rear extension in Dorset in 2026, you’re looking at roughly £2,200–£3,400 per m² all-in (excluding VAT), depending how smart you want it.
A mid-range 20–25 m² kitchen-diner style job usually lands somewhere between £55,000 and £90,000 before professional fees, contingency and the final finishes. Go two-storey, or start adding lots of glass and coastal detailing, and the numbers climb.
That range covers most of what we see around Yeovil, Sherborne, Weymouth, Dorchester and the surrounding villages. Coastal spots and conservation areas often sit towards the higher end.
2026 costs around Dorset – per m²
These figures reflect real jobs in and around Dorchester, Weymouth, Poole, Yeovil, Sherborne and the wider West Country:
| Extension Type |
Basic Spec |
Mid Spec |
Higher Spec |
| Single-storey rear |
£2,200–£2,800 |
£2,800–£3,400 |
£3,400–£4,500 |
| Side return / wraparound |
£2,400–£3,000 |
£2,800–£3,600 |
£3,600–£4,500+ |
| Two-storey rear |
£2,200–£2,800 |
£2,400–£3,200 |
£3,000–£4,000 |
| Loft conversion (dormer) |
£1,500–£2,000 |
£1,800–£2,200 |
£2,200–£2,800 |
All figures are £ per m², all-in build cost, excluding VAT.
Typical total project costs (finished, mid-spec):
- Small single-storey (12–18 m²): £35,000–£70,000
- Medium single-storey (20–30 m²): £60,000–£110,000
- Larger / wraparound (35–50 m²): £110,000–£180,000+
- Two-storey: usually 40–60% more than a single-storey of the same footprint
Conservation areas, AONB sites, awkward access or tricky ground (plenty of that round Yeovil, Sherborne and much of Dorset) commonly add another 10–25%. Better to know that up front than discover it halfway through the dig.
What actually moves the price
It’s rarely just “how many square metres”. A few things quietly make a big difference round here:
- Size – Bigger jobs often cost less per square metre because the fixed costs (foundations, scaffolding, site setup) get spread out.
- Type – Single-storey is usually the simplest and most cost-effective. Two-storey gives you more usable space for the money, but the structure and scaffolding step up.
- Ground – Clay, tree roots and drainage issues are common around Yeovil, Sherborne and large parts of Dorset. They can add thousands if you’re not careful at the design stage.
- Planning – Plenty of smaller rear extensions still fall under Permitted Development. Conservation areas, listed buildings or National Park edges need full applications, extra drawings and a bit more patience.
- Finishes – Bi-folds, roof lanterns, aluminium windows, underfloor heating and a fancy kitchen will push you into the higher bands sharpish. Decide early what you actually need.
- Roof – A warm flat roof (GRP or similar) is popular and sensible on a lot of Dorset extensions. Pitched roofs or complicated detailing cost more, but sometimes suit the house better.
The hidden costs most people forget
The build cost is only part of the story. These extras catch a lot of people out:
- Party wall surveyor (if you’re on a boundary): £800–£2,000
- Structural engineer: £800–£1,500+
- Architect / design fees: £3,500–£8,000
- Planning application fee: around £548 (2026)
- Building Control: £600–£1,200
- Skips, waste and drainage agreements: £600–£2,000
- Contingency: always allow 10–15% — you’ll thank yourself later
Add those up and you can see why a “£70k build” can feel closer to £85–90k by the time the last invoice is paid.
A few sensible rules
- Get a proper measured survey and a fixed-price quote, not just day rates.
- Decide the specification early. Changing your mind halfway through costs money.
- Build in a proper contingency and stage the payments — never large lumps up front.
- Think about whether you’ll need to move out for a bit, or can live through the works.
- Compare like-for-like quotes that cover the same scope, materials and roof system.
A cheap headline number that leaves half the job out isn’t a bargain — it’s a headache waiting to happen.
Is it worth it?
In most of Dorset and the West Country — Yeovil, Sherborne, Weymouth and beyond — the answer is usually yes. A well-designed extension often adds more value than it costs, especially kitchen-diners and extra bedrooms, and it means you can stay in the home and area you already like rather than going through the stress and expense of moving.
Done properly, it should feel like the house always had that extra space — not like an obvious add-on.
Quick answers to the usual questions
How much for a single-storey extension in Dorset in 2026?
Roughly £2,200–£3,400 per m² all-in. A 20–25 m² mid-spec job often totals £55,000–£90,000 before fees and contingency.
Do I need planning permission?
Plenty of smaller rear extensions still fall under Permitted Development. Conservation areas, listed buildings, AONBs or bigger projects usually need full planning. Local knowledge helps here — what’s straightforward in one village can be fiddlier in the next.
How long does it take?
A straightforward single-storey is often 8–16 weeks on site, plus design and planning time beforehand. Two-storey or awkward sites take longer. Weather and material lead times can stretch it a bit too.
What about a kitchen extension?
A 20–25 m² kitchen-diner commonly sits in the £60,000–£110,000 range depending on finishes, glazing and whether a new kitchen is included in the figure.
How much contingency?
10–15% of the build cost is the sensible minimum round here. Ground conditions and old buildings have a habit of throwing up the odd surprise.
Need a realistic local quote or want to talk through roof options (including GRP flat roofs) for your extension in Yeovil, Sherborne, Weymouth or further afield? Give us a shout — we know the ground and the planners, and we’ll give you straight numbers.
