Costing a house extension in Dorset in 2026 is more nuanced than a simple “£/m²” figure. Material prices have stabilised after a turbulent few years, but labour, design fees, planning costs and energy-performance upgrades all affect the bottom line. This guide gives you realistic 2026 prices for the most common extension types in Dorset, plus the hidden costs most homeowners miss.
2026 Average Cost per m² in Dorset
For a typical Dorset home, expect the following all-in build costs (excluding VAT):
- Single-storey rear extension (basic spec): £2,200 – £2,800/m²
- Single-storey rear (mid spec, kitchen-diner): £2,800 – £3,400/m²
- Two-storey rear extension: £2,400 – £3,200/m²
- Side return / wraparound: £2,800 – £3,600/m²
- Glass-box / contemporary spec: £3,400 – £4,500/m²
- Loft conversion (dormer): £1,500 – £2,200/m²
These figures assume Dorchester / Weymouth labour rates. Properties in conservation areas, AONB locations or with difficult access can add 10–25%.
Worked Example: 25 m² Kitchen Extension in Dorchester
| Item | Cost |
| Build cost (25 m² × £3,000) | £75,000 |
| Architectural design & planning | £3,500 – £5,500 |
| Structural engineer | £800 – £1,500 |
| Planning fee | £368 |
| Building control | £600 – £1,000 |
| New kitchen (mid-range) | £12,000 – £20,000 |
| Flooring, decoration, finishes | £4,000 – £7,000 |
| Typical total | £96,000 – £110,000 |
Hidden Costs Most People Miss
- Party wall surveyor (£800 – £2,000 if you share a boundary)
- SAP / Part L energy assessment (£250 – £500)
- Skip and waste removal across the project (£600 – £1,500)
- Drainage build-over agreement with Wessex Water (£300 – £600)
- Temporary kitchen or accommodation if you can’t live in
- Decorating and snagging after handover
- Furniture and curtains for the new space
Where You Can Save Without Compromising Quality
- Standardise window and door sizes — bespoke glazing is the single biggest cost driver
- Reuse existing services where sensible, rather than re-routing the entire heating or plumbing
- Choose mid-range fittings with high-end statement pieces — a single feature splashback or worktop goes a long way
- Plan storage early — built-in joinery is cheaper to integrate than to retrofit
- Get three competitive quotes from local Dorset contractors with verifiable references
Financing a House Extension in Dorset
Most homeowners fund extensions through a combination of savings, a remortgage or a further advance from their existing lender. Some use secured loans for shorter-term funding. We’re not financial advisers — speak to an independent mortgage broker familiar with Dorset property values for tailored advice.
Why Costs Vary So Widely
Two extensions of the same size can vary by £30,000 or more in Dorset. The biggest variables are:
- Glazing specification (bifolds, sliders, rooflights)
- Kitchen and bathroom spec
- Flooring (engineered oak vs LVT vs ceramic)
- Underfloor heating
- Air-source heat pump or other low-carbon heating
- Site access (rear-only access can add days of labour)
Get a Fixed Quote From KAP Woodwork & Building Services
We’ve been pricing and delivering extensions across Dorset for more than 30 years. Every quote we issue is fully itemised, fixed-price wherever possible, and broken into clear stage payments. Book a free site visit and we’ll talk you through what your project should realistically cost.
Related reading: Planning Permission Guide · Project Timeline · Extensions Service