How much does a house extension cost in Cornwall?

The honest answer is that there’s no single figure — but you can work within realistic ranges before you’ve spoken to anyone. In Cornwall, most extensions in 2026 fall somewhere between £1,800 and £4,000+ per square metre depending on specification, and knowing roughly where your project sits on that scale is the most useful thing you can do before investing in drawings or design.

Key takeaways

• Cornwall extension costs typically range from £1,800 to £4,000+ per m² depending on spec
• A 20m² extension might cost £45,000–£80,000+; a 40m² extension could land around £90,000–£160,000+
• Groundworks, steelwork, design fees, and fit-out are where budgets most often fall short
• A 10–15% contingency is sensible on any project
• Early conversations with a builder are about checking financial viability — not locking in a price

What are the current costs per square metre in Cornwall?

Extension costs vary by specification level. As a starting point for 2026, most projects in Cornwall fall into one of three broad bands:

Basic spec — £1,800–£2,400 per m². This typically covers a straightforward single-storey structure with standard materials, no unusual groundworks, and a functional finish. Think plastered walls, basic flooring, and off-the-shelf joinery.

Mid-range — £2,400–£3,000 per m². More considered finishes, better windows and doors, and usually some degree of design input. This is where most family extensions sit when they want something that feels well-built without going bespoke.

Higher-end — £3,000–£4,000+ per m². Structural complexity, premium materials, glazing features, or a challenging site. Kitchen-diners with large roof lanterns, vaulted ceilings, or bi-fold walls into the garden tend to land here.

These figures are consistent with UK industry benchmarks for 2026, adjusted for Cornwall’s labour market, which sits broadly in line with national averages rather than London rates.

What does that mean in real numbers?

Working through those bands with typical project sizes:

A 20m² extension — roughly the size of a large kitchen-diner or two modest bedrooms — might cost anywhere from £45,000 to £80,000 or more depending on spec. At basic end that’s around £45,000–£48,000; mid-range pushes to £55,000–£60,000; a more considered finish with some complexity can easily reach £70,000–£80,000+.

A 40m² extension — a meaningful addition to a family home — could land anywhere from £90,000 to £160,000+. The lower end assumes a simple structure, good site access, and a functional finish. The upper end is a more complex build with premium materials and a more involved fit-out.

One thing that surprises people: smaller extensions often cost more per square metre than larger ones. Fixed costs — groundworks, scaffolding, structural steelwork, connection to existing services — don’t shrink proportionally with the footprint. A 15m² extension carries most of the same structural overhead as a 30m² one.

What drives costs up?

The price per square metre figure is a useful starting point, but it doesn’t tell the full story. Several factors can push a project well above the baseline range.

Groundworks and ground conditions. A flat, accessible plot with good ground conditions is the simplest scenario. Sloping sites, poor drainage, made-up ground, or proximity to trees all add cost — sometimes significantly. Groundworks can’t be fully priced until the site is opened up, which is why contingencies matter.

Structural requirements. Opening up into an existing house almost always involves steelwork. The cost of the structural elements — beams, padstones, temporary propping, engineer’s fees — is fixed regardless of how large the extension is.

Design and professional fees. Architect or designer fees, structural engineer sign-off, planning application fees, and building control charges typically add 10–15% on top of the build cost. These are unavoidable on most projects and are often left out of early budget conversations.

Fit-out. The cost per square metre figures above usually cover the build to a plastered, ready-to-decorate standard. Kitchens, flooring, decorating, lighting, joinery, and external works — patio, drainage, landscaping — are all additions. A mid-spec kitchen alone can add £15,000–£30,000 to the total cost of a kitchen extension.

VAT. Extension work is subject to VAT at 20%. Many builders quote ex-VAT; make sure you’re comparing like for like when you receive quotes.

How to approach budgeting

The most common budgeting mistake is starting with a number that feels achievable and working backwards to justify it, rather than starting with what the project actually costs and deciding whether it makes sense.

A more realistic approach:

Start with a range, not a single number. Use the per-m² bands above to set a floor and a ceiling for your project based on its likely size and spec. If either end of that range doesn’t work for you financially, it’s better to know before you’ve paid for drawings.

Add a contingency of 10–15%. On a £100,000 project, that’s £10,000–£15,000 set aside for unexpected groundworks, price changes during the build, or scope adjustments. The Royal Institution of Chartered Surveyors recommends this as standard practice.

Be clear on your finish level before you get quotes. Two quotes for the ‘same’ extension can differ by £30,000–£40,000 if one assumes a basic finish and the other assumes a fitted kitchen, tiled floors, and landscaping. Agreeing on what’s in and out of scope before pricing avoids that confusion.

When should you speak to a builder?

Early — before you’ve spent money on an architect or drawn up detailed plans.

A good builder can give you a rough sense of whether a project is financially viable for your site in 30 minutes. That’s worth doing before you commit to design fees. It’s not about getting a quote at that stage; it’s about establishing whether the project makes sense at all, and whether the site throws up anything that would push costs beyond a workable range.

Once you have drawings, a proper quote can be put together. But by that point you’ve already spent money, and if the numbers don’t work you’ve limited your options.

Frequently asked questions

How much does a single-storey rear extension cost in Cornwall in 2026?

For a typical 20–30m² single-storey rear extension in Cornwall, expect to budget between £45,000 and £90,000+ depending on spec, site conditions, and fit-out. Basic spec at 20m² starts around £45,000–£48,000 before VAT; a more considered mid-range 30m² extension with kitchen fit-out can reach £80,000–£100,000 or more once VAT and fees are included.

Why do smaller extensions sometimes cost more per square metre?

Because several significant costs are fixed regardless of size. Groundworks, structural steelwork, scaffolding, building control, and connection to existing services cost roughly the same whether the extension is 15m² or 40m². Those fixed costs are spread over fewer square metres on a smaller project, which pushes the per-m² figure up.

Should I get an architect before speaking to a builder?

Not necessarily. Speaking to a builder early — before committing to design fees — lets you check whether the project is financially viable for your site. An architect’s drawings are needed for planning and for proper tendering, but a preliminary conversation with a builder costs nothing and can save you significant expense if the project doesn’t stack up.

What contingency should I allow on an extension budget?

10–15% is the standard recommendation. On a £100,000 build cost that’s £10,000–£15,000 held in reserve. Groundworks are the most common source of unexpected cost — once the ground is opened up, conditions can vary from what was assumed at pricing stage.

Does the extension cost per square metre include VAT?

Usually not. Most builders quote ex-VAT and most published cost guides show figures before VAT. Extension work is subject to 20% VAT. Always check whether a quote includes or excludes VAT before comparing figures.