Why won't you publish a fixed price per square metre?

Because it would be misleading. The same 25m² extension can vary substantially in cost between a level estate plot and an exposed coastal site with restricted access, and quoting an average helps nobody plan.

What is the cheapest way to add space?

Usually converting what you already have — garage, loft or an existing outbuilding — because the structure and enclosure exist. It is not always the right answer, but it is nearly always the cheapest per square metre.

How do design fees work?

We agree a fixed fee for a defined scope after the first visit, so you know what the design stage costs before committing. Build cost is separate and is priced from the drawings.

Should I get a cost plan before planning?

For anything substantial, yes. There is no sense obtaining permission for a scheme that turns out to be well beyond budget, and an early cost check is far cheaper than a redesign.

Cost · Cornwall

What drives the cost of a Cornwall extension — and why two quotes can differ by a third

Rate-per-square-metre figures from national websites are the wrong starting point for a Cornish project, because they assume level access, benign ground and a suburban finish. We would rather explain the variables that move the number than publish an average that is wrong for your site.

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The variables that move the number most

In roughly the order they surprise people.

Ground conditions — rock, made ground from former mining activity, and high water tables all change foundation design
Access — a harbour-side terrace with no vehicle access is a different job from a detached house with a drive
Exposure — coastal sites need higher-specification weathering, fixings and glazing
Glazing area and specification — usually the single biggest discretionary cost
Whether the kitchen or bathrooms move — relocating services quietly doubles the scope
Finish level — the same shell can be finished at very different prices

Comparing quotes properly

Quotes are only comparable if they price the same drawings and the same specification. Without a full plans package and a specification, three builders will each assume a different window, a different floor build-up and a different scope — and the cheapest is usually the one that assumed least.

Where money is well spent and badly spent

Well spent: survey, ground investigation where there is doubt, and insulation and airtightness detailing that you cannot revisit later. Badly spent: rushing to site without a specification, and specifying a rooflight arrangement that requires structure nobody has priced.

Frequently asked

Straight answers.

Why won't you publish a fixed price per square metre?: Because it would be misleading. The same 25m² extension can vary substantially in cost between a level estate plot and an exposed coastal site with restricted access, and quoting an average helps nobody plan.
What is the cheapest way to add space?: Usually converting what you already have — garage, loft or an existing outbuilding — because the structure and enclosure exist. It is not always the right answer, but it is nearly always the cheapest per square metre.
How do design fees work?: We agree a fixed fee for a defined scope after the first visit, so you know what the design stage costs before committing. Build cost is separate and is priced from the drawings.
Should I get a cost plan before planning?: For anything substantial, yes. There is no sense obtaining permission for a scheme that turns out to be well beyond budget, and an early cost check is far cheaper than a redesign.

Want a realistic cost bracket for your project?

A free first visit gets you a banded figure based on your actual site, not a national average.

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