Is a cost per square metre a reliable guide?

Only as a first sanity check. It ignores glazing, ground, access and how much of the existing building is being altered, and those are exactly the variables that separate a straightforward job from an expensive one.

What is the single most common cause of overspend?

Scope creep during the build, closely followed by ground conditions that were never investigated. The first is avoidable by fixing the design before starting, the second by looking at the ground rather than assuming.

Should I get quotes before or after the drawings?

After. Quotes taken from planning drawings alone are estimates dressed up as prices, because each builder is filling in the missing information differently.

Do you give an idea of cost before I commit to fees?

Yes. The free first consultation includes an honest cost band for what you are describing, and if the budget and the brief do not meet, we will say so at that point rather than three months later.

Cost guide · Cornwall

Single storey extension costs in Cornwall, where the money actually goes

Most cost articles give you a rate per square metre and leave it there, which is useless the moment your site is on a slope, your access is a lane, or your rear elevation faces the weather. The cost of a single storey extension in Cornwall is set by a handful of decisions, and once you can see them written down the quotes you receive start to make sense.

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The five things that set the price

Two extensions of identical floor area can differ by a third. These are the reasons, in the order they usually matter.

Ground conditions: granite close to the surface means breaking out, soft made ground near an estuary means deeper or engineered foundations
Glazing: a large sliding opening costs money twice, in the glass and in the structure holding the wall above it
The structural opening into the existing house, which decides whether you need a modest beam or a substantial one with new bearings
Access: a site a lorry cannot reach is a site where every material is handled twice
The roof tie-in, which is where the detailing is fiddliest and where leaks appear if it is improvised

What belongs in the build budget and what does not

Quotes vary wildly because they include different things. A shell price covers foundations, structure, roof, glazing and weather-tightness. It may not cover kitchen units, flooring, decoration, landscaping to make good, or the design fees themselves. Before comparing two numbers, write down what each one excludes.

Why drawings make the price more certain, not less

A builder pricing from a planning drawing has to guess at the insulation strategy, the beam, the drainage and the finish, and a sensible builder prices a guess with a margin on top. A builder pricing from a technical package with details and a specification is pricing the same job as everybody else, which is when quotes become comparable.

Doing the drawings and the build in one place

We draw schemes and we build them. That matters for cost because the detail that would otherwise be a risk item in somebody else's quote has already been resolved on paper by the people who will be standing on the site.

Frequently asked

Straight answers.

Is a cost per square metre a reliable guide?: Only as a first sanity check. It ignores glazing, ground, access and how much of the existing building is being altered, and those are exactly the variables that separate a straightforward job from an expensive one.
What is the single most common cause of overspend?: Scope creep during the build, closely followed by ground conditions that were never investigated. The first is avoidable by fixing the design before starting, the second by looking at the ground rather than assuming.
Should I get quotes before or after the drawings?: After. Quotes taken from planning drawings alone are estimates dressed up as prices, because each builder is filling in the missing information differently.
Do you give an idea of cost before I commit to fees?: Yes. The free first consultation includes an honest cost band for what you are describing, and if the budget and the brief do not meet, we will say so at that point rather than three months later.

Want a realistic figure for your extension?

Book the free first consultation and we will give you an honest cost band before any fees are committed.

Free · No obligation

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