Build costs per square metre in Cornwall, and why the same rate gives two very different budgets

01

What the rate actually covers

A rate per square metre normally describes the building itself measured across its internal floor area: substructure, frame, envelope, roof, internal finishes, services and fitting out. It usually excludes the things that vary most from plot to plot, which is precisely why quoted rates look tidier than final accounts. Before you compare two rates, establish what each one includes, because the gap between them is often definitional rather than real.

• Commonly excluded: site clearance, demolition, retaining structures and difficult access
• Commonly excluded: drainage runs beyond the building line and any off mains treatment plant
• Commonly excluded: driveways, landscaping, boundary walls and utility connections
• Commonly excluded: professional fees, surveys, planning fees and building control charges
• Almost always excluded: value added tax, which behaves differently on new build, conversion and repair

02

The Cornish variables that move the number

Cornwall adds a specific set of pressures. Access is the big one: a lane that will not take an eight wheeler changes the price of every load of concrete, block and slate that arrives, because everything is transferred to smaller vehicles or moved by hand. Exposure is the second, since coastal and moorland sites need better sealed junctions, heavier fixings and materials that survive salt laden wind. Ground is the third, from granite close to the surface to soft estuarine deposits to former mining ground, each of which changes the substructure before a single wall goes up.

03

Shape, storeys and glazing

Cost sits in the envelope, not the floor. A compact two storey form encloses more space for less wall and less roof than a long single storey plan of the same area, which is why single storey work usually carries a higher rate. Corners, changes of level, roof valleys and long runs of glazing all add expensive linear metres. None of this means you should build a shoebox, it means you should know which parts of the design you are paying a premium for and decide deliberately whether the premium is buying you something you value.

04

Turning a rate into a budget

Take the internal floor area, apply a rate honestly matched to the specification you have in mind, then add the excluded items above as separate lines rather than assuming they are absorbed. Add professional fees and statutory charges. Then add a contingency that reflects how much you know about the site: a modern estate house with a level rear garden needs less than a listed cottage with unknown foundations. A budget built this way survives contact with tender returns far better than one built from a single headline figure.

05

Where drawings change the price

The single biggest cost control available to a homeowner is a drawing package detailed enough that three builders price the same job. Where the information is thin, each contractor prices their own assumptions, adds risk to cover the gaps, and you end up comparing three different projects. Where junctions, insulation build ups, structural sizes and finishes are stated, the quotes converge and the differences you see are genuine differences in labour and programme.