Extension cost calculator: how to work out your build cost per square metre

There is no single cost per square metre for an extension, but there is a reliable way to calculate your own number. This guide gives you the method, the current UK ranges and the additions people forget.

The calculation

1. Measure the new internal floor area in square metres.
2. Multiply by a build cost rate per square metre for your quality level and region.
3. Add works outside the extension footprint: knock-throughs, remedial work, redecoration to existing rooms, landscaping and making good.
4. Add kitchens, bathrooms, joinery, flooring and specialist glazing separately. These are not in a shell rate.
5. Add professional fees and statutory costs, typically 10 to 15 per cent of build cost.
6. Add VAT at 20 per cent if your rates were quoted excluding VAT. Most contractors registered for VAT will charge it on domestic extensions.
7. Add contingency: 10 per cent on a new-build extension, 15 to 20 per cent where you are opening up an older house.

Indicative UK build cost rates

These are shell-and-core to finished-but-basic rates, excluding VAT, kitchens and furniture.

• Standard specification, outside London and the South East: roughly 1,800 to 2,400 pounds per square metre.
• Mid specification, most of England and Wales: roughly 2,400 to 3,000 pounds per square metre.
• High specification, or London and the South East: roughly 3,000 to 4,000 pounds per square metre.
• Premium and heavily glazed schemes: 4,000 pounds per square metre and upwards.

Basements and structural underpinning sit outside these ranges and cost multiples more. Loft conversions are usually cheaper per square metre than ground floor extensions, because the shell already exists.

A worked example

A 30 square metre single-storey rear extension, mid specification, outside London.

• Shell: 30 x 2,600 = 78,000
• Structural opening into the existing house, plus making good: 9,000
• Kitchen supply and fit: 18,000
• Flooring, decoration and lighting: 8,000
• Sliding doors and rooflights uplift above allowance: 7,000
• Subtotal: 120,000
• Fees, surveys and statutory costs at 12 per cent: 14,400
• Contingency at 12 per cent: 14,400
• Total excluding VAT: 148,800
• Total including VAT at 20 per cent: 178,560

The headline "30 metres at 2,600" number was 78,000. The realistic commitment is more than double. That gap is where most extension projects go wrong.

Costs people leave out

• Design and engineering fees, and planning or lawful development application fees.
• Building control fees and inspections.
• Party wall surveyor fees, often 1,000 to 2,500 pounds per neighbour if you appoint separate surveyors.
• Site surveys: measured survey, drainage CCTV, asbestos, tree survey where relevant.
• Utility diversions, new consumer unit, boiler upgrade or heating redesign.
• Scaffolding, skips, welfare and temporary protection.
• Build over agreement with the water company where you build near a public sewer.
• Living costs if you move out, or the productivity cost if you do not.

What moves the number most

1. Ground conditions and foundation depth, particularly near trees or on clay.
2. The size of structural openings. A 6m clear opening is a very different steel from a 3m one.
3. Glazing. Large sliding or pivot doors and structural glass are a specification decision with a five-figure swing.
4. Access. A rear garden reachable only through the house adds labour and protection cost to every delivery.
5. Roof form. Flat roof with rooflights is cheaper than a pitched roof with a lantern and complicated junctions.
6. Whether you are also touching services, plumbing and electrics in the existing house.

How to get from estimate to real price

An estimate is only a budget test. To get a comparable price, you need a specification and drawings that every contractor prices identically, then compare on scope, exclusions, provisional sums, programme and payment terms rather than the bottom line.

What to do next

1. Set your total budget, then work backwards to an affordable area rather than picking a size first.
2. Ring-fence contingency and keep it out of the design conversation.
3. Get a designer to produce a specification good enough to price.
4. Send an identical brief to three or four contractors.
5. Compare like for like and check what each price excludes.

BuildWise Workspace turns this into a live budget and programme, and matches you with professionals near you who work at your quality level.