Cost Calculator
Extension cost calculator
Get an indicative cost for a single or double storey extension from its footprint and finish level. A ballpark to plan around, not a quote.
In short
A single storey extension in the UK typically costs from about £2,000 to £4,000 per square metre of footprint, ex VAT, depending on finish.
A double storey extension costs more in total but less per square metre of floor area, because the roof and foundations are shared across two floors.
Enter your footprint and finish to get an indicative range. For a number you can build against, send us your drawings.
Your details
Ground floor footprint of the extension.
Indicative cost
Fill in your details to see an indicative cost range.
This is a rough guide. For a proper, measured estimate you can quote from or build against, send us your drawings.
Get a measured estimateHow this estimate is worked out
The calculator multiplies your extension footprint by a cost per square metre band for your chosen finish. The single storey bands cover one floor over that footprint, while the double storey bands already account for building two floors over the same ground area.
Double storey work usually costs less per square metre of finished floor area than single storey, because the foundations and roof are shared across both floors. The total cost is higher, but you get more space for the money.
Figures are ex VAT and cover the build only. They exclude professional fees, planning, structural design and any specialist groundworks or underpinning. Treat the result as a planning ballpark.
What drives the cost
Glazing and openings
Bifolds, large rooflights, sliding doors and big structural openings are some of the biggest single costs on an extension, and they vary hugely by product.
Kitchen and bathroom content
Most extensions house a new kitchen or bathroom. The fit out, appliances and finishes can swing the budget by tens of thousands.
Structural work
Steel beams, removing walls to open up to the existing house, and foundations near drains or trees all add cost that a footprint alone does not show.
Connecting to the existing house
Matching brickwork, levels, roofs and services to the existing building, plus making good where old meets new, is easy to underestimate.
An indicative ballpark only
These figures are a rough guide based on typical UK rates and are shown ex VAT. They are not a quote and should not be used to price or build a real job. Every project differs on specification, site conditions, access and local rates. For a figure you can actually quote from or build against, send us your drawings and we'll prepare a proper, measured estimate.
Frequently Asked Questions
Ready for a real number you can quote from?
A calculator gives you a ballpark. Send us your drawings and we'll come back with a professional, measured estimate, a fixed fee and a turnaround time.
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