Extensions
How much does an extension cost?
What actually drives the price of an extension, and how to budget sensibly.
There is no single figure that applies to every extension. Cost is driven by size, the complexity of the structure, the specification of finishes, and site-specific factors like access and ground conditions. Get a proper quote based on your own project rather than relying on a generic price per square metre.
Last updated 17 August 2026
Why a single price per square metre is misleading
Extension costs are often quoted as a price per square metre, but this hides more than it reveals. Two extensions of the same size can cost very different amounts depending on the roof design, the number of structural openings, the glazing specification, and whether services like electrics and plumbing need extending.
A single-storey rear extension with a simple flat roof will generally cost less per square metre than a double-storey extension with a complex roofline, purely because of the additional structural work involved.
The main factors that drive cost
Size and configuration matter, but so does what's underneath and around the site. Ground conditions, drainage, and how easily materials and trades can access the site all affect the final figure.
Specification is often the biggest variable within your control: kitchen fittings, flooring, glazing and internal finishes can move the total cost significantly, even on an identical structural footprint.
How to budget sensibly
Start with what you want the space to do, not a target number. A clear brief, room by room, makes it far easier to get an accurate quote and to spot where costs can flex without compromising the parts of the project that matter most to you.
Build in contingency. Most experienced homeowners and professionals recommend setting aside a reasonable percentage of the budget for unforeseen items, particularly on older properties where what's uncovered during construction isn't always predictable.
Have a question about your own project?