Extensions
What drives the cost of a house extension?
The factors that move an extension's price up or down - for published £/m² ranges, see our extension cost guide.
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. For published UK price ranges, see our house extension cost guide; this guide covers what actually moves those numbers up or down for your project.
Last updated 3 June 2026
·By Richard Anslow
Key points
- There's no single accurate £/m² figure - two extensions the same size can cost very differently depending on roof design, structural openings and specification.
- Ground conditions, drainage and site access are easy to overlook but move the price as much as the visible finish does.
- Specification (kitchen, flooring, glazing, internal finish) is usually the biggest variable within your control.
- Build in contingency, particularly on older properties, rather than treating the first quote as the final figure.
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.
FAQs
Where can I see published extension cost ranges, not just what drives them?
Our house extension cost guide sets out the published UK £/m² and total-cost ranges by size, with named industry sources. This guide covers what moves those numbers for your specific project.
Have a question about your own project?