It is tempting to look for a single price-per-square-metre figure for a house extension, but two extensions of the same size can cost very differently once you account for the site, the specification and the structure. Here is what actually moves the number.
Size and shape
Cost does not scale in a straight line with floor area. A simple rectangular extension is usually more economical per square metre than a complicated shape with multiple corners, roof junctions or changes of level, because more junctions mean more labour and more places for things to go wrong.
Structural complexity
- Removing load-bearing walls or chimney breasts, which need steel beams and structural calculations
- Working on sloping sites or close to trees, boundaries or existing foundations
- Underpinning or unusual ground conditions found once digging starts
- How the new roof ties into the existing one
Specification and finish
Two identical shells can end up at very different prices depending on the kitchen, bathroom fittings, glazing, flooring and how much of the existing house is altered to connect to the new space. Builder-grade, mid-range and high-end specifications all carry different costs, and it is worth deciding roughly where you want to sit before you ask for a quote.
Site access and location
A site with easy access for materials and skips is more straightforward than a terraced house with no side access, where everything has to move through the building or over a wall. Party wall work, scaffolding and protecting neighbouring property all add time and cost on tighter sites, which is common across much of London.
The route through planning
Permitted development, a planning application, and a listed building or conservation area all carry different costs and timescales before work can even start. See our guide to planning permission and Building Regulations for the difference between the two.
This guide is general information for homes in England and is not legal or planning advice. Rules can change and differ between properties and local authorities, so always confirm the position with your council or a qualified professional.
The only reliable way to know what your project will cost is a site visit and a written quote. See our extensions page, or request a quote to get a figure for your own project.