Extension costs explained
11 March 2026 — 6 min read
Extensions are usually quoted by the square metre, which is a useful shorthand and a poor predictor. Glazing, groundworks and roof form change the figure dramatically.
The three biggest variables
Glazing: a run of steel-framed sliding doors can account for a tenth of the build cost on its own.
Groundworks: trees, drainage runs and made-up ground can add weeks and considerable expense before anything appears above the soil.
Roof: a flat roof with rooflights and a pitched roof with a lantern are not comparable pieces of work.
Every project is different. If you would like a figure specific to your home, tell us about your project and we will come back to you with a clear next step.
