How much does a single-storey extension cost?

A bigger extension costs more, but not in a straight line. Groundworks, scaffolding and roofing costs are largely fixed regardless of size, so a small extension often costs more per square metre than a larger one. That's why a modest single-storey addition sometimes prices out closer to a bigger one than you'd expect.

Roof type makes a real difference. A simple pitched roof is cheaper than a flat roof with a lantern, though the lantern brings in more light. Groundworks depend heavily on what's underneath, if we hit poor ground conditions or an old drain run, that changes the foundation design. And finishes, bifold doors versus a single set of French doors, underfloor heating versus radiators, add up quickly once you start specifying.

Planning also has a cost, not just in fees but in time. A project that needs full planning permission rather than falling under permitted development adds several months before work can even start.

The only way to get an accurate figure is a site visit and a proper conversation about what you actually want, not a rough estimate over the phone. We quote against a clear specification so you know exactly what's included, and where the room is for choices that change the price.

Want a rough starting figure before you get to that stage? Try our extension cost calculator for a price range based on jobs we've actually built.