How deep do foundations need to be for a single-storey extension?

There's no single legal depth — the honest answer is that building control decides on site, based on the ground your extension sits on. The factors that matter are soil type (shrinkable clay is the big one), nearby trees and hedges, drains crossing the dig, the water table, and any made-up or filled ground.

The process in practice: your builder digs trial holes or the actual trench, and a building control inspector looks at the excavation before any concrete is poured, confirming the depth at which suitable load-bearing ground appears. On straightforward sites that's routine; on clay with trees nearby, or on poor ground, the inspector may require deeper trenches or an engineered solution — which is where a structural engineer earns their fee. This applies whether or not you need planning permission: building regulations approval covers the structure regardless. Budget for the council's building regulations submission and inspection fees, typically £400–£1,200 depending on the council and project size.

Because the depth is unknown until the ground is opened, make sure every quote states an assumed foundation depth and a rate for going deeper, so surprises are priced fairly. Our house extension cost guide breaks down groundworks, and gov.uk's building regulations guidance explains the approval routes.