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.

It's worth knowing the options too, because the type changes the cost as much as the depth. Most extensions use trench-fill — a deep trench filled almost to the top with concrete — or a traditional strip footing with blockwork built up to ground level; trench-fill is quicker and common on deeper digs. Where clay and nearby trees force very deep foundations, a piled foundation with a reinforced ground beam can work out cheaper and less disruptive than digging metres of trench, and a raft can suit poor or made-up ground. Building control, and on tricky sites a structural engineer, will steer which is appropriate.

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.