How Deep Should Foundations Be? A UK Guide

For a standard low-rise domestic build — an extension, a garage, a house — a traditional strip foundation in reasonable ground is typically dug to somewhere between 450 mm and 1,000 mm deep, and in practice most UK footings end up around 900 mm to 1 m. The reason isn't the weight of the building; a house doesn't need a metre of concrete to hold it up. It's about getting below the zone where the ground moves.

Near the surface, soil shrinks and swells with frost, rain and drought, and it's disturbed by tree roots and old topsoil. Go deep enough and you reach ground that stays stable through the seasons. In most of the UK, that "stable" depth for ordinary soils is around 750 mm to 1 m — which is where the rule of thumb comes from.

Takeaway: the metre isn't about load, it's about digging past the layer of ground that moves with the weather.

The single biggest factor is what you're digging into. In rough order of how deep they push you: rock or dense gravel is stable close to the surface and footings can be shallow, sometimes 450 to 600 mm, once you're onto sound bearing strata. Sand and stable non-shrinkable soils typically need around 700 to 900 mm. Firm clay is the awkward one — it shrinks when it dries and swells when it wets, so footings usually go to at least 1 m, and deeper near anything that dries the ground out. Made ground, peat, soft or filled ground is unpredictable, and this is where strip footings often stop being viable and you move to trench fill, raft or piled foundations on an engineer's design.

The load-bearing capacity of the soil sets how wide the footing needs to be; the stability of the soil sets how deep. They're two separate questions, and both matter.

Takeaway: identify your soil before you price the dig — clay and made ground can double the depth a sandy plot would need.

If there's clay and there are trees, the calm "1 metre" answer goes out of the window. Clay soils shrink as roots draw moisture out of them, and that shrinkage reaches surprisingly deep and surprisingly far sideways. The NHBC publishes guidance — its Chapter 4.2 tables — that sets foundation depth based on the clay's shrinkage potential, the tree species' water demand, and the distance from the tree to the building. Under that guidance, a high-water-demand tree like an oak or willow on high-shrinkage clay can push required foundation depths to 2.5 m or more within striking distance of the trunk.

This is not a corner to cut on judgement. If your plot is clay and there are — or recently were — trees nearby, foundation depth is a design decision for the building control officer and often a structural engineer, not a rule of thumb off a website.

Takeaway: clay plus trees means depth is designed, not guessed — assume deeper and get it signed off.

People expect Approved Document A to state a number. It largely doesn't. Approved Document A (Structure) of the Building Regulations, together with the traditional strip-foundation provisions, requires foundations to be taken down to a depth at which the ground has adequate bearing capacity and is not affected by seasonal movement, frost or vegetation — and, for clay soils subject to volume change, it points to the kind of NHBC-style depth tables described above. In frost-susceptible ground, footings should generally be at least 450 mm below finished ground level to clear the frost zone, though seasonal-movement requirements usually take you well past that.

In practice the depth is agreed on site. The building control officer inspects the open trench before any concrete goes in, checks you're onto suitable bearing ground, and signs off the depth. That trench inspection is a hold point — you don't pour until it's passed.

Takeaway: the Regs set the principle — past frost, past seasonal movement, onto firm ground — and your building control officer sets the actual number at the trench inspection.

A single-storey rear extension on a Nottingham semi, trench-fill strip footings, 600 mm wide. The ground is firm clay and there's a neighbour's apple tree about 6 m from the new corner.

Ordinary firm clay with no trees would suggest around 1 m. The tree is moderate-water-demand and within range, so the NHBC tables push the design depth to about 1.5 m at the corner nearest the tree, tapering back as you move away. Footing trench: 600 mm wide by 1.5 m deep. For a 10 m total run of trench fill poured to within 150 mm of the surface (1.35 m of concrete), that's 10 x 0.6 x 1.35 = 8.1 m³ of concrete.

Drop those figures — trench width, depth and length — into the Concrete Footing Calculator and it returns the volume, the number of bags if you're mixing, or the ready-mix quantity, plus a cost. At 8.1 m³ you're firmly into ready-mix territory: a typical 2026 delivered price from a supplier like Travis Perkins or a local ready-mix plant would be in the region of £110 to £140 per m³ for a standard mix, so roughly £900 to £1,150 of concrete before the dig and labour.

Takeaway: on clay near a tree, expect trench-fill depth of 1.5 m and a concrete order that jumps accordingly — 8 m³ is a ready-mix job, not a mixer-and-bags one.

Plenty of DIY advice treats "dig deeper" as the safe default — if in doubt, go down another 300 mm. It isn't always the right call, and it can create problems. Over-deep trenches in unstable ground are a collapse and safety risk, they burn concrete (and money) fast at ready-mix rates, and on a shrinkable clay site a deep footing on one part of the building and a shallow one on another can actually cause differential movement — one corner riding the seasonal swell while another sits still, which is exactly the cracking you were trying to avoid.

The right depth is a consistent, designed depth across the whole foundation for the ground you've got, confirmed at the trench inspection — not the deepest you can be bothered to dig. More concrete is not the same as more safety.

Takeaway: match the depth to the ground and keep it consistent — a uniform correct depth beats a random deep one every time.

Once your depth is agreed, the Concrete Footing Calculator turns trench width, depth and length into a concrete volume, bag count and cost — for strip, pad and post-hole footings.

Disclaimer: This guide is for planning. Foundation depth must be confirmed by your building control officer at the trench inspection. Ground conditions vary; consult a structural engineer if you have any doubt about your soil type or bearing capacity.