Why Foundation Depth Changes on Site — and Why That Is Honest
The drawing proposes a depth. The ground decides it and building control confirms it in the open trench — which is why depth is the most common genuine variation.
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Foundation depth is not fixed on the drawing. The Approved Documents set a frost minimum of 0.45m, but most of London sits on shrinkable clay where the figure becomes 0.75m, 0.9m or 1.0m — deeper still near trees — and the real number is confirmed by a building control inspector standing in the open trench. That is why depth is the most common genuine variation on a London build, and the one worth budgeting for before you start. The estimator prices the substructure explicitly, and our extensions page explains where it sits in the programme.
The number is decided by the ground, not the drawing
A structural engineer proposes a foundation depth from what can be known in advance: published geology, the position and species of nearby trees, the depth of neighbouring foundations, and whatever trial holes have been dug. That proposal is well-informed and it is still a proposal.
What confirms it is the excavation inspection. Building control attends the open trench, looks at the material the foundation will bear on, and confirms the depth before any concrete is placed. It is one of the notifiable stages, it closes permanently the moment the trench is filled, and it cannot be done retrospectively.
This is the honest part of the process that sounds like the dishonest part. When a builder says “we won’t know until we dig”, they are describing the actual mechanism — not preparing you for a price rise.
The figures
| The figure | What it means on site | Source |
|---|---|---|
| 0.45m | Minimum depth to the underside of a strip foundation, for frost, except where founded on rock. Increase in areas of long frost or to reach satisfactory ground. | AD A §2E4 |
| 0.75 / 0.9 / 1.0m | On clay with a modified plasticity index of 10% or more: not less than 0.75m (low shrinkage), 0.9m (medium), 1.0m (high). Increase where trees are nearby. | AD A §2E4 |
| Projection and thickness | Concrete strip foundations should project each side of the wall by at least the depth of the foundation below the wall face; thickness not less than the projection, and never less than 150mm. | AD A §2E2–2E3 |
| 70 kN/m | Combined dead and imposed load should not exceed this at the base of the wall. | AD A §2C24(c) |
| 0.25m | Where the water table can rise to within 0.25m of the lowest floor, drain the ground by gravity or safeguard the building by other effective means. | AD C §3.2 |
| Inspection | Excavation for foundations is inspected before it is covered — this is the inspection that fixes foundation depth. | Building control notifiable stages |
| Notice | At least two clear working days' notice to the building control body before work starts. | Building control notifiable stages |
| Excavation safety | Excavations must be supported so they cannot collapse. | CDM 2015 reg. 22 |
| Party wall | Excavating within 3m of an adjoining structure and deeper than its foundations, or within 6m cutting a 45° line beneath them: 1 month's notice. | Party Wall etc. Act 1996, s.6 |
Approved Documents are statutory guidance to the Building Regulations 2010 for England. Compliance is judged against the Requirements in Schedule 1; your building control body has the final word on your specific building.
Why London makes this worse
Most of London sits on clay, and clay changes volume with moisture. It shrinks as it dries and swells as it rewets, and a foundation sitting inside that zone moves with it. That is the mechanism behind most domestic subsidence in the capital, and it is why the frost minimum of 0.45m is almost never the governing figure here.
Trees make it substantially worse. A mature tree draws moisture out of clay across a radius that depends on its species, its mature height and its distance from the building. Foundations within that influence have to reach below the zone where seasonal movement happens, which on high-shrinkage clay near a large tree can mean well past 1.0m.
Two things surprise people about trees. First, the tree does not have to be in your garden — a neighbour’s, or a street tree, counts. Second, removing the tree can be worse than keeping it. Clay that has been dried by roots for decades will swell as moisture returns, and that heave lifts foundations rather than settling them. On a site where a tree is coming out, that is a conversation with the engineer before it is a conversation with a tree surgeon.
Turn the unknown into a number: trial holes
The single most useful thing you can do before anyone prices your extension is dig trial holes.
A trial hole is a small excavation, usually next to the existing foundation, that shows three things: how deep the existing foundation goes, what the ground is made of, and where the water table sits. It takes a morning. It costs a fraction of one week’s delay.
With trial holes, the engineer designs to measured ground and every contractor prices the same substructure. Without them, each builder is guessing, and the cheapest guess wins the job — which means the cheapest guess is the one that has to be corrected on site, at your expense, in week three.
If you take one thing from this: trial holes before quotes, not after.
How to make a depth change boring
A variation for foundation depth is legitimate. What makes it feel like a scam is when it arrives with no evidence and no agreed rate. Fix that in advance.
Hold a ring-fenced foundation contingency. Not general contingency — a specific allowance for this specific risk, which you either spend on depth or get back. Naming it stops it being absorbed elsewhere.
Hold programme float too. Extra depth is extra days as well as extra money, and a programme with no slack turns a routine variation into a late-delivery argument.
Agree the rate before work starts. A price per additional metre depth, agreed at contract, means a depth change is arithmetic rather than negotiation.
Require photographic evidence. The open trench, dated, with a measure in shot, plus the inspector’s confirmation of the bearing. Any builder doing this properly is already taking those photographs — building control inspections are exactly the moments where a record is worth having.
Ask what the quote assumed. “We’ve allowed 1.0m” is a scope. “Foundations included” is not. Two quotes assuming different depths are not comparable, which is the whole subject of our lesson on what is inside a price.
When depth becomes underpinning
If the dig goes below the underside of an existing foundation — the house’s own, or a neighbour’s — you are no longer just digging deeper. You are removing support from something already standing, and that is underpinning: a different method, a different sequence and a different number.
The threshold matters because it is where a modest depth variation stops being modest. It is worth asking the engineer explicitly, at design stage, how close the proposed dig comes to that line.
The takeaway
The drawing proposes a depth; the ground decides it and building control confirms it. Hold a ring-fenced foundation contingency and programme float, require photographic evidence for any change, and dig trial holes before anyone prices the job. Do that, and a depth variation stops being a shock. It is just the build.
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