How deep do house foundations need to be?
Why depth is about ground movement rather than strength, the minimum depths Approved Document A publishes, and why the honest answer for any particular site is that it depends on what is found in the trench.
- Difficulty
- intermediate
- Time
- 10 min
- Read
- 5 min
- Safety
- warning
Short answer
Depth exists to get the underside of the foundation below the layer of ground that changes volume with the seasons. Approved Document A gives 450mm as a minimum on non-shrinkable ground to avoid frost, and on shrinkable clay not less than 0.75m on low shrinkage, 0.9m on medium and 1.0m on high. Near trees on clay the depth needed is considerably greater and comes from the NHBC Standards tables. The final depth is decided in the trench, not on paper.
This is the question people ask when they are planning an extension, and the reason it never gets a single-number answer is that the number is a property of the ground rather than of the building. The published minimums are floors, not targets, and the phrase every groundworker uses — that the depth is decided when the trench is open — is not evasion. It is how the system works.
Safety
Step by step
- Start with frost, which sets the shallowest case.Water in the ground expands when it freezes and lifts what is above it. Approved Document A takes 450mm below finished ground level as the minimum on non-shrinkable soils for that reason alone. That is the floor, not the answer.
- Then ask whether the soil is shrinkable.Clays with a significant plasticity index change volume as they wet and dry. Where they do, Approved Document A gives minimum depths to the underside of not less than 0.75m on low shrinkage clay, 0.9m on medium and 1.0m on high.
- Then ask about trees, which change everything.A tree draws water out of clay over a wide area and to a considerable depth, and the effect persists after the tree is gone. Where trees are or recently were near, the required depth comes from the NHBC Standards tables, which set it from the species, its mature height and its distance from the foundation. Depths well beyond a metre are routine.
- Add the site-specific reasons for going deeper.Made ground and fill that will compress, a sloping site where the ground falls away, an existing drain run, or a neighbouring foundation that is deeper than yours. Each is a reason the trench keeps going.
- Expect the depth to be confirmed in the trench.The designer sets a depth from the site investigation; the building control inspector looks at what is actually exposed before concrete is placed. If the ground is not what was expected, the depth changes then. This inspection is the reason the depth cannot be finalised in advance.
- Understand why matching a neighbour is not a method.Ground varies over a few metres. A neighbour's extension may have been built before the tree grew, on a different part of the plot, or without any investigation at all.
- Budget for the possibility of depth.Foundation depth is the single most common reason an extension costs more than the quote. A quote that assumes a shallow trench and finds clay and a mature oak will change. Asking a builder what depth their price assumes is a fair and useful question.
Tips
- The figures above are minimums from the guidance, not a design. They tell you what would never be acceptable; they do not tell you what your site needs.
- If a tree has been removed from a clay site and a building is put up soon after, the clay can recover moisture and swell for years. That is heave, and it is why removing a tree before building is not the simplification it looks like.
- Where a foundation has to go deeper than a neighbouring one, the sequencing and support become a real engineering question and often a party wall matter as well.
Common mistakes
- Treating 450mm as the depth for house foundations — It is a frost minimum on non-shrinkable ground. On the clays that cover much of southern and eastern England it is nowhere near enough, and a foundation at that depth would move every summer.
- Pouring concrete before the trench has been inspected — The inspection is the point at which the assumed ground is checked against the real ground. Concreting first means the only way to prove the foundation is to break it out.
- Ignoring a tree because it is in the neighbour's garden — Roots do not respect boundaries and the tables are based on distance, not ownership. A tree over the fence is as relevant as one in your own garden, and it may also be protected.
If it doesn't work
The groundworker says the trench has to go deeper than quoted
Cause: The ground found is softer, more shrinkable or more tree-influenced than assumed — Fix: Ask to see it before it is filled, ask what building control has said, and get the revised depth and price in writing. This is a normal and legitimate variation, not automatically a builder inventing work.
You have hit a drain in the line of the foundation
Cause: An existing or shared drain crossing the excavation — Fix: Stop and get it designed. The foundation may need to bridge the drain with a lintel, and altering or building over a public sewer needs the water company's agreement.
The trench is filling with water
Cause: A high water table, a wet season, or a leak — Fix: It is a design and a safety matter, not something to bail out and carry on with. Concrete placed into standing water does not achieve what was specified, and a waterlogged trench is far more likely to collapse.
Questions people ask
How deep are the foundations of a typical Victorian house?
Often very shallow — sometimes only a few courses of brick corbelled outwards below ground. They have generally performed because the ground beneath them has never been disturbed and the buildings are relatively light and flexible. It is a reason to be careful about what you change, not a reason to worry.
Can I find out the depth without digging?
Local authority building control may hold records for anything post-war, and a structural survey can establish it properly with a supported trial pit. Neither is a DIY exercise.
Do foundations have to be deeper on a slope?
Generally yes, because the ground can move downhill and because the effective cover on the downhill side is less. Sloping sites are one of the standard reasons a design calls for more depth, or for a stepped foundation.
Is it different in Scotland and Northern Ireland?
The technical requirements are set out in the Scottish Technical Handbooks and the Northern Ireland Technical Booklets rather than in Approved Document A, and the local authority verifier or building control body is the approving authority. The engineering principles are the same; do not quote an English Approved Document at a Scottish verifier.