It is one of the most common questions we are asked, and the honest answer is that there is no single depth. The right depth is whatever it takes to reach ground that will not move under your building, and that depends on the soil, the trees nearby, any drains, and your neighbour's foundations. Here is how each one affects it.
Who decides how deep foundations should be?
Building control checks that foundations meet the building regulations, and a structural engineer designs them where the ground or the building needs it. Building control usually inspects the open trenches before concrete is poured, so the depth can be confirmed against the ground actually found. A good builder digs to that agreed depth, not to a rule of thumb.
~0.75mtypical depth to which clay soils shrink and swell with the seasons
Deepernear trees on clay, often significantly
45°load spread from a foundation; drains inside it set a minimum depth
Why does clay soil change foundation depth?
Clay holds water. When it gets wet it swells, and when it dries out it shrinks, and that movement can crack walls and floors built on top. The moisture changes mainly happen near the surface, typically to about 0.75m, so on clay, foundations are taken deeper than that to sit in ground that stays stable through the seasons.
Building in GloucestershireClay is common on sites around Gloucester and Cheltenham. That is why we never price foundations from a rule of thumb: we look at the ground, the trees and the drains first.
How do trees affect foundations?
Trees draw large amounts of water out of the ground, so on clay soils their roots can dry and shrink the ground well below the normal shrink–swell zone. Foundations near trees on clay may need to be significantly deeper than you might expect. If a tree is far enough away, it may have no effect at all, which is why the distance and species matter.
Don't just cut the tree downRemoving or heavily reducing a tree on clay can let the ground swell as it takes up water again, which can damage foundations too. Get advice from a structural engineer or building control before felling.
What if there's a drain near my extension?
A foundation spreads its load into the ground at roughly 45°. Where a drain or sewer sits inside that spread, the excavation should normally go at least as deep as the bottom (invert) of the drain or its trench, so the new foundation does not bear down on it. The alternatives, depending on the situation, include diverting the drain or bridging over it.
What about my neighbour's foundations?
Where you build next to an existing building, the new excavation should normally reach at least the depth of the adjoining building's foundation. Digging close to a neighbour's building can also bring in the Party Wall Act: see do I need a party wall agreement for my extension?
Which type of foundation will I need?
| Type | What it is | Where it's used |
|---|---|---|
| Strip foundation | A strip of concrete in the bottom of the trench, with blockwork built up from it. | Firm ground at a moderate depth. |
| Trench fill | The trench is filled almost to the top with concrete. | Deeper trenches, including many clay sites, where it is quicker than building blockwork deep underground. |
| Raft | A reinforced concrete slab across the whole footprint. | Softer or variable ground, where loads need spreading. |
| Piled | Columns taken deep into the ground, with a beam on top. | Very deep firm ground, poor soils, or very close to large trees. |
From our own work: we build specialist foundation systems, basements and reinforced concrete structures, including on confined sites where temporary earthwork support is needed. We dig to the depth agreed for your ground, and building control signs it off before any concrete goes in.
Planning a project like this?
We can visit the site, explain what your project needs and give you a clear, itemised quote.
Frequently asked questions
Who decides how deep my foundations should be?
Building control checks that foundations meet the building regulations, and a structural engineer designs them where ground conditions need it. Your builder digs to the depth they agree for your site.
Why do trees make foundations deeper?
In clay soils, tree roots draw moisture out of the ground and make it shrink, which can move shallow foundations. Foundations near trees on clay are taken deeper, below the zone the roots affect.
Can I just cut the tree down instead?
Not without advice. Removing or heavily reducing a tree on clay can let the ground swell as it takes up moisture again, which can also damage foundations. Check with a structural engineer or building control first.
What happens if there is a drain near my extension?
Where a drain or sewer runs close to the new foundations, the excavation should normally go at least as deep as the bottom of the drain, so the new foundation does not load it.
Sources and further reading
- Planning Portal: Building regulations, foundations
- GOV.UK: Approved Document A (Structure)
- Planning Portal: Is building regulations approval needed for an extension?
This article is general guidance for England, correct when last reviewed. Rules differ in Wales and can be changed locally (for example by Article 4 directions or planning conditions), so check with your local planning authority or building control before starting work.