Building Foundations Near Trees on Clay

Why clay and trees are a problem together

Clay is the one common soil that changes volume with its moisture content. Only the finest particles, smaller than about two thousandths of a millimetre, actually swell as they take up water and shrink as they lose it, but there are enough of them in a shrinkable clay to move the ground surface up and down through the year. Much of London and the South East sits on London Clay, which is why movement related cracking is such a familiar problem here.

A tree makes that seasonal movement worse. Its roots draw large volumes of water out of the soil to replace what the leaves lose, drying the clay well beyond the trunk and to a surprising depth. In a dry summer the clay under and around the tree shrinks and the ground settles. Build a shallow foundation into that zone and the building rides the clay up and down with the seasons, which is one of the classic causes of subsidence. The defence is simple to state: take the foundation down below the depth where the moisture, and therefore the volume, keeps changing.

The four things that set the depth

Working out how deep is a judgement, not a single number off a chart. The approach set out in NHBC Standards Chapter 4.2 and in BS 5837 for trees weighs up four factors, plus the local climate.

• How shrinkable the clay is. Clays are graded by their volume change potential, from low through medium to high, based on a simple soil test (the plasticity index, measured to BS 1377). The higher the potential, the deeper the foundation.
• The tree's water demand. Species are grouped as high, moderate or low water demand. High demand species such as oak, poplar, willow, elm and eucalyptus draw the most water and reach furthest. If the tree cannot be identified, it is treated as high demand.
• Its mature height. Water uptake follows leaf area, so the design uses the tree's likely mature height, not its current size. A young tree that will grow tall is still a tall tree as far as the foundation is concerned.
• How close it is. The nearer the trunk, the deeper the foundation. This is why foundations are often stepped, deeper on the side facing the tree and shallower away from it, as in the diagram above.

Climate then adjusts the picture. Hot, dry summers dry the clay deeper, so the driest parts of the South East, including much of London, sit at the demanding end of the scale.

How deep, in practice

On clay, a foundation should reach below the seasonal drying zone, and never be shallower than the frost minimum. As a broad guide, the minimum depths by volume change potential look like this.

Typical guidance only, based on the approach in NHBC Standards Chapter 4.2. The right depth for your site depends on the actual soil, tree and layout, and must be confirmed by a structural engineer. It is not a green light to dig to a fixed depth.

Those depths rise quickly where a high demand tree stands close to the wall. A conventional strip or trench fill foundation stays economic down to roughly 2.5 m. Beyond that it becomes slow and expensive to dig, and the sideways pressure of swelling clay on a deep trench becomes a design issue in its own right, so the foundation is usually redesigned as short bored piles with ground beams spanning between them. That is engineer designed work, outside the simple cases in Approved Document A of the Building Regulations.

Heave: the hidden risk when a tree is removed

It is tempting to think the problem goes away if the tree does. It can make it worse. A mature tree that has been quietly drying the clay for decades leaves the ground beneath it desiccated. Remove the tree, and over the following years the clay slowly takes that water back and swells. The ground lifts, and so does anything founded in it. This upward and sideways movement is called heave, and it can crack a building just as badly as subsidence, sometimes worse, because the pressures are large and act upwards.

For that reason a tree removed within about a year of the foundations being laid is treated, for design, as if it were still there. On sites where trees have been taken out, foundations are usually piled and the ground floor is built as a suspended slab with a void or compressible layer beneath it, so the swelling clay has somewhere to go without lifting the structure. The practical message for homeowners is simple: do not fell a mature tree close to the house, or plan to, without taking advice first. Our guide to the signs of subsidence in London homes explains how movement first shows up.

Extensions and existing houses near a tree

Most of our tree and clay enquiries are not new houses but extensions next to an existing home with a tree in the garden or on the pavement. The new foundations have to suit the tree and the clay in their own right, and they also have to sit sensibly alongside the older, usually shallower, foundations of the existing house, so that the two do not move differently and crack at the join. That junction is a common source of cracking and deserves its own design attention.

One thing that is not the answer on shrinkable clay with trees is a raft. A raft spreads the load but still sits near the surface, in the very zone that dries and swells, so it is generally not accepted where there is a shrink and swell risk from trees. The reliable solutions there are a deep foundation or piles.

What EMA Structures does

Getting this right is exactly the kind of judgement a structural engineer is for: balancing the risk of movement against the cost of digging deeper or piling. We inspect the site, identify the trees and their likely mature size, and where needed our own team digs trial pits so we can see and test the clay rather than guess at it. We then design the foundation, whether that is a deep strip, trench fill or piles with ground beams, and produce the calculations Building Control needs. Because we both diagnose and design, and have a site team to carry out the below ground work, the same people take your project from the tree in the garden to a foundation that will not move.

Concerned about a tree near your home?

Tell us what you are planning and what is growing nearby, and send a photo or two. Send us the details and a structural engineer will advise on whether the tree affects your foundations and what depth or type is likely to be needed. An inspection with a written report costs £495 + VAT, see our fees guide, correct as of July 2026.