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. Strictly it is crown volume rather than height that governs how much water a tree draws, and root spread has no fixed relation to height, but mature height is the practical proxy the design tables use.
• 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.

Which trees, and how far

The most useful evidence on which trees cause damage comes from the survey of thousands of implicated trees on clay that Kew carried out after the storm of 1987. Two cautions before the numbers: the distances are records of where damage was found, not safe distances or odds, and the great majority of trees growing closer than these figures never cause any damage at all. They are a starting point for an investigation, never a reason on their own to fell a tree.

Indicative groupings drawn from published records of trees implicated in subsidence on shrinkable clay. They describe where damage has occurred historically, not the risk from any particular tree, and must not be used to decide whether a tree should be removed.

Two things about roots surprise people. They are shallow: in the Kew sample almost half of the structural roots were within a metre of the surface and only around one tree in twenty-five had them deeper than 2 m, so a root you find in a trench may be holding the tree up and should not be cut casually. And their spread bears no fixed relation to the tree's height; a small tree with a dense crown can have wider roots than a taller one with a sparse crown, which is why the design method leans on species and mature height as a proxy rather than on measuring roots.

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.

Whether that happens quickly or slowly depends on how the tree was drying the ground. If the clay was rewetting fully each winter, removing the tree is a small extension of the normal seasonal recovery and the swelling is usually over within a year or two. If the tree had built up a deep, persistent dry zone over decades, the clay can go on taking up water for many years, in extreme cases more than twenty, and the swelling can reach 100 to 150 mm in a high-shrinkage clay. That is why a mature oak, poplar or willow next to an old house is treated with such respect, and why the ground investigation checks for a persistent moisture deficit before anyone decides what to do with the tree. Our guide to heave vs subsidence explains the mechanism, the timescales and the anti-heave underpinning used where it goes wrong.

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. Tying an extension into an existing house is a design question in its own right, and a common source of cracking when it is got wrong.

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.

Managing an existing tree instead of felling it

Where a tree is already implicated in damage to an existing house, felling is the last resort, not the first; our guide to trees and subsidence: prune, fell or underpin walks through that decision in full. Reducing the crown reduces the leaf area and therefore the water the tree draws, and research since 2004 has shown that a proper crown reduction, which leaves a smaller, denser crown, does control uptake, whereas thinning the crown while keeping its size does not and is no longer recommended for this purpose. The catch is regrowth: the tree restores its leaf area within a few years, so the reduction has to be repeated every two to three years for as long as the tree stands, and that becomes the owner's ongoing responsibility once any insurance claim has closed. Root barriers are rarely worth it; roots have been recorded passing beneath barriers nearly 3 m deep and over the top of shallower ones. Before anyone touches a tree, check with the council whether it is protected by a Tree Preservation Order or stands in a conservation area, because unauthorised work to a protected tree, including its roots, is a criminal offence. If the council refuses consent for work to a tree that is proven to be causing damage, compensation can be claimed for the extra cost of the alternative repair. Where the tree belongs to a neighbour or the council, a claim needs to show the damage was foreseeable, not just that the tree caused it, so an engineer's and an arboriculturist's evidence matters from the start. For what a tree-related subsidence repair costs at each stage, see our subsidence repair cost guide.

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 specialist site 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 specialist construction arm to carry out the below-ground structural work under engineering oversight, 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.