How close to a house can I plant a tree?

Subsidence from trees is real but is very largely a clay soil phenomenon. Clay shrinks as it dries and swells as it wets, and a thirsty tree close to a building can pull enough moisture out of clay in a dry summer for that movement to reach the foundations. On free-draining soils that mechanism barely operates, so the sensible planting distance for the same tree can differ by a factor of three depending on what is under the garden.

Step by step

1. Find out what your soil is.Dig a hole. Clay rolls into a sausage when moist and cracks in a dry summer; sand runs through your fingers; chalk is obvious. The British Geological Survey's online mapping gives the underlying geology for any address free of charge. This single fact changes the answer more than the species does.
2. Find out how old the house is and how deep the foundations are.Houses built before roughly the 1950s often have shallow foundations, sometimes less than a metre, and are much more vulnerable on clay. Modern foundations on clay are usually designed deeper, and any newer extension may differ from the original house.
3. Use mature height as the starting rule on clay.For a large, high water-demand species on shrinkable clay — oak, poplar, willow, elm — a distance equal to the tree's mature height is the conventional planning figure. For medium trees, half to two-thirds of mature height. These are rules of thumb, not guarantees; a structural engineer or arboriculturist gives the real answer for a specific site.
4. Know which species take the most water.Willow, poplar, oak, elm and to a lesser extent ash, sycamore and horse chestnut are the high water-demand trees associated with clay shrinkage claims. Birch, rowan, hawthorn, crab apple, amelanchier and most small ornamental trees are far lower risk.
5. Halve the problem by choosing a smaller tree.Most garden situations do not need a large tree. A rowan, crab apple, amelanchier, hawthorn or small ornamental cherry gives structure, blossom, autumn colour and wildlife value at 5 to 8m, and can sit far closer to a building without concern.
6. Think about the drains before the foundations.Roots do not break into sound pipes, but they exploit joints and cracks that are already failing, and old clay drainage runs are the classic case. Keep vigorous trees well away from known drain runs and inspection chambers, and be aware of where they are.
7. Think about the practical nuisances.Gutters full of leaves every autumn, honeydew and sooty deposits on cars under lime and sycamore, fruit dropping on a patio, moss on the north side of a roof, and shade in the one part of the garden that gets sun. These affect people far more often than subsidence does.
8. Consider what the roots will meet.Roots extend well beyond the canopy, mostly in the top 600mm of soil. A tree planted 2m from a patio, a shed base, a drive or a low wall will eventually lift them, and those are the structures that actually get damaged in most gardens.
9. Check for restrictions before planting near a boundary.There is no general law about planting distance from a boundary, but a tree that will overhang heavily, block a neighbour's light or drop onto their roof is a dispute in slow motion. Site it where it works for both gardens.

Tips

• If your house is on clay and has a history of movement, get advice before planting anything large. Insurers take tree proximity into account and a claim history changes the picture.
• Removing a large established tree from clay close to a house can itself cause damage — the clay rehydrates and swells, which is called heave, and it can be as damaging as the shrinkage. Never fell a big tree near a house on clay without professional advice.
• Plant for the tree's mature size, not for the size on the label at the garden centre. Everything is small in a pot.

Common mistakes

• Planting a willow or poplar in a suburban garden — They are the two species most associated with clay subsidence, they grow very large very fast, and they are aggressive around drains. There is almost no domestic garden where either is a good idea.
• Applying clay-soil distances on sand or gravel — It rules out perfectly safe planting for no reason. The shrink-swell mechanism needs shrinkable clay.
• Felling a mature tree near a clay-founded house to be safe — Heave from rehydrating clay is a real and expensive risk. Take advice before removal, not just before planting.
• Ignoring the drains — Root damage to drainage is far more common in gardens than root damage to buildings, and it is usually cheaper to prevent than to fix.

Questions people ask