Trees, TPOs and Foundation Design

Two entirely separate questions get merged into one every week: whether you are allowed to touch the tree, and how deep the tree makes you dig. They have different answers, they come from different places, and neither one predicts the other. A leylandii hedge that nobody has ever protected can set a foundation depth of two metres. A magnificent oak with an Order on it, standing thirty metres away, can change nothing at all. This guide separates the legal question from the engineering one, because merging them is what leads people to fell the wrong tree, or to assume the right one has stopped the project.

The engineering question does not care whether the tree is protected

Depth comes from a set of tables, and the tables have no idea what the council thinks of the tree.

A mature broadleaf moves an extraordinary volume of water out of the soil and into the air across a summer. On shrinkable clay that dries the ground, the clay contracts, and the contraction reaches a good deal further than the branches do. Foundation depth near a tree is set by four variables acting together: the species and its water demand, its mature height rather than its height today, its distance from the wall, and the volume change potential of the soil, which is a laboratory result rather than an opinion. Put those four into the tables and you get a depth, and on the worst combinations the tables run out at about two and a half metres and the answer becomes piles instead.

Those tables are not legislation. Approved Document A requires a foundation adequate for the ground and says the depth must increase to account for vegetation, and then stops. It publishes no species list. The working numbers come from the house warranty providers’ standards, of which the National House Building Council’s are the best known, resting on research published by the Building Research Establishment on low rise buildings near trees on shrinkable clay. Structural engineers use them, building control bodies expect to see them, and lenders and warranty providers require them. None of that makes them law.

So the sentence to hold on to is this: protection status tells you what you may do to the tree, and tells you nothing whatsoever about your foundation. A self seeded sycamore in your own hedge line, which you may legally take out this afternoon with nobody’s permission, may be the thing driving your depth. And the depth is what costs money. How the whole design decision is assembled sits on the foundations guide, and why the soil half of the calculation varies so much across this district is in building on Essex clay.

Three ways a tree can be protected, and they are not the same mechanism

Different legislation, different triggers, different clocks, different consequences for getting it wrong.

The conservation area route is often described as a weaker version of an Order. It is better understood as a trigger. The six week notice exists so the council gets a chance to decide whether it wants to protect the tree permanently, and a notice on a good specimen frequently produces an Order by return. Where nothing comes back, the works you described may go ahead and the permission runs for two years, after which you notify again. The wider designation and what else it changes is covered in extending a house in a conservation area.

The third route is the one nobody checks. Estate housing from the 1980s onwards was routinely permitted with a landscaping condition, and the trees planted under it are tied to the land through the permission rather than by any Order. There is no entry on a tree map to find. It is in the decision notice for the original estate, which is the same document that often removed permitted development rights, and planning conditions are worth reading in full before you plan around anything in the garden.

One regime that does not apply: a felling licence from the Forestry Commission, which does not extend to trees growing in a domestic garden.

What an Order actually prohibits, and the exemptions that exist

The offence is drawn around the tree’s survival, not around the chainsaw.

Under an Order it is an offence to cut down, uproot or wilfully destroy a protected tree, and separately to top, lop or wilfully damage one in a way likely to destroy it. That second limb is the one that reaches building work. Severing structural roots in an excavation can kill a mature tree as effectively as felling it, and it is caught in the same terms. A digger driver working to a drawing nobody checked is a criminal exposure, not a snagging item.

Convicted in the Crown Court, the fine is unlimited, and the court is required to take into account any financial benefit that has resulted or is likely to result from the offence, which is aimed squarely at people who calculate that the tree is worth more gone. Separately, where a protected tree is removed or destroyed there is a duty on the landowner to replace it, enforceable by a tree replacement notice. That duty follows the land, so it can land on a buyer who did nothing.

The exemptions are narrower than the rumours about them:

• Dead trees. Removal is exempt, but you must give the council five days’ notice beforehand, and the replacement duty still applies. Dead is not the same as dying, and it is not a judgement to make yourself.
• Immediate danger. Work needed urgently to remove an immediate risk of serious harm is exempt, with notice given as soon as practicable. A tree that is inconvenient in five years is not an immediate danger.
• Abating a nuisance. Limited, judged against what is actually necessary, and not a general licence to cut back to the boundary.
• Implementing a full planning permission. Work necessary to carry out development for which full planning permission has been granted is exempt. This is the useful one, and it has a trap in it.

The root protection area, which usually decides the foundation type

Once a tree is protected, the argument stops being about depth and becomes about method.

Where a council is assessing an application near a significant tree it works to the British Standard on trees in relation to design, demolition and construction, BS 5837:2012, and it will expect the same documents an architect would provide on a larger site. A tree survey records each tree and grades it, from category A for high quality specimens worth designing around down to category U for those that should go anyway. An arboricultural impact assessment sets out what the proposal does to them. A method statement says exactly how the work will be carried out near them, and protective fencing goes up before any machine arrives, not after.

The number that governs is the root protection area. It is drawn as a circle around the trunk with a radius of twelve times the stem diameter measured at 1.5m above ground, and the resulting area is capped at 707 square metres. Inside that circle the ground is supposed to stay as it is: no excavation, no compaction under vehicles or stored materials, no change of level, and nothing tipped or washed out onto it, cement washings in particular.

Read that alongside a foundation trench and the conflict is obvious. A protected tree close enough to drive your depth to two metres is usually close enough that the trench runs through its root protection area, and the two requirements cannot both be met by digging. Which is why, on a protected tree site, the answer is so often bored piles with a reinforced ground beam spanning between them and a void beneath the floor. It puts the load below the influence of the roots while removing almost all of the excavation, and it is frequently the arboricultural answer and the structural answer at the same time. Piling reads as a punishment on a quotation. Beside a protected tree it is often the thing that makes the extension possible at all, and the cost sits on our groundworks page with the rest of the ground.

When the tree belongs to next door

Your foundation responds to the roots. Your rights stop at the boundary.

You have no right to have a neighbour’s tree removed or reduced because of what it does to your building costs, and the fact that it is setting your depth is not a planning consideration anyone will weigh for you. The design absorbs it.

At common law you may cut back branches and roots that cross the boundary, to the boundary line and no further, and what you cut remains the tree owner’s property and should be offered back. That right disappears entirely if the tree is protected, where the same cut needs consent first. It is also a right worth using carefully rather than fully: severing major roots on one side of a mature tree can destabilise it, and if it then comes down on something, the liability is yours rather than theirs.

Where an existing building is being damaged by encroaching roots, and the tree’s owner has been given proper notice of it, liability for the damage can follow. That is a remedy after the event, though, and it is no use at all to somebody trying to build. The practical move is the boring one: talk to the neighbour early, put what you agreed in writing, and design on the assumption that the tree stays.

The order to do this in

1. Check protection status on your address with the council before any design work starts, for your own garden and for anything mature on the other side of the fence. Ask about Orders, the conservation area boundary, and any condition on the original permission.
2. Identify species and estimate mature height for everything within influencing distance, which is further than most people assume and further than the canopy.
3. Get the soil tested where the ground is uncertain, because the depth tables need the volume change potential and not a guess at it.
4. Let the structural engineer design the foundation from those inputs, before the layout is fixed, since the answer sometimes moves the extension.
5. Where a protected tree is affected, run the arboricultural work and any consent application alongside the planning application rather than after it, and decide deliberately whether to apply for permission rather than rely on permitted development.
6. Get the protective fencing up before the first machine comes through the gate.

We check the tree position on every job before anybody draws anything, because it changes what is possible rather than only what it costs. Where a protected tree is in play we bring the arboricultural and structural work together early, so the foundation design and the tree work are answering the same question. The survey and the quotation cost nothing.

Tree protection is specific to the tree and to the address, and foundation design is a matter for the structural engineer and building control on your own project. Treat this as an explanation of how the two systems work rather than as advice on a particular tree.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.