Tree influence and foundation depth assessment
An assessment of how nearby trees on shrinkable clay affect foundations, with the foundation depths needed to avoid damage.
About this service
Trees take water from the ground, and on shrinkable clay that water loss makes the soil shrink. Foundations that sit within the zone a tree can dry out may move as the clay shrinks in summer and swells back in winter. A tree influence assessment works out how far down that effect reaches at your site and sets the foundation depth needed so new work is not disturbed by it.
Much of north and west London sits on London Clay, and many streets are lined with large plane, oak, lime and poplar trees, alongside mature garden trees and hedges. Recognised industry foundation guidance links the required depth to the tree species, its mature height, its distance from the foundation and how shrinkable the soil is. Applied properly, it is a reliable method that Building Control officers know well.
We identify the relevant trees, including those in neighbours' gardens and on the street, confirm or test the soil's shrinkability and calculate the foundation depth at each part of your proposed footprint. We also cover heave, which matters where trees have been removed or will be. The assessment goes through our three-step quality control process, with final sign-off by our Head of Engineering. Fees start from £450 + VAT.
When you need it
- You're building near trees on clay soil
- Trees have been removed or are planned to be
- Building Control or a warranty provider needs foundation depths near trees
What's included
- Identification of relevant trees and species
- Assessment of soil shrinkability
- Required foundation depths to recognised guidance
- Advice on heave protection
How it works
- Map the treesWe record every tree and hedge within influencing distance, including those beyond your boundary, noting species, height and distance from the proposed foundations, plus any recently removed trees.
- Confirm the soilWe establish whether the clay is of high, medium or low shrinkability, using existing test results, a trial pit sample or laboratory plasticity testing where none is available.
- Calculate depthsUsing recognised guidance, we calculate the required foundation depth around the footprint, which often varies along a single wall as distance from each tree changes.
- Heave and detailingWe specify compressible heave material against foundations and beneath slabs or beams where needed, and advise whether deep trench fill or a piled solution makes more sense.
What you receive
What happens when trees are removed, planted or protected
Removing a tree is not a simple fix for clay movement. Once a large tree is gone, the dried clay slowly takes up water and swells, which can lift foundations and slabs for years afterwards. Recently felled trees therefore still count in the assessment, and heave protection becomes essential. Equally, a young tree that will grow tall must be assessed at its mature height, not its current size. Our London Clay and trees guide explains the mechanism in more detail.
Protected trees add another layer. If a tree has a preservation order or sits in a conservation area, you may not be able to remove or prune it, and foundations near its roots may need a design that limits excavation, such as piles with a suspended beam. Where roots also threaten drains or an existing wall, a root barrier design can be combined with the assessment. The results feed directly into foundation design for your extension or new building.
Frequently asked questions
How deep should foundations be near trees on clay?
It depends on the tree species, its mature height, its distance from the foundation and how shrinkable the clay is. Depths can range from the normal minimum of around a metre to well over two metres close to thirsty species such as oak or poplar. Beyond a certain depth, piled foundations are usually more practical. A site-specific assessment gives the actual figure.
Do neighbours' trees affect my foundation depth?
Yes. Roots do not respect boundaries, so trees in neighbouring gardens and on the street are included in the assessment if they are within influencing distance. You do not need to own or control a tree for it to affect your clay, which is why we survey beyond your boundary as part of the work.
What is heave protection and when is it needed?
Heave protection is a compressible material placed against the sides of deep foundations and under slabs or ground beams. It absorbs the swelling of clay that is rehydrating, for example after a tree has been felled. Without it, swelling clay can push foundations and floors upwards. It is needed wherever trees have been or will be removed, or the clay is desiccated.
Will Building Control accept a tree influence assessment?
Yes, an assessment based on recognised foundation guidance is the normal way to demonstrate suitable foundation depths near trees, and Building Control officers are familiar with it. We send the assessment and calculations to Building Control and liaise with them if they have questions, which helps avoid delays when the foundations are inspected on site.