Trees and subsidence: the evidence councils accept

If your insurer says a protected tree is causing subsidence and the council says the tree stays, you are in the middle of a dispute that gets settled by evidence rather than by opinion. Councils refuse consent to fell protected trees on subsidence grounds regularly, and they do it because the application arrived with a loss adjuster's letter and nothing else.

The evidence that changes a tree officer's mind is a set of four things, gathered over at least one summer.

How trees are said to cause clay shrinkage

Shrinkable clay soils lose volume as they dry and swell again as they wet up. A tree growing in that soil takes water out of it through the growing season, deepening and widening the zone that shrinks. If a building's foundations are shallow and sit in the affected zone, the ground beneath part of the building drops, and the building cracks in a pattern that reflects the movement.

Three conditions have to hold at once. The soil must be a shrinkable clay. The foundations must be shallow enough to be in the zone of influence. And the tree must be close enough and thirsty enough to be causing the drying.

Take any one away and the tree is not the cause. That is what the council will test, and it is why so many claims fail on evidence. Cracking also results from leaking drains, from clay heave after an earlier tree was removed, from thermal movement, from sulphate attack and from ordinary settlement of an extension against an older structure.

The British Geological Survey publishes shrink-swell hazard mapping, and it is a sensible first check. It is not a substitute for site-specific soil testing.

The evidence a council expects before consenting to removal

Councils vary in how they word the request, but the package is consistent.

1. A structural engineer's report describing the damage, the crack pattern, the crack widths against a recognised damage classification, and the engineer's diagnosis.
2. A drainage survey, usually CCTV, excluding leaking drains as a cause or identifying them as a contributing one.
3. Trial pit information giving the actual foundation depth and type, with the soil profile logged.
4. Soil analysis from the foundation depth and below, establishing plasticity, and desiccation profiles where the claim is that the soil is drying.
5. Root identification from samples taken beneath or adjacent to the foundation, linking roots present to the species in question.
6. Level or crack monitoring covering at least a full seasonal cycle, showing a movement pattern consistent with clay shrinkage.
7. An arboricultural report on the tree itself, its condition, its amenity value and the options short of felling.

Applications that supply items one and seven get refused. Applications that supply all seven usually succeed, because the tree officer has something to write a decision on.

Soil analysis, root identification and level monitoring

The three technical items are where most claims fall short.

Soil analysis establishes whether the clay is shrinkable and whether it is currently desiccated. Plasticity index testing categorises the shrink-swell potential. Moisture content profiles taken at intervals down the borehole show whether the soil is drier near the tree than away from it. A single sample proves nothing. The comparison is the evidence.

Root identification means taking root samples from beneath the foundation or from the trial pit and identifying the species, either by microscopic examination of the anatomy or by DNA analysis. Finding roots near a foundation is not the same as finding roots of the tree you want to fell. On a plot with several mature trees and a hedge, identification is the only way to attribute.

Level monitoring is the item that takes time. Precise levelling of studs fixed to the building, read at regular intervals, shows the seasonal pattern of movement. Clay shrinkage produces downward movement through the summer and recovery over winter. Thermal movement and settlement look different. Twelve months of readings is the usual expectation, which is why insurers start monitoring before they apply and why an application made in month three of a claim tends to be premature.

The tree survey side is worth doing to the same standard. A tree survey to BS 5837:2012 records species, dimensions, condition and the root protection area, giving the tree officer a baseline against which to weigh the loss.

Alternatives to felling

A tree officer who refuses consent is usually saying that the applicant has not shown that felling is the only answer. Four alternatives get considered, and a good application deals with each of them explicitly.

Crown reduction on a cycle reduces water demand, but the effect is temporary and disputed. Say what reduction is proposed, on what cycle, and why it will or will not be sufficient.

Root barriers installed between the tree and the building can work where the geometry allows, and they fail where the barrier is too shallow or too short.

Underpinning transfers the foundation below the zone of influence. It is expensive and disruptive and it is often the answer the tree officer prefers, because it removes the problem without removing the tree. Cost alone is not usually enough to reject it, though a comparison of costs is a material consideration in the round.

Superstructure repair and monitoring, accepting some movement, is realistic where the damage is cosmetic rather than progressive.

Where the ground conditions themselves are in question, a Geotechnical Assessment prepared to Eurocode 7 principles gives the engineering basis, and a Structural Survey records the building's condition. Councils expect suitably qualified authors for both, and professional review is strongly advised.

Working with the insurer and the loss adjuster

The insurer's arboriculturist and the council's tree officer are not natural allies, and the homeowner sits between them.

A few things help. Ask the loss adjuster for the full technical file rather than the summary letter, and submit the technical documents with the application, not the correspondence. Ask the insurer to fund the monitoring period rather than pressing for an early application that will be refused. And keep the council informed while the evidence is being gathered, because a tree officer who has known about the claim for a year responds differently from one who receives a demand with a deadline attached.

If consent is refused, the Tree Preservation Order regulations provide for compensation for loss or damage caused by the refusal, subject to exclusions set out in the 2012 Regulations. The rules exclude certain losses and there are conditions on how and when a claim is made, so take advice on the detail before relying on it.

What happens when the evidence is incomplete

An illustrative example. A homeowner reports diagonal cracking to a 1930s bay window in a dry summer. The insurer's report names a protected oak 11 metres away and recommends removal. The council refuses, noting no soil testing, no root identification and no monitoring. Over the following year the insurer installs monitoring, and the drainage survey finds a fractured foul drain three metres from the bay. The drain is repaired, the movement stops over the next cycle, and the oak stays. Had the tree been felled in the first month, the drain would still have been broken and the homeowner would have lost a mature tree for nothing.

Felling a protected tree without consent is an offence under section 210 of the Town and Country Planning Act 1990, and the court is directed to have regard to any financial benefit arising from it. An insurance-driven felling with no consent is not a defensible position, whatever the loss adjuster has written.

Start the monitoring, order the soil work, and apply when the file is complete.

PlanScope drafts tree, structural and geotechnical planning documents structured to UK planning policy, with a "to be confirmed" flag wherever site investigation results are missing, so your engineer and arboriculturist complete them. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode to see which reports your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.