Tree related subsidence and consent to reduce the tree

Tree related subsidence on a shrinkable clay soil is one of the few situations where a local planning authority will consider consent to fell or heavily reduce a protected tree. It will not do so on an assertion. The evidence has to establish three things: that the building is suffering clay shrinkage subsidence rather than some other movement, that the tree is a material cause of it, and that nothing short of the proposed works will stop it.

Which means the tree preservation order consent application is the last step, not the first. It follows a monitoring period that usually runs through a full seasonal cycle.

How tree related subsidence is identified

Clay shrinkage subsidence happens where a desiccating soil loses moisture, shrinks, and takes the foundation down with it. The mechanism needs a shrinkable clay, a period of moisture deficit, and shallow foundations.

The diagnostic signs are reasonably consistent. Cracking that is diagonal and tapers, usually wider at the top. Movement that worsens in late summer and recovers partially in winter, which is the seasonal signature that distinguishes clay shrinkage from most other causes. Damage concentrated on the elevation nearest the vegetation. And a foundation depth that is shallow relative to the depth of soil drying.

Geology is the first check. Published mapping shows the distribution of soils with high shrink-swell potential, and a property on a low plasticity soil is unlikely to be suffering clay shrinkage whatever the crack pattern looks like.

Proximity matters but is not determinative. Influence distance is conventionally related to the mature height of the species, and water demand varies considerably between species, so a high water demand broadleaf at 12m can be implicated where a lower demand species at 6m is not.

The evidence an insurer assembles

An insurer investigating a claim will usually commission the following, and the authority will expect to see it.

• A structural report describing the damage, its distribution and the crack widths, with a damage category.
• Trial pits or boreholes establishing the foundation type and depth and the soil profile.
• Laboratory testing of the soil, including plasticity and moisture content with depth, to show whether desiccation is present and to what depth.
• Root sampling from the boreholes, with roots identified to species, which is the evidence linking a particular tree to the drying.
• Level and crack monitoring over a seasonal cycle, with readings at defined intervals and a plotted record.
• An arboricultural report on the tree, its condition, its amenity contribution and the effect of the proposed works.

The root identification and the monitoring record are what carry the application. A claim without root identification leaves the authority free to conclude that a different tree, or no tree, is responsible.

Note that a survey carried out in winter has limits on what it can say about a tree's condition and its canopy, a point we deal with in winter inspection limits on a tree survey.

Applying for consent where the tree is protected

Two routes apply depending on the protection.

Where a tree preservation order is in force, you apply to the local planning authority for consent to carry out the works, supported by the evidence. The authority normally has eight weeks to decide, and a refusal or a conditional consent can be appealed to the Planning Inspectorate.

Where the tree is in a conservation area but not subject to an order, you give the authority six weeks' notice of the intended works. The authority can respond by making an order, which converts the notice into a consent application. If it does nothing within the six weeks, the works may proceed.

Highway trees and trees in other ownership are a different problem. Where the implicated tree belongs to the council or a neighbour, you have no standing to apply for consent, and the route is a request to the owner supported by the same evidence, with the civil nuisance position in the background.

What the council weighs against the damage

The authority is balancing the amenity value of the tree against the harm to the building, and it will look for the least drastic effective option.

Four questions shape the officer's recommendation.

1. Is the causal link established, or is the application relying on proximity alone.
2. Would reduction, rather than removal, arrest the drying. Reduction of the crown reduces water demand, although the effect is temporary and the works have to be repeated.
3. Would an engineering solution work instead. Underpinning or a root barrier removes the problem without removing the tree, and an authority is entitled to prefer that where it is practicable.
4. What is the amenity contribution of this particular tree, judged on its visibility, condition, size, species and contribution to the character of the area.

Cost is relevant but it is not the authority's primary concern, and an application resting mainly on the comparative cost of underpinning tends not to succeed on its own. The stronger argument is practicability: a root barrier that cannot be installed because of services, or underpinning that is not feasible given the construction, is a reason the alternative fails.

Alternatives to removal

Three alternatives are worth costing before the application, because the authority will ask about all three.

Crown reduction on a cycle reduces leaf area and water demand. It needs committing to as a programme, with an interval stated, and it should be offered as a management plan rather than a single operation. Authorities respond well to a proposal that keeps the tree and manages the risk.

Root barriers intercept roots between the tree and the foundation. They work where there is room to excavate a trench to sufficient depth without destabilising the tree or cutting through services, and they do not work where the tree is close to the wall.

Underpinning takes the foundation below the zone of seasonal moisture change. It is the definitive answer and the most expensive, and it raises its own questions about adjacent structures and about the extent of the works.

Where any of these involves excavation near the tree, the root protection area constraints are the same as on a construction site, and the approach in utility trenching close to trees on a development site applies.

Compensation where consent is refused

The tree preservation order regulations provide a compensation route. Where an authority refuses consent, or grants it subject to conditions, a person who suffers loss or damage in consequence may claim compensation from the authority.

Three points to be aware of. A claim has to be made within a limited period after the decision, so the deadline needs noting on the day the decision arrives. Certain losses are excluded, and the Regulations set out the exclusions and any financial threshold, so read them rather than assuming the whole loss is recoverable. And the claim is a matter of evidence like any other, which means the ongoing damage after the refusal has to be recorded and quantified.

This is general guidance, not legal advice. Take advice on a compensation claim, and check the current Regulations, before relying on any of it.

Running the claim and the application together

The sequencing is where homeowners and landlords lose time.

The insurer's investigation sets the pace, and the application cannot sensibly be made before the monitoring is complete. So the practical approach is to tell the local planning authority early that a claim is under investigation, ask whether it has a preferred process for subsidence cases, and submit the application with the full evidence pack rather than in instalments.

Two further things help. Offer replacement planting in a position where it will not cause the same problem, which turns a loss of amenity into a managed change and is easier for an officer to recommend. A short Landscape Strategy covering species, position and establishment does that job. And ask the authority's tree officer for a site meeting with the arboriculturist present, because the amenity judgement is made on site rather than from a desk.

A tree survey prepared to the relevant standard, covering the implicated tree and anything else within influencing distance, keeps the application from being refused on the ground that another tree may be responsible.

Enter your postcode on PlanScope to see which arboricultural documents a protected tree application is likely to need.

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