Trees, subsidence and the planning answer

If a protected tree is blamed for cracks in your house, the planning answer is rarely "cut it down". Councils refuse consent for the removal of a protected tree on subsidence grounds far more often than homeowners expect, and they refuse it because the application arrives without the evidence that links that particular tree to that particular crack. Trees subsidence planning cases are won on monitoring data, soil chemistry and root identification, not on photographs of the crack.

There is a second reason removal is not automatically the fix. On shrinkable clay, taking out a mature tree lets the soil rehydrate and swell, which can lift a foundation that was built when the tree was already there. That is heave, and it does more damage than the original settlement in some cases. A sensible engineer will say so before you apply.

What tree-related subsidence actually is

Shrinkable clay soils change volume as their moisture content changes. London Clay, the Gault, Weald, Oxford, Lias and Kimmeridge clays all do this to varying degrees, and the British Geological Survey mapping will tell you whether your postcode sits on one of them.

A large tree in summer draws a great deal of water from the soil around it. On clay, that drying causes the ground to shrink, and a shallow foundation sitting in the affected zone settles with it. In winter the soil rehydrates and the foundation partially recovers, which produces the characteristic pattern: cracks that open in late summer and close in spring, widening slightly year on year in dry summers.

Two things follow from that pattern. First, a single inspection proves nothing, because you cannot tell seasonal movement from progressive failure in one visit. Second, the tree species matters, since high water demand species such as poplar, willow, oak and elm affect soil at greater distances than a birch or a magnolia of the same height.

The evidence the insurer wants, and the evidence the council wants

These overlap but they are not the same, and homeowners often submit the insurer's pack to the council and wonder why it is not enough.

An insurer's investigation typically involves crack monitoring and level monitoring over at least a full seasonal cycle, trial pits to establish the depth, width and condition of the foundation, soil sampling to measure plasticity and the desiccation profile with depth, and root sampling with laboratory identification of the species found beneath the foundation. The BRE digests on low-rise building foundations in shrinkable clay soils and on the influence of trees on house foundations are the technical references the engineer will work to.

The council needs all of that plus two further things. It needs the arboricultural side: the tree's amenity value, its condition, its contribution to the street and whether it is the only tree capable of causing the effect. And it needs to see that less drastic options have been considered and rejected with reasons, because the authority's job is to weigh amenity against the harm being claimed.

That is why the strongest applications pair an engineer's report with an arboricultural report rather than relying on either alone. A Structural Survey sets out the building's condition and the foundation evidence, and councils expect a suitably qualified author for it with professional review strongly advised.

Applying for consent on a protected tree

Where the tree is covered by a Tree Preservation Order you apply to the council for consent under the Town and Country Planning (Tree Preservation) (England) Regulations 2012. In a conservation area with no TPO, you give six weeks' notice of the proposed works instead, and the council may respond by making an Order.

The Regulations contain exemptions, and this is where people get into trouble. There is an exemption for work urgently necessary to remove an immediate risk of serious harm, and one for works necessary to prevent or abate a nuisance. Subsidence damage can amount to a nuisance in the legal sense, but the exemption turns on that legal test rather than on how serious the cracking feels. Felling a protected tree on that basis without taking advice is how homeowners end up prosecuted.

Apply instead, and say in the application:

1. Which tree, with a plan showing its position relative to the damaged elevation and the distance measured.
2. What the monitoring shows, with the data covering at least one full seasonal cycle.
3. What the trial pits and soil tests found, including foundation depth and the desiccation profile.
4. What the root identification confirmed, species by species.
5. What alternatives were considered, and the engineer's reason for rejecting each.
6. What replacement planting is offered, with species and position.

The Regulations also contain provisions allowing a claim for loss or damage caused by a refusal or by a conditional consent, subject to conditions and time limits. That is worth knowing about, because it changes the tone of the conversation with an authority that is minded to refuse without engaging with the evidence.

Alternatives to taking the tree down

Three are available in practice, and all three need commitment rather than a one-off job.

Cyclical crown reduction lowers the tree's leaf area and so its water demand. It works only if it is repeated on a fixed cycle, usually every few years, and only where the tree tolerates the pruning. An agreement to a programme of works, recorded in the consent, is better for everyone than repeat applications.

Root severance with a physical barrier cuts the connection between the tree and the soil beneath the foundation. The barrier must extend below the rooting depth to be effective, the excavation itself has to be designed so it does not destabilise the tree, and on a narrow urban site there is often nowhere to put it.

Engineering the building rather than the tree is the third route. Underpinning to a depth below the zone of seasonal moisture change removes the sensitivity altogether, and where the tree has high amenity value this is frequently the outcome the council will press for. Whether the insurer agrees to pay for it is a separate argument.

Building near existing trees

For a new extension or new house on clay near trees, the foundation design is the answer rather than the tree. Industry foundation guidance for low-rise housing sets depths by reference to the soil's volume change potential, the water demand category of the species and its mature height, and on a bad combination that can mean deep trench fill or piles and ground beams.

Heave precautions come with it: compressible material against the face of deep foundations, slip membranes, and void formers beneath ground beams so that swelling soil lifts into a void rather than against the structure.

That design then has to coexist with the arboricultural constraint. The root protection area calculated under BS 5837:2012 limits where you can excavate, and a deep foundation close to a tree is exactly the conflict our note on retaining walls and level changes in a root protection area deals with. An Arboricultural Impact Assessment setting out root protection areas, the construction method and any no-dig solution should be prepared alongside the foundation design, not after it.

Where a planning condition requiring tree retention sits alongside a later wish to remove the tree, the two consents can pull in opposite directions, as our note on when a planning condition and a TPO consent conflict explains.

If consent is refused

There is an appeal to the Secretary of State against the refusal of consent for works to a protected tree, determined by the Planning Inspectorate and usually dealt with by written representations. It is a rehearing of the merits, so new evidence can be submitted, and the quality of the arboricultural case matters as much as the engineering, as our note on arboricultural evidence at appeal sets out.

Where the tree stands on someone else's land, there is also a civil route. The House of Lords in Delaware Mansions v Westminster City Council (2001) confirmed that a landowner can recover the cost of remedial works caused by encroaching tree roots in nuisance, subject to notice to the tree's owner. That is a legal claim rather than a planning one, and it needs a solicitor.

Start the monitoring now if you have not already. A year of data gathered while you wait is the difference between an application the council can grant and one it has no choice but to refuse.

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 on PlanScope to see which reports your project is likely to need.

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