Tree risk assessment methods and planning decisions

A tree risk assessment answers a different question from the one a condition survey answers. Amenity surveys ask how much the tree contributes and how long it is likely to contribute for. Tree risk assessment in planning asks how likely a failure is, what it would hit, and whether the harm is enough to justify removal against the protection the tree carries. Councils turn down risk cases every week, and the reason is almost always the same: the applicant described defects without ever quantifying the risk.

If you are writing the report that supports an application to fell a protected tree, the structure of the argument matters more than the length of it.

Why risk sits apart from amenity

A Tree Preservation Order is made on amenity grounds, and an application to fell is decided by weighing the loss of amenity against the reasons put forward. Safety is one of those reasons, but it has to be evidenced on its own terms rather than asserted.

There is also a duty running underneath. Occupiers owe a duty of care to lawful visitors under the Occupiers' Liability Act 1957 and a narrower duty to others, and the courts have consistently treated that as a duty to take reasonable care rather than a duty to eliminate risk. Reasonable care means a system of inspection proportionate to the setting, not felling anything with a cavity.

The exemptions matter too. The Town and Country Planning (Tree Preservation) (England) Regulations 2012 allow works to remove an immediate risk of serious harm without consent, and allow dead trees to be removed on notice. Both are narrow. Relying on the urgency exemption for a tree that has been declining visibly for three years is how enforcement cases start.

The structured methods in common use

Four approaches account for most reports submitted to English authorities. They differ in how they express the answer, not in what they consider.

All of them multiply three things: the likelihood that the part fails, the likelihood that a target is present when it does, and the severity of the outcome given the size of the part. Say which method you used, say why, and report the inputs. A reviewer cannot test a conclusion whose inputs are hidden.

Whatever the method, the report should record the inspection date, the weather and ground conditions, whether the inspection was from ground level only, and what could not be seen. An honest limitation section carries more weight with a tree officer than a confident one.

Target occupancy changes the answer more than the defect does

This is the part most reports underplay. The same partially decayed limb over a rarely used footpath and over a primary school entrance produces two entirely different risk figures, and the difference is occupancy rather than biology.

Record occupancy properly. How many people or vehicles pass through the target area, for how long, at what times of year. A hedge-line tree over a field used for winter grazing is not the same as one over a car park.

And occupancy can be changed. Moving a bench, rerouting a desire line, fencing a zone under a crown or closing a path seasonally are all legitimate risk controls, and an authority is entitled to ask why they were not considered before felling was proposed.

Defects that matter and defects that do not

Cavities are the classic false positive. A hollow stem with a sound residual wall can be mechanically adequate, and a large basal cavity in a low-target setting is often a habitat feature rather than a hazard.

What deserves attention is different: significant root plate movement, recent lean with soil heaving, included bark unions on large co-dominant stems with cracking, active decay fungi associated with root or butt rot, hanging or detached limbs, and cracks that extend through the cross-section. Fungal identification matters because the decay strategy differs by species, and a bracket at the base of a mature beech is not the same news as one on a poplar.

Where decay is suspected and the target is high, non-invasive testing before a felling recommendation is proportionate. Decay detection and static pulling tests cost a fraction of the value of a mature tree.

Subsidence claims are a separate and frequently confused issue, and the evidence needed there is different again, as trees and subsidence claims on neighbouring property sets out.

Presenting a risk case in an application

Authorities expect a sequence, and applications that follow it are decided faster.

1. Identify the tree against the order or the conservation area boundary, with a plan.
2. State the method, the inspection level and the limitations.
3. Describe the defects, with photographs and any test results.
4. Quantify the target occupancy with actual observations.
5. Set out the options considered, from monitoring through to reduction and finally removal.
6. Justify why the chosen option is the least intervention that manages the risk.
7. Offer replacement planting where removal is proposed.

The options step is where most reports fail. A report that jumps from defect to felling without showing that crown reduction, bracing, target management or a shortened inspection interval were considered reads as a pre-decided outcome. Where reduction is proposed, the specification has to be written properly rather than left as a percentage, and pruning specifications and writing them properly covers what a tree officer expects to see.

Where an authority refuses a risk case

The recurring grounds are worth listing because they are so consistent. No structured method at all. Occupancy assumed rather than observed. Decay reported from a visual inspection with no attempt at quantification where the consequence would be severe. No consideration of remedial works. A report written by the contractor who would carry out the felling, with no separation between assessment and commercial interest.

That last one is worth taking seriously as a reviewer. Independence is not a formality here. Where the same firm assesses and quotes, expect the authority to give the report less weight, and expect an objector to say so.

One more failure mode. Risk reports produced to justify clearing a development site, dated conveniently close to the application, receive close attention. If the tree really is dangerous, the report should read the same whether or not there is a scheme, and the arboricultural impact assessment should reach the same conclusion independently.

Monitoring as the usual outcome

In most cases the proportionate answer is not removal. It is a defined inspection interval, a written trigger for reassessment, and a small amount of target management. Set the interval against the defect and the target, record who holds the duty, and diarise it.

Where a tree is being kept through construction, the risk assessment and the protection strategy have to be written together, because the works themselves change failure likelihood. That interaction is covered in retaining a tree through a construction programme.

Enter your postcode on PlanScope to see which tree and landscape documents your site is likely to need.

This is general guidance, not legal advice. Tree risk assessment should be carried out by a suitably qualified arboriculturist, and councils usually expect a suitably qualified author. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.