A tree risk assessment is a structured inspection that grades how likely a tree is to fail and how serious the consequences would be, so that a landowner can act proportionately and evidence their duty of care. It is a separate exercise from a planning tree survey — it exists to protect people and property from trees, not to get an application validated.
When you need a tree risk assessment
Unlike a tree survey for planning, a risk assessment is driven by duty of care rather than a validation list. Anyone who owns or manages land with trees owes a duty — under common-law negligence and the Occupiers' Liability Acts 1957 and 1984 — to take reasonable care that those trees do not injure people or damage property. A documented, proportionate inspection is the recognised way to show that duty has been met if a branch ever falls.
Typical triggers include:
- Trees overhanging a public highway, footpath, railway or right of way.
- Trees near buildings, car parks, play areas, schools or workplaces where people gather.
- Trees on a boundary where a neighbour has raised concern about safety or overhang — see our note on overhanging branches and TPOs.
- A portfolio duty — councils, estates, landlords, managing agents and highways authorities inspecting stock at intervals.
- After a storm, drought or visible defect — cracks, fungal brackets, deadwood, sudden lean or root heave.
- Buying, selling or insuring a property with significant trees — often prompting a tree survey for a mortgage, and closely linked to trees and subsidence where clay soils are involved.
The principle throughout is proportionality: the law does not expect a zero-risk landscape, only reasonable steps matched to the level of risk and the number of people exposed. A veteran oak in a locked rural field with no target beneath it may reasonably be left alone; the same tree over a school gate is a different question entirely.
What the assessment covers
An assessment is usually a Visual Tree Assessment (VTA) carried out from the ground, looking at each tree systematically for signs of ill health, structural weakness and defects that make failure more likely. The inspector records:
- Species, size and location, and what sits beneath the tree (the "target" — a road, a bench, a bedroom).
- Physiological condition — vigour, crown dieback, foliage density, disease.
- Structural condition — cavities, decay, cracks, included bark, weak or bark-included forks, lean, root-plate movement, and fungal fruiting bodies (a key indicator of internal decay).
- The likelihood of failure, the size of the part that could fail, and the value and occupancy of the target below it.
- Recommended action — from "no action" through crown reduction, deadwooding or bracing to felling — and a re-inspection interval.
Where the ground-level VTA flags something inconclusive, a more detailed Level 2/3 investigation may follow: a climbing (aerial) inspection, or decay detection such as a resistance-drilling probe or sonic tomograph that maps sound wood versus internal cavity. Good practice is to escalate only where the initial inspection justifies it — over-instrumenting every tree is neither proportionate nor cheap.
The three main methods: VTA, QTRA and VALID
UK practitioners generally work within one of three recognised frameworks. They are complementary, not competing — the choice depends on the site and the client's needs.
| Method | What it is | Best suited to |
|---|---|---|
| VTA (Visual Tree Assessment) | The underlying inspection technique: a systematic visual read of a tree's "body language" and defect symptoms. Most assessments are built on it. | Almost all inspections; the foundation the numeric methods sit on |
| QTRA (Quantified Tree Risk Assessment) | A proprietary, numeric method expressing risk as a probability of harm per year (e.g. 1/1,000,000), combining part size, probability of failure and target value. | Owners who want a defensible, consistent numeric threshold across many trees |
| VALID | A more recent risk-assessment system emphasising probability, transparency and calibrated judgement, with free and licensed tiers. | Practitioners wanting a modern, evidence-weighted, openly documented framework |
QTRA deliberately reframes the question from "is this tree safe?" (which no tree ever perfectly is) to "what is the probability of harm, and is it tolerable?" It sets the risk against thresholds an owner can adopt as policy — which is why it is popular with local authorities, utilities and large estates managing thousands of trees on a consistent basis.
VALID works on similar logic — quantifying likelihood and consequence — but stresses calibrated, transparent judgement, and its free tier has widened access to structured risk recording. Whichever framework is used, the deliverable should be the same in spirit: a clear grade per tree, a recommended action, and a re-inspection date.
Worked example: a car-park lime after a wet winter
A managing agent's inspector finds a mature lime overhanging a busy office car park with a bracket fungus at the base and slight crown thinning. Under QTRA the "target" scores high — cars and people beneath it much of the working day — so even a modest probability of a large-limb failure pushes the annual risk of harm above the tolerable threshold. A resistance drill confirms basal decay. The report recommends a crown reduction to cut both the sail area and the size of the part that could fail, bringing the risk back within tolerance, plus a 12-month re-inspection. Had the same tree stood over a rarely-used rural lane, the low target value would likely have justified monitoring rather than surgery — the identical defect, a different proportionate answer. That target-weighting is exactly what separates a real risk assessment from a blanket "cut it back to be safe".
What you get and what it costs
The output is a tree risk report — a schedule of trees with condition, risk rating, recommended works and inspection intervals, usually with a location plan and photographs. This is a different document from the arboricultural report produced for planning, though a consultant can supply both from one visit.
Costs vary with the number of trees, access and travel. Single-tree or small-garden inspections sit at the lower end of the market, while portfolio and estate surveys are priced per tree or per day and fall as the count rises. Because there is no fixed tariff, get an itemised quote — see our guide to tree survey costs and what affects the price for indicative ranges and the factors that move it. Be wary of a suspiciously cheap "drive-by" survey; our note on cheap tree surveys and hidden costs explains why a thin report is a poor evidence trail if a claim is ever made.
Risk assessment vs planning tree surveys — don't conflate them
This is the single most useful distinction to hold on to:
- A BS5837 tree survey protects trees from development. It grades trees A/B/C/U for retention, calculates each tree's root protection area, and feeds an arboricultural impact assessment submitted with a planning application.
- A tree risk assessment protects people and property from trees. It grades likelihood and consequence of failure and drives a management programme.
They answer different questions, use different grading systems, and are commissioned for different reasons. Our guide on tree survey vs arboricultural survey untangles the wider terminology if the labels blur.
Safety works and protected trees — status still bites
A crucial trap: a hazardous tree can still be legally protected. A Tree Preservation Order or conservation-area designation does not lapse because a tree has become a risk. The exemptions — a genuinely dead tree, or urgent works to remove an immediate danger — are narrow, and the dead-tree route requires five working days' prior notice to the council except in a true emergency. Cutting first and explaining later is how landowners with a real safety concern still end up prosecuted. Before any works to a tree that might be protected, confirm status with a TPO check and read pruning or felling a tree with a TPO. If the work is justified, document the danger with dated photographs before you cut.
Requirements and protections are set locally — check your council
The duty of care is national, but the statutory tree-protection regime that gates your safety works is administered locally. Whether the specific tree carries a TPO, whether it sits in a conservation area, how quickly the council's tree officer will make a fresh order on a tree someone wants gone, and what evidence the authority expects before it accepts an "urgent danger" claim all vary from one local planning authority to the next. Two identical hazardous trees either side of a borough boundary can face very different consent hurdles.
PlanWatch tracks live tree-related planning activity per council — new TPOs, conservation-area works notices and applications turning on trees — so you can see how protective a given authority is before you act. Compare local pages such as Leeds, Manchester, Lambeth and Bristol, or start from the tree-surveys hub and search your own postcode. This guidance is England-centric: the common-law duty of care and Occupiers' Liability principles apply across Great Britain, but the statutory tree-protection regimes differ in Scotland, Wales and Northern Ireland — always check local rules before acting.
Choosing an inspector
There is no statutory licence to inspect trees, so credentials matter. For risk and condition work, look for a Professional Tree Inspection (LANTRA) certificate, QTRA or VALID licensing, or membership of the Arboricultural Association or Institute of Chartered Foresters, alongside professional indemnity insurance. A well-evidenced report from a properly qualified inspector is what stands up if a claim is ever made — our guide on how to choose an arboricultural consultant covers the checks. Use our directory to find a tree surveyor who covers risk assessment as well as planning work.