Tree risk assessment and target zones on a development site
A layout that puts a play area under a mature tree has raised its risk without touching it. How target occupancy drives inspection intervals, and what to hand over at the end of the build.
Tree risk on a development site is managed rather than removed. An arboriculturist answers three questions: how likely failure is, how big the part that could fail is, and what sits underneath it when it comes down. That last element, the target, is the one development changes most, because a layout that puts a terrace and a play area under a mature beech has raised the risk without touching the tree.
How risk is framed rather than removed
The legal backdrop is duty of care rather than a tree-specific statute. An occupier owes a duty under the Occupiers' Liability Act 1957 to visitors, and a narrower duty to trespassers under the 1984 Act. Where the site is a workplace or is controlled by a business, section 3 of the Health and Safety at Work etc. Act 1974 also applies to people who are not employees.
None of that demands a zero-risk site. The National Tree Safety Group's guidance on common sense risk management of trees, published with the Forestry Commission, sets the tone the courts and the Health and Safety Executive have followed: trees give substantial benefit, the overall risk they pose is low, and a proportionate inspection regime discharges the duty.
For a developer this cuts both ways. A proportionate regime costs little to run. But the word proportionate means the effort has to be matched to the target, and the target is what your layout creates.
Target occupancy and why layout changes it
Occupancy is how often a person or a valuable object is present within falling distance. Quantified methods in UK arboricultural practice, including Quantified Tree Risk Assessment, combine occupancy with the probability of failure and the size of the part that could fall, and produce a risk of harm figure that can be compared against a tolerability threshold.
Occupancy is measured in hours, and it moves by orders of magnitude between land uses:
| Situation under the canopy | Typical occupancy |
|---|---|
| Arable field or unmanaged scrub | very low, a few hours a year |
| Rear garden of a house | moderate, concentrated in summer |
| Estate road or parking court | moderate but transient |
| Children's play space or seating area | high, sustained |
| Building footprint | continuous |
Drop a bench under a tree and you have changed its risk category. This is the single most useful thing a design team can take from an arboricultural report, and it is the reason the tree information should reach the layout designer while the layout can still change.
An illustrative example. A 1970s parkland oak sits mid-site on a 60-unit scheme. In the first layout it stands in a mown green with a footpath at the canopy edge, and the arboriculturist accepts a three-yearly inspection. In the second layout the green becomes a toddler play area with equipment under the crown. The tree has not changed. The inspection interval drops to annual, a partial crown reduction appears in the management plan, and the play equipment has to move 4 m to clear the likely failure zone.
Likelihood of failure against the consequence
Two separate judgements sit behind a risk rating, and clients regularly conflate them.
Probability of failure is a structural judgement about a specific part: a decayed union, an included bark fork, a cavity at the base, a lean with recent root plate movement. It is informed by a visual inspection, and where something is found, by decay detection or an aerial inspection.
Consequence is about size and energy. A 300 mm diameter limb at 12 m is a different proposition from a 60 mm twig at the same height, and a whole-tree failure is different again. Two trees can share a probability of failure and sit in entirely different risk bands because of the part involved.
The practical consequence for a submission is that the arboricultural report should name the affected part as well as the tree. Recommendations that say "monitor" without saying what is being monitored are the ones that unravel at handover.
Documenting a reasonable inspection regime
What protects a developer, and later a management company, is a written regime that was followed. The record matters as much as the interval does.
A defensible regime has five elements: a zoning of the site by target occupancy, an inspection interval per zone, a stated competence level for the inspector, a template that records what was looked at and what was found, and a route for acting on findings with dates. Keep the records for the life of the trees, and hand them over.
Competence deserves a sentence of its own. A ground-level visual inspection by a trained person is sufficient for most trees in most zones, but a tree flagged for detailed assessment needs an arboriculturist with the relevant qualification and, where climbing is involved, the appropriate work-at-height arrangements. An Arboricultural Impact Assessment prepared to BS 5837:2012 gives you the condition and category data to build the zoning on, though BS 5837 is a design and construction standard rather than a risk methodology, and it will not do the risk assessment for you.
Risk the new development creates around an old tree
Development changes trees as well as targets. Three mechanisms recur.
Exposure changes when shelter is removed. A tree that grew inside a group and is retained as a single specimen has a crown shaped for shelter it no longer has, and the first winter after clearance is when the failures happen. Phased removal, or retaining a shelter belt, is cheaper than the alternative.
Root environment changes with level alterations, compaction and severance for services. A tree can look unchanged for five years and then decline, which is exactly the window in which a management company takes over. Our note on discharging a tree protection condition without delay covers the mechanics of getting the protective measures signed off.
Water regime changes with new hard surface and new drainage. Trees on shrinkable clay that lose their moisture deficit pattern can respond with crown dieback, and a soakaway placed inside a root protection area can do it quickly.
Managing risk during construction
During the build, the target is your own workforce, and that brings the 1974 Act squarely into play. The site arboricultural method statement should include a pre-commencement inspection of all retained trees within falling distance of working areas, welfare units, material stores and site accommodation.
Two rules save most of the trouble. Do not site a welfare cabin under a retained crown, however convenient the shade is. And re-inspect after any storm with gusts above the local threshold your principal contractor sets, before people go back under the canopy.
Damage found during construction needs recording at the time, with photographs, because a claim about who damaged what is unwinnable a year later.
Handover to the future occupier
The point at which a developer's risk becomes someone else's is the point most often mishandled. A management company that inherits trees with no record starts from scratch, and will usually over-prune to feel safe.
Hand over a tree pack: the BS 5837 survey, the retained tree plan with identification tags, the occupancy zoning, the inspection regime with the next due date, the record of all works carried out during construction, and a note of which trees are protected by a Tree Preservation Order or sit in a conservation area, because works to those trees need consent or notification under the Town and Country Planning (Tree Preservation) (England) Regulations 2012.
Where the trees are part of a wider planting scheme secured by condition, the Landscape Strategy should cross-reference the same document set, and the crown work specification matters more than most people expect, as our piece on specifying crown work so a contractor prices it correctly sets out.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Councils usually expect an arboricultural author with a relevant qualification, so professional review of any tree document is strongly advised, and quantified tree risk assessment is specialist work in its own right.
Enter your postcode on PlanScope to see which tree and landscape reports your site is likely to need.
Frequently asked questions
Is there a legal duty to inspect trees on a development site?
There is no tree-specific statute, but the Occupiers' Liability Acts 1957 and 1984 impose duties to visitors and trespassers, and section 3 of the Health and Safety at Work etc. Act 1974 applies where a business controls the premises. The National Tree Safety Group guidance sets the proportionate standard that regulators and courts have followed.
What is target occupancy in tree risk assessment?
Occupancy is how many hours a person or valuable object is present within falling distance of a tree, expressed as a rate. It is one of three inputs in quantified methods, alongside the probability of failure and the size of the part that could fall. Changing land use under a canopy changes occupancy by orders of magnitude without changing the tree.
Does a BS 5837 survey include a risk assessment?
No. BS 5837:2012 is a design, demolition and construction standard that categorises trees by quality and sets root protection areas, so it gives you the condition data a risk assessment builds on. The risk assessment itself is a separate exercise using a recognised method, and it needs restating whenever the layout changes what sits under the crown.
How often should retained trees be inspected after occupation?
The interval follows the target rather than the tree. Trees over sustained high-occupancy areas such as play space or seating are commonly inspected annually, while trees over low-occupancy ground can go several years between inspections. Set the intervals by zone, state the inspector's competence level, and record each inspection on a template that survives a change of managing agent.
What should be handed over to a management company?
A tree pack containing the BS 5837 survey, the retained tree plan with tag numbers, the occupancy zoning, the inspection regime with the next due date, records of all works done during construction, and identification of any trees covered by a Tree Preservation Order or standing in a conservation area, since those need consent or notification.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.