Tree categorisation A to U in a BS 5837 survey

If a tree survey has landed on your desk with a column of letters against each tree, this is what those letters mean. Under BS 5837:2012, a qualified arboriculturist grades every tree on or adjacent to the site as U, A, B or C. Category U trees cannot reasonably be kept for more than about ten years and carry no retention value. Category A trees are the high quality ones your layout should be built around. B and C sit between the two. The BS 5837 tree categories drive the tree constraints plan, and the constraints plan is what your architect should be designing to before a single window position is fixed.

That matters for a one to nine unit scheme because the grading is the argument. A refusal on tree loss usually turns on removing something a case officer thought was worth keeping.

What A, B, C and U actually mean

The cascade in BS 5837:2012 works in a fixed order. The surveyor first asks whether the tree can be retained at all. If it cannot, because of irreversible decline, serious structural defect or disease, it is category U and drops out of the assessment. Everything that survives that first question is then graded on quality and on estimated remaining contribution, expressed in years.

The years are not a life expectancy. They are a judgement about how long the tree will keep making a worthwhile contribution in its current setting, which is why the same tree can be graded differently on a woodland edge and in a suburban back garden.

One trap for self-builders: a category U grading is not a licence to get the chainsaw out. A hollowing over-mature ash with woodpecker holes and lifted bark is exactly the sort of tree that supports roosting bats, and disturbing a roost is an offence under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017. If the arboriculturist flags potential roost features, you need a preliminary roost assessment and, if warranted, a bat survey by a licensed ecologist before removal.

The subcategories that explain why a tree matters

A, B and C each split into three subcategories, so you will see A1, B2, C3 and so on. Subcategory 1 covers mainly arboricultural qualities: form, vigour, the tree as a specimen. Subcategory 2 covers landscape qualities, including screening, visual amenity and contribution to a group or an avenue. Subcategory 3 covers cultural values, including conservation, historic, commemorative or other significance.

So a straggly hawthorn in a hedge line might be C1 as an individual and still be part of a B2 group if the hedge screens the site from a lane. A veteran oak on a former parkland boundary can be A3 for its historic association even where its crown is patchy. U trees have no subcategory, because the assessment stops at retention.

The subcategory is where you argue. If a tree is graded B2 purely for screening, a replacement planting scheme that restores the screen within a few seasons is a serious response. If it is A1 or A3, you are usually redesigning rather than negotiating.

Reading the survey schedule column by column

The schedule is the appendix everyone skips. It is also the document a case officer or the council's tree officer will read most closely, so check it before it goes anywhere near the portal. Expect these columns:

1. Tree reference number, matching the tag on site and the plan.
2. Species, in common and Latin form.
3. Height in metres and stem diameter in millimetres, measured at 1.5 m above ground level.
4. Branch spread, usually given for four compass points, and crown clearance above ground.
5. Age class: young, middle aged, mature, over-mature or veteran.
6. Physiological condition and structural condition, each with observed defects.
7. Preliminary management recommendations, independent of any development.
8. Estimated remaining contribution in years, banded as under 10, 10 plus, 20 plus or 40 plus.
9. Category and subcategory.
10. Root protection area, in square metres, with the radius from the stem.

The root protection area is where the schedule turns into a design constraint. For a single stem tree the default RPA is a circle with a radius of 12 times the stem diameter, capped at 707 m², adjusted in shape where soils, structures or existing surfacing justify it. Two mature limes can therefore write off a strip of your plot before you have drawn anything, because that is the area within which the arboriculturist will resist excavation, level changes, storage and vehicle movement.

Also look for the survey date and the surveyor's qualifications. Councils generally expect a suitably qualified author, and many will treat a schedule more than a year or two old as stale. Check with your local planning authority rather than assuming.

How category feeds the design and the case officer's view

The sequence that works is survey, then constraints plan, then layout, then impact assessment. Done in that order, the Arboricultural Impact Assessment reports on a design that already respects the RPAs of the A and B trees, and the officer has very little to push back on.

Done in reverse, you get the familiar problem: a plot layout that puts a driveway across an A2 oak's rooting zone, followed by an impact assessment full of caveats and a tree officer consultation response recommending refusal. The NPPF (December 2024) is clear in its design chapter that existing trees should be retained wherever possible and that development should contribute to tree cover; most adopted local plans carry a policy on trees and landscaping that a tree officer will cite directly.

In practice, loss of a C tree rarely troubles anyone if replacement planting is proposed. Loss of a B tree needs a reason and mitigation. Loss of an A tree on a small residential site is the sort of thing that gets a scheme called in or refused, and it is worth testing at pre-application stage before you spend money on drawings.

Illustrative example: two category B limes and a U ash

This is an illustrative worked example, not a real site.

A 0.09 hectare backland plot for two dwellings has three notable trees. T1 and T2 are common limes on the northern boundary, each roughly 14 m tall with stem diameters around 550 mm, good vigour, some historic topping wounds. The surveyor grades both B2: moderate quality, at least 20 years remaining contribution, valued mainly for the screening they give to the neighbouring garden. Their RPA radius is 12 times 550 mm, so 6.6 m from each stem.

T3 is a 9 m ash with a thinning crown, dieback in the upper third and a basal cavity. It is graded U, with a preliminary management recommendation to fell on safety grounds regardless of the development.

The design consequence is straightforward. The two plots shift 1.5 m south, the parking area moves onto a no-dig cellular surface outside both lime RPAs, and the ash comes out, subject to a bat check because of the cavity. The impact assessment then records no loss of retained trees, incursion into RPA limited to a no-dig construction, and two replacement trees. That is a much shorter consultation response than the alternative.

Ash dieback and why categories move between surveys

Ash dieback is the main reason a tree drops a grade, or falls to U, between one survey and the next. A tree recorded as B1 three years ago with a full crown can present with significant crown loss and secondary honey fungus at the base today. Neither the earlier surveyor nor the later one was wrong.

The practical point for a scheme that has stalled: if your consent is a year or two old and you are about to start on site, get the schedule refreshed before you rely on it. If a tree has moved to U in the meantime, you need that recorded by the arboriculturist rather than assumed by a groundworker, particularly where a planning condition ties you to a specific tree protection plan.

TPOs, conservation areas and the extra layer of consent

Category is a technical grading. Legal protection is separate, and the two do not line up. A category C sycamore can be covered by a Tree Preservation Order made under the Town and Country Planning (Tree Preservation) (England) Regulations 2012, and a category U ash in a conservation area still needs notice before works.

Where a TPO applies, works need the council's consent unless an exemption applies, and dead or dangerous tree exemptions have notice requirements of their own. In a conservation area, section 211 of the Town and Country Planning Act 1990 requires six weeks' written notice to the council for works to trees above 75 mm stem diameter measured at 1.5 m, giving the authority the chance to make a TPO. Where trees are removed as part of an approved planning permission, the position differs again, so ask the tree officer in writing rather than relying on a rule of thumb.

Check the constraints early. Councils publish TPO and conservation area layers on their own mapping, and national datasets are being consolidated on planning.data.gov.uk. If either applies, say so in the covering documents and in your survey brief, because a tree officer who finds an unmentioned TPO tree on your layout will read the rest of the submission with more suspicion.

When you are assembling the pack, the tree evidence usually sits alongside ecology, drainage and design documents; the full list of report types shows which of those a minor residential scheme typically triggers. Enter your postcode on PlanScope to see which reports your site is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Tree surveys and impact assessments are expected to be prepared by a suitably qualified arboriculturist working to BS 5837:2012, and a PlanScope draft is a starting point for that person rather than a substitute. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.