Root protection area: how the zone is calculated

A root protection area is the zone of soil around a tree that has to stay undisturbed if the tree is expected to survive the build. BS 5837:2012 sets the root protection area definition used by every local planning authority in England: a circle whose radius is 12 times the stem diameter, measured at 1.5 m above ground level, with the resulting area capped at 707 m². A 400 mm stem therefore carries a radius of 4.8 m. Draw it wrong on the layout and the tree officer's objection writes itself, usually two days before the eight week determination date.

What the zone is meant to protect

The area is a proxy, not a survey of where roots actually are. Roots rarely form a neat circle. They follow water, air and loose soil, so they run under old lawns and away from compacted hardstanding, and on a tight urban plot they may be entirely on one side of the stem.

BS 5837 accepts that imprecision and asks for a defensible minimum instead. The figure is meant to hold enough undisturbed rooting volume, and enough of the soil structure that supports it, for the tree to stay stable and physiologically viable through construction and for years afterwards. Compaction is the usual killer. A tracked excavator crossing wet ground once can close the soil pores that the fine roots depend on, and nothing visible happens above ground until the third summer.

That delay matters commercially. A tree that dies two years after practical completion is still a breach of the condition that required its retention.

The 12 times stem diameter calculation

Measure the stem diameter at 1.5 m above ground level, multiply by 12, and that is the radius in the same units. The whole calculation belongs in the schedule of the tree survey that accompanies the application, alongside the BS 5837 category (A, B, C or U) for each tree.

The cap is the part people miss. Past a stem of roughly 1.25 m the radius stops growing, because the standard accepts that a mature tree's rooting extent becomes hard to justify as a simple multiple. So a veteran-sized oak on an ordinary planning application gets 707 m² and no more, unless the tree qualifies for the wider buffer described further down.

Multi-stem trees and awkward measurements

Multi-stem trees need a combined diameter before the multiplier is applied. For a tree with up to five stems, take the square root of the sum of the squared diameters. For six or more, take the mean diameter, square it, multiply by the number of stems and take the square root of that.

Two field situations catch people out. Where a tree forks below 1.5 m, measure immediately below the swelling caused by the fork rather than at the standard height, because a reading taken across the fork inflates the diameter and produces a zone no officer will believe. Where the ground slopes, the measurement point is taken relative to the natural ground level at the stem, which on a bank means measuring from the upslope side.

And if the tree sits just off site, in a neighbour's garden or on highway land, the calculation is identical. The area does not stop at the red line.

Reshaping the zone without shrinking it

BS 5837 allows the shape to change where there is evidence for it. An existing basement wall, a road, a river or a band of made ground will have already stopped roots going one way, and the area can be redistributed to the side where rooting is plausible. What the standard does not allow is a smaller area. Offset it, stretch it, make it kidney-shaped, keep it contiguous with the stem, and keep the square metres.

Incursion is a different question from reshaping. The standard treats an incursion of less than 20 per cent of the area as potentially acceptable where an arboriculturist demonstrates the tree will stay viable and the lost area is compensated for in adjacent soil. That judgement is the arboriculturist's, made against the species, age, condition and soil, and it should appear in the arboricultural impact assessment with reasoning rather than as an assertion.

Anything larger is a design change, and it costs less to make it at sketch stage than at committee.

What the zone does to a layout

Treat the root protection areas as fixed obstacles on the constraints drawing, at the same moment as the flood zone and the sewer easement. A few consequences follow almost every time:

• Foundations inside the area need a pile and beam or screw pile solution, designed with the arboriculturist, not a strip trench.
• New hard surfaces inside the area have to be no-dig, built up on a cellular confinement system above existing ground level, with the finished level change designed to work at thresholds.
• Service runs cannot be trenched through the area, so the drainage and utility strategy has to route around it or use a single supervised entry under the guidance for trenchless techniques.
• Level changes are the quiet risk. Adding 300 mm of made ground over roots suffocates them as effectively as digging them out.

Illustrative example: a plot with a category B lime of 550 mm diameter on the northern boundary carries a 6.6 m radius, roughly 137 m². On a 9 m wide plot that removes most of the northern third from conventional construction. Moving the footprint 2 m south costs a designer an afternoon. Redesigning after a refusal costs a client a determination cycle, and where the scheme is already registered an extension of time agreement is usually the way to avoid a refusal on the officer's statistics.

Veteran trees and the wider buffer

Ancient and veteran trees sit outside the ordinary arithmetic. Government standing advice for ancient woodland, ancient trees and veteran trees asks for a root protection area of 15 times the stem diameter, and then for a buffer zone measured at least 15 m from the edge of the canopy, whichever produces the larger area. On a wide-crowned field oak that buffer can be several times the BS 5837 circle.

The policy weight differs too. Loss of, or deterioration to, ancient or veteran trees is treated as irreplaceable habitat in the National Planning Policy Framework (edition published 17 August 2026, so check the current text and policy code before citing it), and the bar for permitting harm is set deliberately high.

Old trees also tend to carry features that matter to other constraints. Loose bark, rot holes and split limbs are bat roost potential, which is why a bat survey and the arboricultural work should be commissioned together rather than in sequence. Bat emergence surveys run from May to September, so a late instruction costs a season.

Showing the zone on drawings

Officers look for three drawings and lose patience quickly when they are merged into one. The tree constraints plan shows every retained tree with its root protection area and canopy spread over the survey base. The tree protection plan shows the same areas with fencing lines, ground protection and the construction exclusion zone marked. The arboricultural method statement then describes sequencing, supervision and who signs off each stage.

Label each area with the tree number and the radius so the figure can be checked against the schedule without a scale rule. Where an area has been reshaped, say so on the drawing and give the reason in a note. Where an incursion is proposed, hatch it and cross-refer to the paragraph of the impact assessment that justifies it. A Design and Access Statement that explains why the layout sits where it does, in tree terms, saves a round of questions.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified arboriculturist before relying on it. A BS 5837 survey has to be carried out by a suitably qualified arboriculturist, and councils will not accept a calculation made from a photograph.

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