Root protection area: how the BS 5837 radius is set

The root protection area calculation in BS 5837:2012 is arithmetic you can do on the back of a drawing: multiply the stem diameter by 12 to get a radius in metres, swing that circle from the centre of the stem, and cap the resulting area at 707 m². A stem of 450 mm measured at 1.5 m above ground gives a radius of 5.4 m and an area of about 92 m². That figure is the minimum area of rooting soil the standard expects to be left undisturbed, and it is the number a tree officer checks first when your drawings arrive.

Most arguments about trees on small sites are really arguments about this circle. Get the measurement and the geometry right at survey stage and the rest of the tree survey follows. Get it wrong and the scheme either loses a wall or gains a condition nobody can comply with.

What the area represents

The root protection area is a notional area, not a map of actual roots. Roots rarely sit in a tidy disc. They follow water, air and the easiest route through the soil, which on a terraced site often means straight under the neighbouring garden rather than into compacted made ground. BS 5837 accepts that imprecision and sets a default area that, in normal conditions, holds enough undisturbed soil to keep the tree alive and stable after construction.

Two things follow from that. The area protects soil volume and soil structure, so compaction by a tracked excavator damages it as surely as a trench does. And because the figure is a minimum rather than a target, an arboriculturist can justify a larger area for a veteran tree, a tree on thin soil over rock, or a tree already stressed by previous works.

The standard pairs the area with a root protection fence. Default protective fencing in BS 5837 is 2.3 m high weldmesh panels on a scaffold framework, set on the line of the area and treated as a construction exclusion zone for the duration of the works.

Working the radius out from stem diameter

Measure stem diameter at 1.5 m above ground level, not at the base. For a single-stem tree, radius in metres equals stem diameter in metres multiplied by 12. A 300 mm stem gives 3.6 m. A 650 mm stem gives 7.8 m, an area of roughly 191 m².

Multi-stem trees need a combining step before the multiplication, and this is where surveys go wrong. For a tree with two to five stems, BS 5837 takes the square root of the sum of the squares of each stem diameter. Above five stems, it uses the square root of the mean stem diameter squared multiplied by the number of stems. A three-stem birch at 200 mm, 180 mm and 150 mm combines to about 310 mm, so the radius is 3.7 m, not the 2.4 m you would get by averaging.

Trees on sloping ground are measured perpendicular to the lean rather than vertically. Coppice stools, hedgerow trees growing as a line and trees with buttressed bases all need a judgement call recorded in the survey schedule, because a case officer cannot audit a number whose basis is not stated.

Illustrative example. A 1930s semi has a mature oak in the rear garden with a stem diameter of 820 mm at 1.5 m. The radius is 9.84 m, so the area is about 304 m². The proposed single-storey rear extension sits 7 m from the stem. The scheme survives, but it needs a method statement rather than an optimistic drawing.

The 707 m2 cap and when it bites

The 707 m² cap corresponds to a radius of 15 m, and it applies to trees with a stem diameter above about 1.25 m. Beyond that point the formula keeps growing while the standard stops. A 1,400 mm stem would give a radius of 16.8 m and an area of 887 m², so the area is reduced to 707 m² and the radius drawn at 15 m.

The cap is a practical limit on the circle, not a statement that a very large tree needs less soil. Arboriculturists working on ancient and veteran trees often set the area by canopy spread plus a buffer instead, and Natural England and Forestry Commission advice on ancient woodland and veteran trees uses buffer distances rather than the BS 5837 formula. Say which method you used and why.

Reshaping the circle without shrinking it

The circle can be redrawn. BS 5837 allows the shape to be modified where soil conditions, topography, existing structures or known root distribution mean the roots are unlikely to be evenly spread, provided the total area stays the same and the substituted land is capable of supporting roots.

That last clause is the one people skip. Offsetting a circle onto an existing concrete yard, a highway or a neighbour's building adds area on paper and nothing underground. An offset is defensible where the tree stands next to a Victorian basement wall and the only available rooting soil lies the other way. It is not defensible as a drafting trick to clear a building line.

Where part of the area has to be taken, the standard treats encroachment below 20 per cent of the area differently from larger losses: compensatory area should be provided next to the retained area so the total soil volume holds up. Above that, the case moves from mitigation to a question of whether the tree can be retained at all, which is a different conversation with the council and a different set of refusal risks.

Building inside the area

Work inside the area is not automatically refused. It has to be specified, sequenced and supervised. The usual package is an arboricultural impact assessment, a tree protection plan and an arboricultural method statement, with the method statement tied into the permission by condition.

A workable method statement names the plant, the access route, the sequence and the supervision points. It specifies hand digging or air spade excavation near retained roots, states the maximum root diameter that may be cut and who authorises cutting beyond it, and sets out what happens if a root larger than that is found. Conditions requiring approval of details before any work starts on site are common, so build the discharge timetable into the programme rather than treating it as paperwork for later.

Service runs are the quiet killer. A single drainage trench across the area can sever more root than the foundations do, and drainage layouts are often fixed after permission by a different consultant who has never seen the tree plan.

Ground protection and no-dig construction

Where access or hard surfacing has to cross the area, protection replaces exclusion. Load spreading over existing ground levels is the principle: a geotextile separation layer, a granular or cellular confinement layer sized for the wheel loads expected, and a surface that keeps plant off the soil itself. Three-dimensional cellular systems laid on geotextile over the existing level are the standard answer for a drive within a root protection area.

The detail that matters is the level. No-dig means no dig. If the finished surface has to meet an existing threshold, somebody will propose a shallow excavation to make the levels work, and the design will quietly stop being no-dig. Resolve the levels at design stage with the topographical survey in front of you.

Permeability and gas exchange matter as much as load. A sealed surface over the area starves the soil, so permeable blocks or porous asphalt with an open-graded sub-base do the job that dense bitmac cannot.

Showing it on the drawings

Councils reject tree information for presentation as often as for substance. The area should appear on the same drawing as the proposal, at a stated scale, with the radius dimensioned and the tree reference numbers matching the survey schedule. Show the fence line, the ground protection extents and any offset shape with its area stated in m², so the officer can check the arithmetic without redrawing anything.

Validation lists differ by authority, and some ask for the tree protection plan as a separate drawing rather than a layer on the site plan. Check the local list before you submit, because an invalid application costs more time than the drawing did. If you are assembling the wider document set, the full list of report types shows which of them sit alongside the arboricultural package.

A de minimis argument about a small incursion will sometimes carry, and the thinking behind de minimis in planning judgments is worth understanding before you rely on it. But an unexplained circle, drawn at the wrong radius, with no method statement behind it, loses that argument before it starts.

Enter your postcode on PlanScope to see which reports a site with retained trees is likely to need, then send the arboricultural work to a qualified arboriculturist for sign-off.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.