Root protection area: how the BS 5837 figure is calculated

A root protection area is the minimum area of ground around a tree that has to stay undisturbed if the tree is to survive the building work next to it. BS 5837:2012 treats it as a layout design tool rather than a scientific map of every root: the area judged to hold enough roots and rooting volume to keep the tree viable. On a drawing it starts life as a circle with a radius of 12 times the stem diameter, and your layout has to work around it. Miss that at sketch stage and the arboricultural report comes back three weeks later saying the driveway cannot be built where you have drawn it.

The area matters because it is the thing a tree officer measures. Canopy spread is what everyone looks at, but roots rarely mirror the crown, so the protection area is calculated from the stem instead.

What the protection area is for

Two separate risks sit behind it. The obvious one is severing roots during excavation. The quieter one is compaction: a tracked excavator, a stack of blocks or a month of site traffic over a rooting zone closes the soil pore structure, and a tree can decline for five or six years afterwards before anyone connects the two events.

So the protection area is drawn to cover both. Inside it, the soil structure is treated as part of the tree.

It is also the line that conditions get written around. Where a scheme is approved with trees retained, the decision notice will usually tie the works to an approved tree protection plan and arboricultural method statement, which makes the protection area contractually enforceable rather than advisory. Breaching it is then a breach of condition, and on a protected tree it can be a separate offence.

Calculating the radius and the area

Stem diameter is measured at 1.5 m above ground level. On sloping ground it is taken on the upslope side, and where the tree flares or forks low the standard sets out adjusted measuring points.

For a single stem, the radius is the stem diameter multiplied by 12. The area is then capped at 707 m², which is reached at a stem diameter of 1,250 mm, so even a veteran oak does not generate an infinite circle.

Multi-stem trees use a combined diameter. For two to five stems, take the square root of the sum of the squares of each stem diameter. Above five stems, take the mean stem diameter and multiply it by the square root of the number of stems. Either way the result is then multiplied by 12 in the usual way.

Hedges and groups are handled as a group protection area rather than as a string of overlapping circles, which usually produces a wider and more defensible zone along a boundary.

Redrawing the circle without shrinking it

The circle is a starting assumption, and the standard expects it to be modified where there is arboricultural reason to think roots have gone somewhere else. A Victorian retaining wall on one side, a 1960s road construction, a watercourse, shallow soil over rock: all of these direct rooting, and a circle that ignores them is a worse description of reality than an offset shape.

Two rules keep this honest. The modified shape must have the same area as the circle it replaces. And it must remain plausible as a description of where roots actually are, which means pushing the area out in the direction the roots have had to grow rather than simply sliding it off the development footprint.

That second rule is where most disagreements with a tree officer start. An arboriculturist who offsets an area away from a proposed building without explaining the physical constraint that caused it has not modified the protection area, they have reduced it. Expect the officer to ask for the reasoning, and expect them to rely on their own standing advice from consultees where the council publishes it.

What cannot happen inside the zone

Once fenced, the protection area becomes a construction exclusion zone. The default list of prohibited activity is longer than people expect:

1. Excavation of any kind, including service trenches, soakaways and inspection chambers.
2. Changes in soil level, whether cut or fill, including raising a lawn with imported topsoil.
3. Storage of materials, spoil, skips, site huts, fuel or portable toilets.
4. Vehicle or plant movement, including a single reversing delivery lorry.
5. Mixing or washing out cement, and any discharge of alkaline water.
6. Fires and the siting of generators.

The fencing itself has a specification in BS 5837:2012, and the default is a 2.3 m high scaffold framework with weldmesh panels, braced and signed. A lighter barrier is possible where the arboriculturist justifies it. The detail of that choice is worth getting right first time, because it is the item most often picked up on a site inspection, and the tree protection barrier specification under BS 5837 is usually reproduced in full as a condition.

Where work really has to happen inside the zone, the route is an arboricultural method statement with named supervision and hold points, not an informal agreement on site.

No dig construction and ground protection

Hard surfacing over a rooting zone is possible, and it is built upwards rather than downwards. The principle is no excavation below existing grade beyond removing turf and soft landscaping, then a geotextile separation layer, then a three-dimensional cellular confinement system filled with clean angular no-fines stone, then a porous wearing surface. Load spreads across the cells instead of bearing on the soil, and water and air still reach the roots.

Temporary access uses the same logic with reusable components: a compressible layer with proprietary track panels or scaffold boards on a scaffold frame above it.

Neither solution is a free pass. Both change finished levels, which affects thresholds, drainage falls and the junction with an existing footway, so they belong in the design at the point the layout is fixed. A no dig driveway that arrives as an afterthought usually sits 150 mm above the garage floor.

An illustrative frontage driveway

Take a 1930s semi with a sycamore in the front garden, protected by a Tree Preservation Order, with a stem diameter of 450 mm. This example is illustrative. The radius is 5.4 m and the area 92 m², and the front garden is only 7 m deep, so the whole of the proposed driveway and the new crossover sit inside the protection area.

The workable answer is a no dig build-up across the full driveway, a protection area that is offset towards the open garden rather than reduced, fencing to the back edge of the works during construction, and the spoil from the dropped kerb handled off site. Works to the tree itself, even light crown lifting for headroom, need separate consent from the council because of the Order. The vehicle crossing is a separate application to the highway authority and can be refused for reasons that have nothing to do with the tree.

What makes that package persuasive is the sequencing: the arboricultural information arrives with the application rather than after a holding objection.

How the area is shown on drawings

A tree officer is looking for three related drawings. A tree survey plan records each tree with its reference number, stem diameter, canopy spread and BS 5837 category. A tree constraints plan overlays the protection areas and shadow patterns onto the existing site. A tree protection plan then shows the approved layout with fencing lines, ground protection extents, access routes, service runs and any supervised incursions.

Dimension the protection area on the plan. An undimensioned circle invites the officer to scale it off and reach a different answer. Show cross-sections wherever levels change inside or beside the zone, and mark the categories using the colour convention in the standard so the retained trees read clearly against the ones proposed for removal.

All of that sits inside an Arboricultural Impact Assessment, which most councils list as a local validation requirement where trees are on or adjoining the site. Councils expect a suitably qualified author, and professional review is strongly advised before submission. The companion detail on tree protection plans and the fencing specification covers how those drawings are usually assembled.

Enter your postcode on PlanScope to see which reports a site with retained trees 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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.