Root protection area: the BS 5837 circle explained

The root protection area is the circle drawn around a retained tree that the design has to work around. It is defined in BS 5837:2012 as the area needed to contain enough rooting volume to keep the tree viable, and for a single-stem tree the radius is 12 times the stem diameter, capped at an area of 707 m².

That cap matters. A veteran oak with a 1.5 m stem would otherwise generate an 18 m radius, and the standard stops the calculation at the equivalent of a 15 m radius.

What the area is protecting

Not roots as such. The root protection area protects rooting volume, which is soil that roots can occupy and draw water and air from. Roots themselves extend well beyond it in most species, and a tree can survive losing some. What kills a tree over five to ten years is compacted, waterlogged or removed soil across a large enough share of that volume.

That is why the standard treats compaction from tracked plant, storage of materials and changes in ground level as damage on the same footing as cutting roots. A garden left as a site compound under a mature beech does more harm than a carefully hand-dug trench.

The area also sits alongside two other constraints that do not appear on the same drawing. Shade, which affects how habitable the rooms next to the tree will be. And future growth, because a tree assessed today will be wider in twenty years and the design should not put a wall where the crown is heading.

Deriving the radius from stem diameter

Stem diameter is measured at 1.5 m above ground level, and the calculation is straightforward for a single-stem tree: radius equals 12 times that diameter.

Multi-stem trees use a combined stem diameter derived from the individual stems rather than the largest one, and the standard sets out separate methods depending on whether the stems divide above or below 1.5 m. Get that wrong on a multi-stemmed field maple and the area can be out by half.

The figures are a starting point. An arboriculturist can adjust where soil conditions, past root severance or existing hard surfacing mean the actual rooting environment differs from the default assumption, and that adjustment has to be explained rather than asserted.

Reshaping the area without shrinking it

The circle is a default, not a description of where the roots are. Where root distribution is likely to be asymmetric, because there is a building on one side, a road, a watercourse or a change in soil, the shape can be modified.

The rule is that the area is retained. Push the boundary in on one side and it has to come out on another to hold the same square metres, and the resulting shape should still be contiguous with the stem and plausible as a rooting environment. A crescent squeezed into a strip 1 m wide and 40 m long satisfies the arithmetic and fails the purpose.

Councils challenge reshaping more than any other part of a tree report, so the justification needs evidence: a trial trench, a ground-penetrating survey, a record of existing surfacing, or a clear statement of the historic constraint that pushed the roots one way. Reshaping asserted without evidence tends to be refused, and the design has to change instead.

What can and cannot happen inside it

Inside the protected area, the default is that nothing happens. Construction exclusion is the point of it.

Where something must happen, the options in ascending order of risk are:

1. Hand digging under arboricultural supervision, with roots over a set diameter retained.
2. Ground protection to spread load, allowing access without excavation.
3. No-dig construction, using a cellular confinement system laid on a geotextile above existing ground level, which builds up rather than digging down.
4. Pile and beam foundations, spanning the area on isolated points positioned to miss significant roots.

What is not acceptable is a strip foundation trench, a service trench cut across the area, a level reduction, or storage and mixing on unprotected ground. Level changes are underrated as a risk. Raising ground by 300 mm over a root system suffocates it as effectively as removing 300 mm.

Services are the recurring problem on small sites. Drainage runs, incoming water and electricity all want to cross the shortest line, and the shortest line is usually under the tree. Route them outside the area or use directional drilling, and settle it at design stage rather than when the groundworker is on site.

Fencing, ground protection and the method statement

Protective fencing is the physical expression of the area on site. The standard specifies a fence to a defined construction rather than a line of plastic mesh, and it should be erected before any plant arrives and stay until work is finished.

Three documents carry this through planning. A tree survey records the trees, their category and their protection areas. A tree constraints plan overlays them on the site so the design can respond. An arboricultural method statement, with a tree protection plan, says exactly how the works near retained trees will be carried out, and it is usually secured by condition.

Councils expect a suitably qualified author for this work and professional review is strongly advised, particularly where a Tree Preservation Order or a conservation area is involved. The Arboricultural Impact Assessment page sets out what the report covers and who normally prepares it. Where planting is proposed to offset losses, that sits in the Landscape Strategy rather than the arboricultural report.

An illustrative example: a garden room beside an oak

Take a 5 m by 3 m garden room proposed in a rear garden with a mature oak of 700 mm stem diameter roughly 6 m away. This example is illustrative.

The protection radius is 8.4 m, so the whole footprint sits inside it. Conventional strip foundations are out. Three routes remain.

Move the building. Shifting it 3 m to the far corner takes most of the footprint outside the area and turns the problem into a construction access question rather than a foundation one.

Build on the surface. A cellular confinement base laid over geotextile on the existing ground, with a lightweight timber structure above, keeps excavation to nothing and is the usual answer on this scale.

Or span it. Screw piles positioned after hand-dug inspection, carrying a beam, can work where the ground has to stay at existing level, though it costs more than the building.

Whichever route, the drawing that persuades an officer is the one showing the protection area, the fencing line, the access route for materials and the position of every service. Enter your postcode on PlanScope to see which tree and landscape documents your site is likely to need, or browse the full report list first.

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.