Modifying a root protection area on a constrained plot

BS 5837:2012 does not require a root protection area to be drawn as a circle and left there. Section 4.6 sets the area by calculation, as the equivalent of a circle whose radius is twelve times the stem diameter for a single stem tree, capped at 707 m². Then it says the shape should be modified where site conditions mean roots are unlikely to be present in part of that circle, or are likely to be distributed asymmetrically, provided the same total area is retained. That distinction, reshaping rather than reducing, is what makes development possible on constrained urban plots without losing the tree.

Case officers and tree officers see far more applications that shrink the area than applications that offset it properly. So the burden is on you to show your working.

When BS 5837 allows reshaping

The standard anticipates three situations where the notional circle does not describe where the roots actually are.

Physical barriers. A basement wall, a deep foundation, a retaining structure, a service trench backfilled with granular material, a canal or a river channel. Roots do not grow through a 1930s brick retaining wall with a concrete footing, and pretending the circle extends beyond it is a fiction that helps nobody.

Soil and ground conditions. Made ground, compacted formation layers, shallow soil over rock, or a water table that limits rooting depth will push the root system laterally in a particular direction.

Existing surfaces and structures. A building that predates the tree occupies volume the roots were never in. A building that postdates the tree may have severed roots on that side already, which is a different argument and needs evidence rather than assumption.

The rule that governs all three is simple. The area does not shrink. If you remove 30 m² on the constrained side, you add 30 m² elsewhere, in a direction where rooting is plausible and where the ground will actually be protected.

The investigation evidence you need

An offset proposed on judgement alone rarely survives a tree officer. What carries it is site evidence.

Trial trenching by hand or by air excavation, carried out under arboricultural supervision, is the usual method. Dig on the constrained side, record what you find at what depth and diameter, photograph it, and plot the findings on the tree protection plan. Air excavation with compressed air is preferred where roots are expected, because it exposes them without severing them.

Ground penetrating radar has a place on large or high-value trees and is less useful on made ground and shallow services. Treat it as supporting evidence rather than the primary case.

Record three things for each trial pit or trench: its position on a dimensioned plan, the depth and diameter of any roots encountered, and the ground profile including any obstruction. A finding of no roots in a correctly located trench is evidence. A finding of no roots in a trench two metres from where the plan shows it is nothing.

Time the investigation sensibly. Trial excavation near a protected tree may itself need consent where a Tree Preservation Order applies, and works to a tree in a conservation area need a section 211 notice with six weeks' notice. Do not open a trench first and ask afterwards.

Existing structures and hard surfaces inside the area

The most common legitimate offset on an urban plot is around an existing building. Where the house has stood for decades within the notional circle, the volume beneath it holds no roots, and the standard supports excluding it.

Two cautions. First, a shallow raft or a strip footing does not exclude the ground beyond it, so exclude the building footprint rather than a generous margin around it. Second, existing hard surfaces are not automatically root-free. A permeable gravel drive over uncompacted subgrade may hold a substantial root system. A block-paved surface on a thick compacted sub-base usually does not. Investigate rather than assume, and say which you did.

Where the offset relies on a structure being replaced, the sequencing matters. Demolishing the old building will expose the ground, and roots that had been excluded by the structure may now be present at the perimeter. Set that out in the arboricultural method statement rather than discovering it on site.

Keeping the total area the same

The arithmetic should appear in the report as well as on the drawing. State the calculated area, the area removed by each modification, the area added, and the resulting total. A case officer who can check the sum in thirty seconds is a case officer who can recommend approval.

Where you add area, add it somewhere that will be protected in practice. An offset that pushes the area into a neighbour's garden you have no control over is worthless. An offset into an area that will become the construction compound is worse than worthless.

The added area should also be plausible in rooting terms: on the same side as the tree's likely resource, away from compacted ground, and within a distance the species can realistically occupy. A 12 metre offset in one direction on a semi-mature birch is not credible.

Presenting the justification to the case officer

Package the argument so it can be read in one pass:

1. The tree schedule with stem diameters, species, category under BS 5837 Table 1, and the calculated root protection area for each retained tree.
2. A tree constraints plan showing the unmodified areas.
3. The investigation record: trench positions, findings, photographs, dates and who supervised.
4. A tree protection plan showing the modified areas with the arithmetic annotated.
5. An arboricultural method statement covering the works within or adjacent to each modified area, including any no-dig construction, the protective fencing specification and the supervision regime.

The arboricultural impact assessment is where all of this belongs, and councils usually expect a suitably qualified author for it. Professional review is strongly advised before submission.

Where the scheme also involves new planting to offset canopy loss, run the landscape strategy alongside rather than afterwards, because the two documents contradict each other surprisingly often on what gets planted where.

An illustrative example

A mid-terrace plot in a suburb has a category B lime with a 600 mm stem diameter on the rear boundary, giving a calculated root protection area of about 407 m² from a 7.2 metre radius. The existing 1930s rear addition and its footings sit inside that circle, along with the party wall foundations either side. Hand-dug trial pits under supervision find no roots beneath the addition and a dense root plate on the garden side. The report excludes the footprint of the addition and the two party wall zones, totalling 38 m², and adds an equivalent 38 m² to the garden side and along the boundary, all of which falls inside land the applicant controls and will be fenced. The new extension sits on a piled and beamed foundation designed with the arboriculturist. This example is illustrative and the figures are calculated for the purpose of showing the method.

Conditions that follow a modified area

Expect the permission to bind you to the document. A condition requiring development in accordance with the approved arboricultural method statement and tree protection plan is standard, and where areas have been modified the condition usually adds pre-commencement fencing to be erected and inspected before any material operation, arboricultural supervision at named stages with written records submitted to the authority, and a restriction on storage, mixing, level changes and service routing within the protected area.

Read the supervision condition before you accept it. A requirement for written confirmation to the authority at each stage means the arboriculturist stays engaged throughout the build, which needs to be in the fee and the programme. Applicants who treat the assessment as a one-off document find out the hard way, usually when a discharge of conditions application is refused because the records were never kept.

Damage to a protected tree is a criminal matter, and the penalties for damaging a protected tree are serious enough to be worth reading before a contractor starts.

When the tree cannot be retained

Sometimes the honest conclusion is that no reshaping works. The tree is category U and in decline, or the area cannot be modified without pushing it into land outside the applicant's control, or the only viable building position sits over the root plate.

Say so early, and say it in the assessment rather than leaving the officer to infer it. A well-argued removal with replacement planting, a clear statement of the amenity value lost and a compensatory scheme is a stronger application than a strained retention that everyone can see will fail during construction. Where the tree is protected, removal needs consent under the Tree Preservation Order or a section 211 notice in a conservation area, and the council can refuse.

Clearing trees before applying, in the hope of removing the constraint, backfires reliably. Site clearance before a planning application tends to produce enforcement action and a refusal rather than a clean site.

This is general guidance, not legal advice. BS 5837:2012 should be read in full and applied by a suitably qualified arboriculturist, and you should check the position with your local planning authority before relying on anything here. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.