Modifying a root protection area for an asymmetric crown
Crown shape reflects light and pruning, not rooting. What actually justifies an offset root protection area, when to commission the investigation, and what reviewers reject.
An asymmetric crown is weak evidence for an asymmetric root system, and that is the first thing to settle before asking an arboriculturist to modify a root protection area. Crowns go lop-sided because of light competition, prevailing wind, past pruning and highway clearance. Roots respond to soil, moisture and physical obstruction, which are different variables. A modification to the default circle under BS 5837:2012 has to be argued from the ground, not from the canopy.
That does not make a modification wrong. On constrained urban sites it is frequently the only way a scheme works, and the standard anticipates it.
Why the default circle is a starting point
The default root protection area is derived from stem diameter alone: a circle with a radius of twelve times the diameter measured at 1.5m, capped at 707 m². It is a proxy, chosen because it is quick and repeatable, and it assumes nothing about the site.
On an open lawn that assumption is reasonable. On a site where the tree stands 1.5m from a Victorian party wall with a basement behind it, the circle is describing rooting volume that physically cannot exist.
The standard therefore allows the area to be modified in shape where soil conditions, the tree's morphology and the presence of existing structures or surfaces justify it. The total area should be maintained, and the modified shape should stay contiguous with the stem.
What actually justifies an offset shape
Work through the constraints in order of how easily they can be evidenced.
Physical exclusions are the strongest. A basement wall, a lift pit, a retaining structure, an adopted carriageway with a full depth compacted sub-base, a culverted watercourse: all of these exclude rooting in a way that can be shown on a section. Photographs of the construction, a trial pit log, or the highway authority's typical construction detail will carry the argument.
Soil and ground conditions come next. Shallow soil over rock, a perched water table, made ground with a high stone content, or severe compaction from decades of vehicle parking all skew rooting, and a soil description from trial pits gives something concrete to reason from.
Direct investigation is the strongest evidence and the one clients resist paying for. Hand-dug or air-spaded trenches perpendicular to the stem, logged by depth and root diameter, answer the question rather than modelling it. Ground-penetrating radar can map rooting without excavation, with interpretation caveats that should be stated rather than buried.
What does not justify a modification, on its own: crown shape, a lean, the age of the tree, or the fact that the building has to go somewhere.
Levels are worth checking separately. Where a site has been cut or filled since the tree established, the current ground surface tells you little about where the rooting horizon sits, and a modification drawn from present-day contours can be wrong in both directions. Old survey drawings and historic aerial photography often show the earlier profile. Ask for them before the trial pits are dug, because they decide where the pits should go.
Sequencing the work against the design programme
The expensive mistake is commissioning the arboricultural work after the layout is fixed.
A better order is simple. Get a BS 5837 survey and tree constraints plan at concept stage, with the default areas plotted. Test the layout against them and find out where the conflicts are. Where a conflict is unavoidable and there is a plausible physical reason to think rooting is restricted, commission the investigation before the planning drawings are finalised. Then draw the modified area, and design to it.
Doing it the other way round produces the pattern reviewers recognise instantly: a modified root protection area whose outline follows the proposed building line to the millimetre.
Budget the investigation as a design cost rather than a planning cost. Two trial trenches and a half day of arboricultural time are minor against the cost of relocating a building or losing a mature tree that the council then refuses to see felled.
What goes on the drawings
Reviewers need to see both figures and both lines.
- Plot the default root protection area as a dashed circle, with its area stated in square metres.
- Plot the modified area as a solid outline, with its area stated in square metres, and show it contiguous with the stem.
- Annotate each removed segment with the reason and a reference to the evidence.
- Show the section through any structure relied on as a physical exclusion.
- Repeat the modified line on the tree protection plan as the construction exclusion zone.
- Mark any area within the modified zone where no-dig construction is proposed.
No-dig construction deserves its own note on the drawing rather than a mention in the text. A cellular confinement system laid over a geotextile on the existing grade, with the build-up above rather than below finished level, is the standard solution for access and parking within a rooting zone. It changes levels at the edge of the area, and those levels have to work with the rest of the scheme.
Carry all of it into the arboricultural impact assessment and tree survey, because a drawing without the reasoning behind it is easy for an officer to set aside.
Where an authority will reject the modification
Tree officers see a lot of these, and the reasons for rejection repeat.
Evidence that is asserted rather than gathered. Modified areas that have quietly lost area in the reshaping. Shapes drawn as narrow corridors that no longer function as protection. Reliance on a patio or thin driveway as an exclusion, when roots commonly run beneath such surfaces. Modifications applied to veteran trees, where the diameter-based circle is the wrong tool altogether and a canopy-based zone with a buffer is more appropriate.
The other frequent objection is a modification that is arboriculturally defensible but leaves no room for construction. If the crane pad, the scaffold, the material storage and the site welfare all sit inside the protected zone, the modification has solved the drawing and not the site.
Where a modification is refused, the practical response is usually more evidence rather than more argument. Occasionally it is a redesign, and it is better to reach that conclusion at planning stage than at discharge of conditions.
Checking the modification against the soil evidence
One honest uncertainty worth stating. Root mapping in heterogeneous urban fill is difficult, and two surveyors can read the same radar data differently. That is a reason to present limitations openly, not a reason to avoid the technique.
Where the scheme also involves shrinkable clay and nearby buildings, the soil investigation serves two purposes at once. Foundation design and root protection are looking at the same ground, and the questions raised by tree-related subsidence at a neighbouring property draw on the same site data. Commission one investigation that answers both rather than two that answer neither properly.
Heritage adds a third layer. Where the tree contributes to the setting of a listed building, the argument for retention is stronger and the tolerance for encroachment lower, so the modification needs to be conservative.
Landscape proposals should follow the same logic: new planting positions, surfacing and levels all have to respect the modified area, and the landscape strategy is where that gets resolved.
Enter your postcode on PlanScope to see which supporting documents a scheme with trees on or near the site 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.
Frequently asked questions
Does a lop-sided crown mean the roots are lop-sided too?
Not reliably. Crowns become asymmetric through light competition, wind exposure, past pruning and highway clearance. Roots respond to soil structure, moisture and physical obstruction. A modification to the default root protection area has to be argued from ground conditions and obstructions, supported by investigation, rather than inferred from canopy shape.
When should the tree investigation be commissioned?
Before the planning drawings are finalised. Get a BS 5837 survey and constraints plan at concept stage, test the layout against the default areas, and commission trial trenching where a conflict is unavoidable and there is a physical reason to think rooting is restricted. Investigation after the layout is fixed looks design-led to any reviewer.
What is no-dig construction?
A build-up laid above existing ground level rather than excavated into it, typically a cellular confinement system over a geotextile, used for access roads and parking within a rooting zone. It avoids severing roots and avoids compaction. It raises finished levels at the edge of the protected area, so it has to be coordinated with the wider site levels.
Can the total root protection area be reduced?
Reshaping and reducing are separate arguments. BS 5837:2012 allows the shape and position to be modified where evidence supports it, with the total area maintained. Proposals that reshape and lose area at the same time are commonly rejected, because only one of the two changes is usually evidenced.
Why do tree officers reject modified root protection areas?
Most often because the evidence is asserted rather than gathered, the area has quietly shrunk, or the shape follows the proposed building line exactly. Reliance on a thin driveway as an exclusion also fails, since roots run beneath such surfaces. Veteran trees attract rejection because the diameter-based circle is the wrong tool.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.