A Root Protection Area (RPA) is the minimum area around a tree that must be left undisturbed to keep it alive and stable. It is the single most important number a BS5837 tree survey produces, because it determines where you can — and can't — build. Every retained tree on a scheme gets an RPA plotted on the tree constraints plan, and any part of your proposal that lands inside one has to be justified.
What the RPA actually represents
The RPA is a notional below-ground area — a planning proxy for the roots you can't see. It is designed to safeguard enough of the rooting environment (soil volume, structure and moisture) that the tree survives construction and continues to thrive afterwards. Inside it, the four things that kill trees on building sites all become a risk: excavation that severs roots, compaction from tracked machinery and stored materials, level changes that either bury or expose roots, and contamination from cement washout, fuel or spoil. The RPA turns all of that into one manageable line on a drawing.
Crucially, the RPA is a below-ground concept and does not always match the canopy above. A tree's crown spread is surveyed separately (in four directions) because shading and physical clearance are different constraints from rooting. It is entirely possible for the RPA to extend well beyond the branches, or for the canopy to overhang land the roots don't reach.
The BS5837 formula
The RPA is a circle whose radius = 12 × the stem diameter, with the stem diameter measured at 1.5m above ground level (this is the diameter at breast height, or DBH). The resulting area is capped at 707m² — equivalent to a 15m radius circle — for trees above roughly a 1.25m stem diameter.
| Tree type | RPA rule |
|---|---|
| Standard single-stem tree | Radius = 12 × stem diameter (at 1.5m), area capped at 707m² |
| Multi-stem tree | Combined notional stem diameter, then 12 × that figure |
| Veteran / ancient tree | 15 × stem diameter, or canopy edge + 5m, whichever is larger |
| Ancient woodland | Minimum 15m buffer from the woodland boundary (more if impacts extend further) |
The veteran and ancient-tree enhancement matters because those trees are treated as irreplaceable habitat under national planning policy — the bar for harming them is far higher, and the RPA is deliberately generous. Our guide to how to calculate a root protection area works through the arithmetic step by step.
Worked examples
- A young C-category tree, 200mm (0.2m) stem diameter: RPA radius = 12 × 0.2 = 2.4m (about 18m²). A small constraint you can usually design around easily.
- A mature B-category tree, 500mm (0.5m) stem: RPA radius = 12 × 0.5 = 6m (about 113m²). Now a serious constraint on a small garden.
- A large mature A-category oak, 900mm (0.9m) stem: RPA radius = 12 × 0.9 = 10.8m (about 366m²). This can swallow most of a modest plot.
- A veteran oak, 1.2m stem: the veteran rule gives 15 × 1.2 = 18m radius — well over 1,000m² — unless the canopy-plus-5m figure is larger still.
Use the calculator below to work out the RPA for any tree, then read on for what it means for your project.
How the stem diameter is measured
The formula is only as good as the diameter that feeds it, and BS5837 is specific about how that is taken. The stem diameter is measured at 1.5m above ground level on the uphill side of the tree. Where that isn't practical, the standard sets out alternatives:
- Forked below 1.5m — measure just below the fork or point of swelling.
- Multi-stem tree — measure each stem, then combine them into a single notional diameter (the square root of the sum of the squared diameters for the largest stems), so the RPA reflects the whole tree rather than one leg of it.
- Tree on a slope or with buttressing — the surveyor adjusts the measurement point to avoid an artificially inflated reading.
This is one reason a survey by a suitably qualified arboriculturist matters: an optimistic or careless diameter feeds straight through to an RPA a tree officer can pick apart. Our guide to the data points in a survey schedule covers exactly what gets recorded.
When the RPA becomes a polygon
The circular RPA is the starting point, not a fixed shape. Where rooting is demonstrably asymmetric, the arboriculturist can re-plot the RPA as a polygon of the same area, redistributing it to where the roots realistically are. The classic case is a street tree with a building or an adopted road hard against one side: roots can't be growing under the existing structure, so the protected area is shifted to the open ground where they must be. The rules are strict — the total area is preserved, the shift has to be justified on the ground (not just to free up developable land), and it shouldn't move more than about a fifth of the area. A polygon plotted purely to make a scheme fit is exactly the sort of thing a tree officer will reject.
What the RPA means for your design
The design principle is simple: keep development out of the RPA wherever you possibly can. Inside it, you should avoid excavation, changes in ground level, stored materials and compaction from vehicles and plant. The RPA is fenced off during construction as a construction exclusion zone, shown on the tree protection plan, and the barrier goes up before any machinery arrives on site.
The best-value move is to let the RPAs shape the layout from the very start. BS5837 is design-led: the survey and constraints plan are meant to be produced before the scheme is drawn, so roads, foundations and drainage runs are routed around the RPAs rather than through them. Discovering, at arboricultural impact assessment stage, that your proposed extension footprint sits squarely inside a protected oak's RPA is how projects end up in expensive redesign — or refusal.
When you can't avoid the RPA
Sometimes the site simply doesn't allow a clean miss, and some incursion is unavoidable. This is where an arboricultural method statement earns its fee. It sets out the engineered precautions that let limited works proceed without unacceptable harm:
- No-dig cellular confined surfacing for driveways, paths and patios over an RPA — a load-spreading raft built up above existing ground with no excavation. See no-dig construction near trees.
- Special foundations — screw piles, mini-piles or beam-and-pier systems that carry loads past the roots with minimal ground disturbance. See piling foundations near trees.
- Hand-digging and root-sensitive excavation with an arboriculturist supervising, so roots above a set diameter are retained and cleanly cut where unavoidable.
- Ground protection — temporary track-mat systems that spread load where site traffic must cross the RPA.
There is a firm rule of thumb behind all of this: keep hard surfacing to a minority of any RPA and keep it porous, so the soil below still breathes and drains. Our guide on whether you can build in a root protection area covers what councils will and won't accept.
Worked scenario: a driveway over an RPA
A homeowner wants to widen a driveway that runs beside a neighbour's mature lime. The lime has a 600mm stem, so its RPA radius is 12 × 0.6 = 7.2m (about 163m²), and roughly a third of that circle reaches under the proposed driveway. A standard dug-out-and-tarmac driveway would sever roots and compact the soil — almost certainly a refusal, and if the lime carried a TPO, a criminal matter too. The workable route is a no-dig build-up: geotextile laid on the existing ground, a cellular confinement layer filled with sharp angular stone, and a porous surface on top, with no excavation inside the RPA and no level change at the tree. That design goes into an arboricultural method statement, the RPA is fenced during the build, and the arboriculturist confirms the works afterwards. The full picture is in our guide to tree surveys for driveways and hardstanding.
Off-site and neighbouring trees
One of the most common — and costly — mistakes is surveying only the trees on your own land. A tree in a neighbour's garden whose RPA crosses the boundary into your site is a material constraint, and the survey must capture it. So too are trees just outside the site whose canopies overhang the works. An application that ignores an off-site tree's RPA is a classic reason for a survey being rejected at validation, and if that neighbouring tree also carries a Tree Preservation Order, cutting its roots without consent is a criminal offence.
RPA rules are applied locally — check your council
The BS5837 formula is national and consistent — 12 × stem diameter is the same in Cornwall as in Cumbria. What varies is how each local planning authority applies and polices it. Requirements are set locally: your council's validation list decides when arboricultural information (including plotted RPAs) must accompany an application, how much detail it wants upfront, and how strict its tree officer is about justifying any incursion. Some authorities want no-dig and foundation details resolved before permission; others accept the principle at application stage and pin the detail to a pre-commencement condition.
Because the tree officer's expectations genuinely differ, always read your own authority's local validation list and, where a tree is significant, ask the officer directly before finalising a layout. PlanWatch tracks live tree-related planning activity per council, so you can see how RPA and tree issues are actually being handled near you — browse examples in Leeds, Bristol and Manchester, or search your own postcode. Note too that this guidance is England-centric; Scotland, Wales and Northern Ireland apply BS5837 within their own statutory frameworks.
Next steps
Start with what a root protection area is for the plain-English version, price the work on our cost guide, and if any tree might be protected, run a TPO check before you touch a single root. When you're ready, a BS5837 survey will plot every RPA on your site properly.