BS 5837 root protection areas: calculating and designing around them

A retained oak is not a decoration on a layout; it is a constraint with a measurable radius. BS 5837:2012 is the standard that turns a tree into a dimension you can design against. Put the root protection area on the base plan before the roads and it shapes the scheme rather than wrecking it at validation.

What BS 5837:2012 is, and what it is not

BS 5837:2012, Trees in relation to design, demolition and construction — Recommendations (BSI) is the reference every UK planning authority reaches for on trees. It is a recommendation, not legislation, and its force comes from adoption: validation lists ask for reports "in accordance with BS 5837:2012", and conditions enforce it. It does not decide whether a tree may lawfully be felled; that is a separate regime, covered below. The standard is paywalled, so no clause text is quoted here: what follows is the method as councils and consultants publish it, and where they differ, that is said.

When a tree survey is needed

The trigger is usually the authority's validation list, not the standard. Most ask for one where trees stand on the site or close enough to be affected by it, and "close enough" varies — Leeds City Council's guidance note defines adjacent trees as those within 12 times their estimated stem diameter beyond the boundary, more for ancient trees. The sequence matters: a tree survey of what is there, an arboricultural impact assessment of the scheme designed against it, then a tree protection plan and method statement.

Calculating the root protection area

The RPA is the plan area around a tree that should be left undisturbed, drawn as a circle centred on the stem. The BS 5837:2012 method, as councils publish it:

• One stem: radius = 12 × the stem diameter, measured over bark at 1.5 m above ground level.
• Two to five stems: combine them first — the combined diameter is the square root of the sum of the squares of the individual diameters — then multiply by 12.
• More than five stems: the combined diameter is the square root of (mean diameter² × number of stems), then multiply by 12.
• The area is capped at 707 m², a radius of about 15 m. There is no minimum.

A 450 mm single stem gives a 5.4 m radius, about 92 m². A twin stem of 300 mm and 400 mm combines to 500 mm — √(300² + 400²) — giving 6.0 m and 113 m². One oddity worth knowing: because the formula changes at the five-to-six stem boundary, adding a small sixth stem can make the calculated RPA smaller. The circle may also be reshaped where rooting is demonstrably asymmetric, but council guidance is consistent that the total area does not change — it is not a route to a smaller number.

Airship Site Designer records the survey against each existing or retained tree — tag, stem diameters, height, crown spread, category and TPO flag — and draws the RPA as a dashed circle in the category colour, on its own DXF layer. A missing stem means no circle at all, because half a survey gives an RPA that is too small. See the documentation.

Tree categories A to U, and what they mean for a layout

BS 5837:2012 sorts surveyed trees into four categories, as councils reproduce the standard's cascade chart:

• U — in such condition that they cannot realistically be retained as living trees, in the context of the current land use, for longer than about 10 years.
• A — high quality, with an estimated remaining contribution of at least 40 years.
• B — moderate quality, at least 20 years.
• C — low quality in adequate condition with at least 10 years, or young trees with a stem diameter below 150 mm.

Each also carries a subcategory: 1 arboricultural, 2 landscape, 3 cultural including conservation — so a tree reads as B2 or C1.

Category A trees are fixed points, and designing a plot into one is an argument you will lose; category B is where the real negotiation happens; category C is tradeable. But a category is an arboricultural judgement, not a planning decision — a C tree in a conservation area still has statutory protection. And the 2005 edition's category R for removal does not exist in the current standard, so a report using one is out of date.

What the RPA actually constrains

Inside the RPA the default is a construction exclusion zone, fenced before plant arrives rather than after. Method statements typically prohibit excavation, regrading, materials storage, vehicle movement, site huts, cement washout and service trenching. Level changes count both ways: raising ground over a root plate is as damaging as digging. Building inside an RPA needs an arboricultural case and usually a pile-and-beam or similar special foundation.

Hard surfacing is where most schemes collide. The widely applied default position is that new permanent hard surfacing should not exceed 20% of the RPA, above which a no-dig, permeable construction is expected. The baseline differs between authorities — some measure 20% of the currently unsurfaced ground in the RPA, others 20% of the whole — and Surrey Heath Borough Council cites BS 5837 clause 7.4.2.3 for it. Treat 20% as the point where you will be asked a question, not as a pass mark. No-dig construction builds entirely above existing grade — typically a cellular confinement system over geotextile with permeable no-fines backfill.

Airship's design review, on Pro plans and above, then flags dwellings reaching into an RPA, hard surfacing over more than 20% of one, retained trees without stem measurements, and protected trees marked for removal — not an arboricultural impact assessment, but the conflicts found early.

TPOs and conservation areas: a separate regime

A tree preservation order is made under section 198 of the Town and Country Planning Act 1990 (England and Wales; Scotland uses the 1997 Scottish Act). Cutting down or destroying a protected tree without consent is an offence under section 210.

The interaction with planning permission catches people out. Regulation 14(1)(a)(vii) of the Town and Country Planning (Tree Preservation) (England) Regulations 2012 exempts work "so far as such work is necessary to implement a planning permission" — but expressly excludes outline planning permission. An outline consent whose layout shows the tree gone does not authorise felling it.

In a conservation area, trees not already under a TPO are protected by section 211: six weeks' written notice before works. The council cannot approve or refuse it; it can make a TPO within the six weeks, or let it run. Regulation 15 of the same Regulations exempts trees whose diameter does not exceed 75 mm, measured over bark at 1.5 m above natural ground level, and 100 mm where cutting down is solely to improve the growth of other trees. On a multi-stemmed tree, any single stem over the threshold ends the exemption.

Neither regime depends on BS 5837. A U tree under a TPO still needs consent to fell; an A tree with no designation can lawfully come down before an application is made. Airship draws tree preservation zones, conservation areas and ancient woodland from planning.data.gov.uk with the other UK planning constraint layers, though it holds only what councils have published.

Designing around retained trees, not around validation

The common failure is sequence. The layout is drawn, the survey is commissioned to support the application, the RPAs land across six plots, and something gives: usually the trees, sometimes the unit count, always the programme. The cheap order is RPA circles on the base plan before the road alignment is fixed: trees push roads, roads push plots. A housing estate layout generated with retained trees in it as obstacles never has the argument.

Where an RPA lands matters as much as how large it is. Verges, front gardens and public open space absorb one; rear gardens are the worst place for it, because sheds, extensions, hardstanding and permitted development rights erode it over twenty years. Street trees are a separate exercise — see the guide to estate roads and Manual for Streets.

The standard is a floor, not a verdict. Whether a 40-year oak really lives beside a terrace two metres outside its RPA is a judgement for an arboriculturist on site.