Root protection areas in an Arboricultural Impact Assessment

A root protection area calculation under BS 5837:2012 starts with one number: the stem diameter measured at 1.5 m above ground. Multiply that diameter by 12 and you have the radius of a circle, drawn from the centre of the stem, that the standard treats as the minimum rooting volume to be left undisturbed. The area is capped at 707 m² for a single-stem tree, which is the same as a 15 m radius. Everything else in an Arboricultural Impact Assessment, from the constraints plan to the method statement, hangs off that figure.

Get it wrong at the survey stage and the layout you spend three weeks drawing is the layout the tree officer refuses.

How the area is derived from stem diameter

Measure the diameter over bark at 1.5 m. On sloping ground, measure on the upslope side. Where the stem forks below 1.5 m, or where there is a swelling or old wound at that height, BS 5837 sets out alternative measuring positions, and the report should say which one was used and why.

For multi-stem trees the standard combines the stems rather than taking the largest. Two to five stems are combined as the square root of the sum of the squared diameters. Above five stems, the mean squared diameter is multiplied by the number of stems and the root of that is taken. Both routes produce a single equivalent diameter that then goes through the same 12-times multiplier.

Two practical points follow. The circle is a minimum, not a target, and the cap at 707 m² means a veteran oak and a slightly larger veteran oak end up with the same nominal area even though their rooting differs. Veteran and ancient trees are dealt with separately in national policy anyway, with buffers usually set well beyond the BS 5837 figure.

When the circle is in the wrong place

Roots do not grow in circles. They follow water, air and the path of least resistance, which on an infill plot usually means away from a 1960s garage slab and towards an unmade verge.

BS 5837 allows the area to be redrawn as a polygon where site conditions indicate that rooting has happened asymmetrically, provided the total area stays the same and the shape remains plausible. The evidence for that has to be in the report. Acceptable evidence includes existing hard surfacing of known construction depth, a retaining wall, a watercourse, a building with deep foundations, or the results of exploratory hand digging or air-spade trenching.

What does not work is redrawing the shape because the house will not otherwise fit. Tree officers see that argument weekly, and the giveaway is a polygon that pushes precisely to the edge of the proposed footprint. If you have no physical evidence of where roots are absent, keep the circle and argue the incursion honestly instead.

Plotting constraints before the layout is fixed

The tree constraints plan exists to be drawn early. It overlays the protection areas, the current and likely future canopy spread, and the shade pattern cast at a chosen time of year onto the topographical survey, before anyone has committed to a house position.

A useful constraints plan shows four things: the stem positions surveyed to the same coordinate system as the topographical survey, the protection area for each retained tree, the canopy at survey date with an allowance for growth, and the retention categories from the tree survey. Category U trees, which are in such condition that they cannot reasonably be retained beyond about ten years, are shown but carry no design constraint. Category A trees carry the most.

Architects who draw the layout first and commission the tree survey afterwards almost always pay for it twice. The second version of the site plan is the one that gets submitted.

Incursions and how much is defensible

Some encroachment is normal on tight sites. BS 5837 contemplates it directly, and the usual working position is that where encroachment is unavoidable and stays below about a fifth of the protection area, and the tree is in good physiological condition, it can be acceptable if compensatory area is provided elsewhere next to the existing protection area.

That test has three limbs and applicants routinely satisfy only the first. Unavoidable means you have shown the alternatives and why they fail, not that the client prefers this one. Good condition comes from the tree survey, so a category C tree in declining condition does not get the same latitude. Compensatory area has to be real space that can be kept free of construction, not a strip of adopted highway verge.

The tone of this section matters more than the arithmetic. Set out the percentage, name the works causing it, state the compensating area and then say what residual risk remains to the tree. An assessment that reports a 14 per cent incursion and explains the mitigation reads as competent. One that reports no impact at all on a plot with three mature limes reads as a document nobody checked.

No dig construction and cellular confinement

Where a driveway, a parking bay or a footpath has to sit inside the protection area, the standard route is a no dig build-up over the existing ground level rather than an excavated sub-base.

The principle is simple. Existing grade is left alone, a geotextile separation layer goes down, a three-dimensional cellular confinement layer is filled with clean angular granular material, and a permeable surface finishes it. Load is spread across the cells rather than driven into the soil. Levels rise, which is why the detail has to be coordinated with thresholds, drainage falls and any adjacent adopted surface before it reaches the planning drawings.

Three details get missed. Kerb upstands need a haunching solution that does not involve a trench. Surface water has to be able to leave the build-up without being piped through the rooting area. And the construction sequence has to keep plant off the soil before the cells go down, which usually means working outwards from the edge on temporary ground protection. The tree protection plan and fencing specification should show all of that, and the method statement should say who supervises it.

There is a fuller treatment of the build-up detail and its limits in our note on no dig construction over root protection areas.

Foundations, services and level changes

Foundation design near retained trees answers two separate questions, and only one of them is arboricultural.

The first is root damage: how much soil comes out, over what area, and at what distance from the stem. A pile and beam solution with a suspended slab removes far less soil than a mass-fill trench, which is why it appears in so many method statements on constrained plots. The second question is clay heave and shrinkage, which is a structural and building control matter driven by soil plasticity, species water demand and the mature height of the tree. On shrinkable clay the pile depths and heave precautions are set by the structural engineer against industry foundation guidance, and the arboriculturist's role is to supply the species, height and distance data.

Services are the quiet failure. Drainage runs, incoming water, electricity ducts and soakaways are designed late, get no arboricultural input, and then have to cross the protection area in an open trench. Either route them outside the area on the drainage strategy, or specify thrust boring at a depth agreed with the arboriculturist, and show the route on the protection plan rather than leaving it to the groundworker.

What the assessment says about each tree

The impact assessment is a tree-by-tree account, not a general statement of intent. For every tree that interacts with the proposals it should record the reference number and species, the retention category, the calculated area and whether it was modified, the specific works that affect it, the percentage and location of any incursion, the mitigation, and the residual effect on that tree.

Then it should be honest about losses. Trees removed for arboricultural reasons, trees removed to enable the development, and trees affected but retained are three different categories, and conflating them is the fastest way to lose the tree officer's confidence. Replacement planting should be quantified by species and size rather than described as enhancement.

Councils expect a suitably qualified author for this work, and professional review is strongly advised before submission. Where a Tree Preservation Order or a conservation area is involved, the works themselves need consent or notice under the Town and Country Planning (Tree Preservation) (England) Regulations 2012, which is a separate process running alongside the application. You can see how the assessment sits alongside the other documents a constrained site needs in the full report index.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified arboriculturist 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.