How the biodiversity metric treats trees on a site

The statutory biodiversity metric treats trees in two quite different ways, and confusing them produces a gain plan that does not add up. Trees standing alone in grassland, gardens, streets or car parks are recorded as urban tree habitat, measured by canopy area and scored on their own condition criteria. Trees inside a woodland, hedgerow or scrub parcel are part of that habitat and are not counted again. Getting the trees biodiversity metric treatment right starts with drawing that boundary on the plan.

The second thing to understand is that the metric is unsentimental about replacement planting. A mature tree removed today and replaced with three whips generates a shortfall that lasts decades.

How individual trees are recorded

Urban trees are entered through the metric's tree module, which estimates canopy area from trunk diameter measured at breast height. The tool groups trees by size band, and canopy area converts into a habitat area that then behaves like any other area habitat in the calculation.

Two consequences follow. Canopy is what counts, so a spreading open-grown oak carries far more area than a narrow upright of the same trunk diameter. And because the module works from diameter, the tree survey has to record diameter accurately for every tree in the assessment, not just for the ones being removed.

Where trees grow in a group with touching canopies, the metric handles overlap so the same square metre is not counted twice. Your ecologist and your arboriculturist need to agree, on a plan, which trees are being treated as individual urban trees and which fall inside another habitat parcel.

That agreement is worth putting in writing. The two disciplines survey to different standards for different purposes, and a site walked separately in March and July by people who never compared plans produces two incompatible sets of tree numbers.

Condition assessment for urban trees

Urban trees carry their own condition criteria rather than the general habitat condition sheets. The assessment looks at the health and structure of the tree, whether the species suits its position, the rooting environment and soil volume available, physical damage, and the quality of the surrounding ground surface.

A semi-mature lime in a generous grass verge with an unsealed rooting area scores well. The same tree in a tarmac pit with a compacted root zone and repeated crown reduction scores poorly, and the unit value falls accordingly.

That creates an opportunity most schemes miss. Improving the condition of retained trees generates units without planting anything: de-compacting and mulching root zones, replacing sealed surfaces with permeable ones within the root protection area, removing competing structures, and putting a sensible management regime in place. On a constrained site, that uplift can be the difference between an on-site solution and buying off-site units.

Trees within other habitat parcels

A tree inside a woodland parcel is part of that woodland and is not entered separately. The same applies to standards within a hedgerow, which are recorded through the hedgerow module rather than as urban trees.

Double counting is the error to watch for, and it runs both ways. An assessor who enters woodland by area and then adds every mature tree within it as an urban tree inflates the baseline, which makes the post-development figure harder to reach. One who omits the trees in a scrub parcel that is being cleared understates what is being lost.

Ancient and veteran trees sit outside this entirely. They are irreplaceable habitat, and their loss cannot be traded through units or credits at all, whatever the metric might otherwise calculate.

Why replacement rarely balances

Two multipliers punish new planting.

Difficulty of creation reflects how hard the habitat is to establish. Time to target condition reflects how long it takes to get there, and a newly planted tree needs decades before it reaches the canopy area and condition of the tree it replaced. The metric discounts future units for that delay, so the units delivered on paper are a fraction of the units a mature tree carried.

The practical arithmetic is unforgiving. Replacing one large, well-conditioned tree can require a number of new trees that simply does not fit on a developed site, along with the soil volume and unsealed surface each of them needs to reach good condition. Run the numbers before the layout is signed off rather than after.

Nor does clearing early help. The baseline is taken from the pre-development condition including any degradation carried out to avoid the requirement, as the note on clearing a site before you apply explains.

Retaining trees as a design decision

A tree that is properly retained appears in both the baseline and the post-development figure, so it costs nothing in units. That makes retention the most efficient move available.

Retention has to be real, though. A tree whose root protection area is largely built over, or whose crown has to be cut back hard to clear a new roof, is not in the same condition after development, and an honest assessment records the drop. Building over a root protection area and then claiming unchanged condition is the single most common weakness in a tree-related gain plan.

Retention also has consequences for the scheme that need testing early, including the effect of a retained canopy on daylight to new habitable rooms, which is covered in the note on retained trees and daylight to habitable rooms.

So the design conversation is about soil volume and distance rather than about the tree line on a drawing. Give retained trees an unsealed rooting area that survives construction, keep foundations and services outside the protection area, and specify tree protection fencing that stays up for the duration of the build.

Service runs are the usual failure. Drainage, gas and electricity all want the shortest route, and the shortest route crosses root protection areas. Fix the service corridors on the layout at the same time as the tree retention plan, or expect the arboricultural method statement to become a negotiation about damage that has already happened.

What the assessor needs from the tree survey

A BS 5837:2012 survey provides most of it, but not all, so brief the arboriculturist on the metric requirement at the outset.

The tree survey should supply species, trunk diameter at breast height, height, crown spread taken in four directions, age class, physiological and structural condition, the BS 5837 category, and the root protection area. On top of that, the metric assessor needs a canopy extent plan rather than crown spread figures alone, a clear schedule of which trees are retained, removed and pruned, and a note of the ground surface and rooting conditions around each retained tree.

Deliver both surveys at the same time. A Biodiversity Net Gain Assessment built on a tree survey that recorded only the trees near the development footprint will have to be redone, and the statutory metric must be completed by a competent person under the Environment Act 2021 regime.

The planting response then belongs in a landscape strategy that specifies soil volumes, pit design and a management regime long enough to reach the condition the metric assumed. Specifying a standard tree in a 900 millimetre pit and expecting good condition in year 30 is how gain plans fail their monitoring reports.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional 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. For specialist technical reports, qualified professional input may be required.