Evidencing a tree canopy cover policy

A tree canopy cover policy asks you to show what percentage of the site sits under tree crowns now, what percentage will sit under them once the scheme is built and grown, and how you got from one figure to the other. Most refusals on canopy policy are not about planting too few trees. They are about a calculation the officer cannot audit.

Canopy cover is measured as crown area projected vertically onto the ground, expressed as a percentage of site area. That is a different accounting system from biodiversity net gain, which counts habitat units and treats urban trees as a habitat type with its own condition criteria. A scheme can pass one and fail the other, so run both and reconcile them in the text rather than assuming the metric answers the canopy policy.

Where canopy cover targets come from

Canopy targets almost never sit in national policy. The National Planning Policy Framework (August 2026 edition) supports tree-lined streets and the retention of existing trees, but it sets no percentage. The number comes from the adopted local plan, a supplementary planning document on trees or green infrastructure, or in London from borough policy sitting under the London Plan's urban greening provisions.

Read the policy wording before you calculate anything, because the phrasing controls the method. Watch for four things: whether the target applies to the site area or the developable area, whether it is a target or a minimum, what time horizon it uses, and whether existing canopy that is retained counts towards it. Policies differ on every one of these, and an assessment written against the wrong reading of the policy is wasted work.

Where the policy is in an SPD rather than the plan, say so and give it the weight an SPD carries. Officers sometimes treat SPD figures as though they were development plan policy.

Measuring existing canopy on a site

Two routes get used, and the better assessments use both.

The survey route takes crown spread measurements from the tree survey prepared to BS 5837:2012, which records crown radii at the four cardinal points for each tree. Those four radii define a polygon or an ellipse, and summing those areas across the survey gives existing canopy. Treating every crown as a circle from a single radius overstates canopy on asymmetric trees, which is exactly the trees along a boundary that has been pruned back from a neighbour.

The remote route uses recent aerial imagery or LiDAR to digitise canopy polygons, or point-samples an aerial photograph across a grid. It is faster, it captures off-site trees whose crowns overhang the boundary, and it is the only practical method on a large site. Its weakness is date: imagery flown in winter under-reads deciduous canopy badly.

Where crowns overlap, count the overlapping area once. Double counting is the commonest arithmetic error in these submissions and it is easy for a reviewer to spot on the plan.

Projecting canopy from new planting

New planting is projected, not measured, so the projection needs a stated basis. Set out for each proposed tree the species, the nursery stock size at planting, the expected mature or target-age crown diameter, and the source of that expected diameter.

Councils that set canopy targets usually state the age at which new planting should be assessed. Where they do not, say which age you have used and why. Assessing at 25 years is a common convention and is defensible for standard urban species. Assessing at full maturity flatters the result and invites the officer to discount it.

Then apply an honest survival assumption. Not every tree planted survives to the assessment year, and a landscape condition requiring replacement of failures within five years does not guarantee a mature crown at year 25. Some assessments apply a percentage reduction for expected losses. If you do, state the figure and its basis rather than burying it.

Time horizons used in the calculation

Present three columns, not one.

The at-completion figure is often uncomfortable, because it dips below the existing figure on any site where mature trees come out. Show it anyway. An officer who finds that the dip was hidden will distrust the rest of the submission, and the honest presentation lets you argue for the mitigation that closes the gap: larger nursery stock, additional planting off the developable footprint, or a commuted sum where the policy allows one.

Retained trees and their contribution

Retained trees carry the calculation on most sites, so their contribution has to be credible. A tree retained inside a root protection area that the construction programme then compromises does not deliver canopy in year 25, and a reviewer who reads the arboricultural method statement alongside the canopy schedule will notice the mismatch. Keeping retention realistic is the same discipline as retaining a tree through a construction programme.

Allow for growth on retained trees where they are young, and allow for decline where the survey categorises them as U or as C with limited remaining contribution. A veteran oak in category A is not going to add crown area over 25 years in the way a semi-mature lime will.

Off-site trees whose crowns overhang the site are a judgement call. If the policy measures canopy over the site, overhanging crown counts, but only while it exists, and you have no control over a neighbour's pruning. Show it separately rather than folding it into the total.

Presenting the calculation for audit

Make the numbers checkable. That means a schedule, keyed to the tree numbers in the BS 5837 survey and to the planting plan, with one row per tree.

1. Tree reference, matching the survey and the planting schedule.
2. Species and BS 5837 category for existing trees.
3. Crown radii or projected crown diameter, with the source stated.
4. Calculated canopy area in square metres, and the geometry used.
5. Status: retained, removed or proposed.
6. The scenario or scenarios the row contributes to.

Under the schedule, give site area, total canopy area for each scenario, and the resulting percentage. On the drawing, show canopy polygons hatched by scenario with overlaps resolved. A reviewer should be able to pick three rows at random, measure them on the plan and get your answer.

Cross-refer the canopy figures into the Landscape Strategy rather than leaving them stranded in an arboricultural appendix. The landscape document is where the officer looks for the planting that delivers the target.

Where the policy is unachievable on a small site

Some sites cannot hit the number. A backland plot with a building footprint covering most of the developable area, or a scheme constrained by a root protection area and an adopted highway, will fall short whatever is planted.

Say so early and argue it on the policy's own terms. Set out the maximum achievable figure with the calculation behind it, show what was tried and why it failed, and offer what the policy allows in substitution. That is usually one of three things: planting in the highway verge under a separate agreement with the highway authority, a financial contribution to off-site planting where the council has a mechanism for it, or additional green infrastructure that delivers the policy's underlying purpose through green roofs and hedgerow rather than canopy.

What does not work is asserting that the target is unachievable without showing the arithmetic. A canopy schedule that stops at the answer the applicant wanted is the fastest way to a request for further information.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which tree and landscape reports your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.