Worked example: canopy cover percentage for a site

Canopy cover is the proportion of a site covered by the vertical projection of tree crowns onto the ground, expressed as a percentage of site area. A growing number of local plans set a target for it, and the canopy cover percentage calculation is simple arithmetic once you have a tree survey, provided you resolve three things first: which trees count, how overlapping crowns are treated, and whether the figure is measured today or projected to maturity.

It is not the biodiversity metric and it does not substitute for one. A site can hit a canopy target and still show a biodiversity loss, and the reverse happens too.

What canopy cover measures and why policies use it

The measure answers a question that biodiversity units do not: how much of the ground is shaded by tree crowns. That matters for summer surface temperatures, for surface water interception, for air quality and for how a street feels at pavement level.

Because it is a plan area measure, it rewards large spreading crowns and takes no account of condition, species or habitat value. A mature sycamore in poor condition contributes more canopy than six healthy young field maples. That is a known weakness of the metric and it is worth saying so in the report rather than letting an officer point it out.

Policies express the target in two main ways. A fixed percentage of site area to be achieved, often with a higher figure in a residential scheme than in an industrial one. Or a no net loss requirement measured against the existing canopy, sometimes with an uplift. Read the policy wording closely, because the two produce very different design responses on a site that is already well treed.

Measuring existing canopy from survey data

The input is a tree survey carried out in accordance with BS 5837:2012, which records crown spread for each tree, usually in four cardinal directions and sometimes in eight.

From that data there are two ways to calculate area. The quicker method treats each crown as a circle using the mean crown radius, giving an area of pi times the radius squared. The better method plots the recorded spreads as a polygon for each tree and takes the polygon area, which matters for lopsided crowns next to buildings. For a small site the circle method is acceptable if you say that is what you did. For a scheme where the target is marginal, use the polygon method.

Groups and hedgerows need a decision rather than a formula. A group recorded as a single item in the survey has a mapped extent, and the usual approach is to take the mapped canopy outline of the group rather than to estimate individual stems. Hedgerows managed below a stated height are normally excluded, and the policy or local guidance should say which.

A Tree Survey prepared to BS 5837 is the foundation of the whole exercise, and councils usually expect a suitably qualified author with professional review strongly advised. Where the survey has been drawn in a mapping package, the canopy areas can be extracted directly, which removes most of the arithmetic error.

A worked calculation for a two hectare site

Illustrative example. The figures below are invented to show the method, not taken from a real site.

A two hectare site, so 20,000 m² of site area. The tree survey records 24 individual trees and two groups. Taking the polygon areas for each crown and the mapped outlines for the groups, the total crown projection comes to 2,600 m², of which 180 m² overlaps between adjacent crowns and is counted once only. The net existing canopy is therefore 2,420 m².

Existing canopy cover is 2,420 divided by 20,000, which is 12.1 per cent.

The proposed layout removes six trees with a combined net canopy of 540 m², leaving 1,880 m² retained, or 9.4 per cent. New planting comprises 48 trees. Taking the mature crown spread assumed for each species from the planting schedule, the projected new canopy is 1,760 m², with 120 m² of assumed overlap at maturity counted once, giving 1,640 m² net.

Projected canopy at maturity is 1,880 plus 1,640, which is 3,520 m², or 17.6 per cent of site area. Against an existing figure of 12.1 per cent that is an increase, and against a policy target of 15 per cent it complies, but only on the assumption that every new tree reaches its assumed mature spread.

Report all three figures: existing, retained and projected. A report that gives only the projected figure invites the question you have avoided answering.

Projecting future canopy at maturity

The projection is where the judgement sits, and it is where a sceptical officer or objector will push.

State the assumed mature crown spread for each species and where the figure comes from. State the time horizon, which is commonly 20 to 25 years rather than true maturity, because a target met in 80 years is not a planning benefit anybody alive will see. And state the assumed survival rate, or explain why you have assumed full survival, since street trees in hard landscape have a well known attrition problem.

Rooting volume is the honest constraint. A tree planted in a 2 m² tree pit over a car park deck will never achieve the crown spread assumed for the same species in open ground, and a canopy projection that ignores soil volume is not credible. Give the soil volume provided per tree alongside the assumed spread.

Then be explicit about what secures the planting. A landscape condition requiring replacement of failures for five years is the usual mechanism, and the figures in the canopy calculation only hold if that condition exists and someone enforces it.

Which trees count and which do not

Decide the rules at the start, write them down, and apply them consistently to the existing and the proposed figures.

• Trees within the application site boundary count. Trees wholly outside it do not, even where they are protected.
• Crowns overhanging the boundary from adjoining land are usually excluded, although some policies count the overhanging portion. Check and say which rule you applied.
• Overlapping crowns within the site are counted once, because the metric is ground area covered.
• Trees to be removed are excluded from the proposed figure, including those removed for reasons unconnected with the development.
• Dead, dying and dangerous trees are normally excluded from the baseline, which has the perverse effect of improving the apparent performance of a scheme that removes them.
• Shrubs, hedges below the stated height and climbers are excluded.

That fifth point deserves care. Excluding a large dead tree from the baseline lowers the existing figure and makes the proposal look better, and an objector will notice. Report the figure both ways where it makes a material difference.

Reporting before and after figures

Present the calculation so that someone else can reproduce it. That means a table with one row per tree or group, giving the survey reference, the crown area, whether retained or removed, and the source of the figure, with the overlap deduction shown as a separate line.

Add a canopy plan. Existing crowns in one tone, retained crowns in another, proposed crowns at maturity in outline, and the site boundary clearly drawn. An officer who can see the three states on one drawing will accept the percentage without recalculating it.

Keep the canopy figures consistent with every other document in the pack. A tree protection plan that retains a tree the canopy calculation has removed is the kind of inconsistency that undermines both, and the discipline set out for a tree protection plan to BS 5837 applies equally here.

Where the metric conflicts with layout efficiency

Canopy targets and plot efficiency pull against each other, and pretending otherwise produces schemes that fail at reserved matters.

Three conflicts recur. Trees need rooting volume, and rooting volume competes with foundations, drainage runs, services and parking. Large species need distance from windows, and a layout designed to the minimum interface distances leaves nowhere to put them. And adoptable highway layouts constrain where a tree can stand, which pushes planting into private gardens where long term retention is least certain.

The design responses that work are structural rather than cosmetic. Retain the existing large trees, because one mature crown is worth many new ones and retention costs no land that the crown was not already occupying. Create a small number of generous planting zones with shared soil volume rather than many isolated pits. Use the public open space and the drainage features as planting opportunities, which a Landscape Strategy can set out alongside the canopy figures.

And test the canopy figure at concept stage rather than after the layout is fixed. The calculation takes an afternoon. Redesigning a scheme that lands two percentage points short takes considerably longer.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Browse the PlanScope report types to see which tree and landscape documents a site usually needs. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.