Canopy cover targets in a green infrastructure plan

Canopy cover targets in a green infrastructure plan are measured as a percentage of site area shaded by tree crowns at a stated point in the future, usually 20 or 25 years after planting, and the figure stands or falls on soil volume rather than on the number of trees you draw. Most plans that get queried by an officer have the planting schedule right and the rooting environment wrong. Get the two reconciled before the drawings are issued, because a canopy target promised over a podium slab with 300mm of substrate will not be delivered, and a condition requiring it will sit on the permission anyway.

What the plan is actually asked to deliver

A Green Infrastructure Plan pulls together the green and blue elements of a scheme and shows how they perform against the development plan. Canopy cover is one of three or four numbers it carries, alongside an urban greening score where that policy applies, a drainage contribution and a biodiversity position.

The policy hooks vary by authority. Some adopted local plans set a canopy cover percentage for new development directly. Others set a replacement ratio for trees removed, or require a net increase without naming a figure. London boroughs tend to work through the London Plan, which sets an urban greening factor target score of 0.4 for predominantly residential development and 0.3 for predominantly commercial development, with a separate policy on trees and woodland.

Read the wording closely. A target expressed as cover at maturity is a different undertaking from cover at 25 years, and a target measured against site area is different from one measured against publicly accessible areas only.

Measuring canopy cover and setting the baseline

The baseline comes first, because almost every policy is written as an increase. Two methods are in general use.

Aerial imagery analysis, sampling points over an orthophoto and classifying each as canopy or not, gives a site percentage and a confidence interval. It works at site scale and above, and it is the method behind most published borough figures. For a single site the alternative is to measure crown spread directly from the tree survey, which you need anyway under BS 5837:2012, and to plot crown projection polygons.

Use the survey. A BS 5837 survey records crown spread by radius in four directions, which converts to a crown projection area for each tree, and the sum of those areas over the site area is your baseline canopy cover. Overlapping crowns get counted once.

For the proposed figure you need a mature crown spread for each species and a growth assumption. Nursery stock data and the species' known mature dimensions give the end point. Stating the assumption is what makes the number defensible, so set out the species, the expected crown diameter at the assessment year and the source.

Urban greening factor scoring where it applies

Where an urban greening factor policy is in force, canopy cover and the score are related but not the same thing. The factor is a weighted area calculation: each surface cover type on the site carries a factor between 0 and 1, the area of each is multiplied by its factor, and the total is divided by site area.

Tree canopy contributes through the planting area beneath it rather than as a separate line in most versions of the table, so a tree in a 1m² pit does little for the score even if its crown will eventually shade 60m². That is the single most common misreading of the policy.

Take the factor values from the adopted plan rather than from a summary, including this one. The London Plan carries the authoritative table for London and several authorities outside London have adapted it with different weightings. Our note on the urban greening factor in a green infrastructure plan goes through the arithmetic in detail.

Green roofs, walls and their weight in the total

Green roofs earn a factor and almost never earn canopy. An extensive sedum roof carries no trees, cannot carry trees, and should be presented as what it is: a contribution to the greening score, to attenuation and to habitat, with no effect on the canopy figure.

Intensive roofs on structural slabs can carry small trees where the substrate depth, the irrigation and the wind exposure allow it. The limiting factors are usually dead load and maintenance access rather than planning policy. Green walls score, need irrigation and a maintenance contract, and attract officer scepticism because so many have failed. If you propose one, condition the maintenance.

Soil volume as the real constraint

Here is the number that decides whether a canopy target is honest. Rooting volume has to be matched to the crown you are promising, and the widely used rule of thumb among arboriculturists and landscape architects is around 0.6m³ of soil for every square metre of mature crown projection.

Run it through once. A tree with a 10m mature crown diameter has a crown projection of roughly 79m², which implies something in the order of 47m³ of available soil. A standard 1.2m by 1.2m tree pit, 1m deep, offers 1.44m³. The gap is why street trees in hard paving stall at a fraction of their species' potential. Treat that calculation as illustrative, and check the figures for your species and soil against current arboricultural guidance.

The practical answers are shared rooting trenches running between pits, structural soils and soil cells under paving, continuous planting beds instead of discrete pits, and, where the slab governs, accepting smaller species rather than drawing large ones.

Those ranges are indicative of typical UK practice rather than standards; the volume for any given tree follows from the crown you intend it to reach.

Reconciling canopy with drainage and biodiversity

The same square metres get counted three times in most applications, which is fine if the design works and dishonest if it does not.

Tree pits that double as rain gardens need an inlet, a kerb detail, a free draining soil specification and an overflow, and the planting has to tolerate both drought and inundation. Coordinate this with the Drainage Strategy rather than describing it twice in different terms, because a reviewer who finds two inconsistent descriptions of the same feature will query both.

Biodiversity net gain runs on the statutory metric and its own habitat classifications. Canopy cover is not a metric input, so a scheme can hit a canopy target and still fail on biodiversity units, or the reverse. The Environment Act 2021 regime expects the metric to be completed by a competent person, and a draft is a starting point for that person rather than a substitute.

Conditioning and monitoring delivery

Canopy targets are delivered over decades and planning conditions are enforced over years, which is the structural weakness of the whole exercise. Authorities handle it in a few ways, and it is worth proposing the wording yourself.

1. A landscape and soils condition requiring planting pits, rooting volumes and soil specifications to be approved before the relevant superstructure works, not just before occupation.
2. A planting schedule condition tying species, stock size and location to an approved drawing.
3. A replacement condition covering trees that die, are removed or become diseased within five years, replaced in the next planting season with the same species and size.
4. A landscape management plan secured for a set period, with an obligation under section 106 where the land will transfer to a management company.

What you should resist is a bare condition requiring a percentage canopy cover by a stated year with no soil specification behind it. It is unenforceable in practice and it hides the design failure rather than fixing it.

Reports drafted on PlanScope are AI-assisted and should be reviewed before submission, and we do not guarantee planning approval. For specialist technical reports, qualified professional input may be required. Browse the 66 PlanScope report types to see how a green infrastructure plan sits alongside the landscape and drainage documents a scheme needs.