Evidencing canopy cover targets in a local plan area

A tree canopy cover planning policy asks for a percentage of the site to sit under tree crowns, and the argument is almost never about whether you agree with the target. It is about the horizon, the assumptions and the arithmetic. An officer needs to see which trees you counted, what crown diameter you assumed for each, at what age, and how that converts to a percentage of the site area. Submit that as a checkable table and the condition discussion is short. Submit a landscape drawing with a sentence claiming compliance and expect a request for further information.

Where canopy cover policies come from

There is no national canopy percentage. The National Planning Policy Framework expects new streets to be tree-lined and existing trees to be retained wherever possible, and the current edition should be checked for the policy reference because the Framework has been renumbered twice since 2023. Everything quantified sits below that.

Local plans and supplementary planning documents are where the numbers live, and they take several forms. Some authorities set a site canopy percentage for new development. Some set a net gain in canopy against the existing baseline. Some require a number of trees per dwelling or per parking space. And in London, the Urban Greening Factor under London Plan Policy G5 works differently again, scoring all surface cover types against a target of 0.4 for predominantly residential development and 0.3 for predominantly commercial, with tree canopy scoring highly among the options.

Read the policy wording closely before you calculate anything. A target expressed as canopy cover at maturity is a very different obligation from canopy cover on completion, and a net gain target on a site with mature trees may be unachievable without retention.

How canopy is measured, and at what horizon

Canopy cover is the area of ground covered by tree crowns seen from above, expressed as a percentage of the site area. Overlapping crowns count once. Most authorities take the site area as the red line boundary, though a few exclude areas outside the applicant's control, which is worth confirming.

The horizon is the variable that changes the answer most. Three are in common use: canopy at planting, canopy at a stated interval such as 25 years, and canopy at maturity. A 12 to 14 centimetre girth nursery stock tree might have a crown of 2 metres on the day it is planted and 8 to 10 metres at 25 years, so the same scheme can be 4 per cent or 22 per cent depending on the horizon chosen.

State your horizon in the first line of the calculation and use it consistently. Where the policy is silent, 25 years is the interval most authorities accept, and it has the advantage of being a period the applicant's management obligations can realistically cover.

Species choice and mature crown assumptions

The crown diameter you assign to each species is the assumption an officer or a consultee arboriculturist will test. Use published ultimate crown spread data, cite the source in the table, and keep the assumptions conservative for trees in constrained pits.

Species selection also decides whether the canopy survives to the horizon you claimed. A scheme relying on a single species across 40 street trees carries a real risk from pest and disease, and several authorities now ask for a diversity limit such as no more than 10 per cent of one species. Mixed ages and a mix of large, medium and small canopy species reads as a scheme designed by someone who expects to be judged on the outcome.

Counting retained trees against the target

Retained trees usually count, and they are the least costly canopy you will ever provide. Existing crown spreads come from the Tree Survey prepared to BS 5837:2012, which gives crown spread by cardinal direction for each tree, so the areas can be taken straight from the survey data.

Two conditions apply in practice. The tree has to be retained in a condition where it can grow on, which means the root protection area is respected and the soil is not compacted during construction. And the tree has to be outside the pressure points that lead to later removal, such as a crown overhanging a new garden or a canopy that will shade habitable rooms. Councils have seen enough trees felled in year five under permitted rights to be sceptical about retained canopy that is in the wrong place, which is the same argument covered in retained trees and daylight to habitable rooms.

Where a retained tree is category U or in decline, count it honestly or leave it out. Claiming canopy from a tree the survey describes as having a remaining contribution of less than 10 years invites a challenge to the whole table.

Soil volume is the limiting factor

This is the part most submissions skip, and the part that determines whether the canopy you drew ever happens. A tree crown is supported by rooting volume, and the commonly used rule of thumb in urban tree design is around 0.6 cubic metres of usable soil for every square metre of projected canopy at maturity.

Run that backwards on a street tree and the numbers get uncomfortable. A crown of 8 metres diameter covers roughly 50 m², which on that rule of thumb calls for around 30 cubic metres of soil. A standard 1 metre by 1 metre by 1 metre tree pit gives 1 cubic metre. The gap is why so many street trees stall at 4 metres and stay there.

The design responses are well established: continuous soil trenches shared between trees, structural soils or cellular systems under paving, rain gardens that double as rooting volume, and verge planting in preference to pits. Whichever you use, show the volume per tree on a section or a schedule. A canopy claim without a rooting volume schedule is an assertion, and an arboriculturist reading it will say so.

Securing delivery by condition or agreement

Canopy that appears on a drawing and not on site is the failure mode authorities are guarding against, so expect the mechanism to be scrutinised as closely as the calculation.

The standard package is a landscape condition requiring planting in accordance with an approved schedule, in the first planting season after completion, with a replacement clause covering losses for five years. That is adequate for a small scheme. On anything larger, or where the canopy target is doing policy work, authorities increasingly want a longer commitment through a management plan and a planning obligation, which is the route described in long term tree management secured by planning agreement.

Three drafting points are worth attention. Specify sizes and root types, because a semi-mature tree and a whip both count as one tree in a bare schedule. Tie the replacement clause to like for like species and size, not to any tree. And where soil volumes and irrigation are the difference between survival and failure, put those in the approved details, because a replacement clause that simply re-plants into the same failing pit achieves nothing.

Presenting the calculation so an officer can check it

One table, on one page, with the workings visible. For each tree: reference, species, retained or new, crown diameter assumed, the age at which that diameter applies, the crown area in m², and the source of the crown spread figure. Then a site area figure, a total canopy area, the percentage, and the policy requirement it is measured against.

An illustrative example. On a 0.42 hectare site, four retained oaks give 310 m² of existing canopy taken from the BS 5837 survey, and 26 new trees are assumed to reach a combined 640 m² at 25 years using published crown spreads, with overlaps deducted. The total of 950 m² against 4,200 m² of site area gives about 23 per cent cover. These figures are illustrative and each crown assumption would need to be evidenced for a real site.

Cross reference the table to the planting plan and to the Landscape Strategy, so the same numbers appear in both. Where you fall short of the target, say by how much and why, and say what you are offering instead. Officers have discretion, and a short honest paragraph about a constrained site with a 6 metre building line is treated better than a calculation that quietly assumes a 15 metre crown on a tree in a paving pit.

This is general guidance, not legal advice. Check the requirement with your local planning authority, because canopy policies, horizons and measurement conventions differ between local plans.

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