Evidencing compliance with canopy cover policies

A canopy cover policy in a planning application is answered with two numbers and the working behind them: the canopy area on the site now, and the canopy area the scheme will produce at a stated point in the future. Everything contentious lives in the second figure, because projected canopy depends on species, rooting volume and time, and an optimistic projection is the easiest thing for a tree officer to take apart. Present both numbers as areas in square metres and as a percentage of site area, and show the assumptions.

Where canopy cover requirements come from

Four sources, and a scheme may face several at once.

Local plan policy is the main one. A growing number of authorities set a canopy cover target for new development, expressed either as a percentage of site area or as a requirement for no net loss with a stated uplift. Some set it borough-wide as a strategic target and apply it through a development management policy.

National policy supports tree-lined streets in new residential development, which is a different requirement from a canopy percentage and is assessed on the drawn layout rather than on an area calculation. Supplementary planning documents, particularly householder and urban design guides, often carry the detailed expectations on species, spacing and pit design. And in London, Policy G5 of the London Plan applies the Urban Greening Factor, with target scores of 0.4 for predominantly residential development and 0.3 for predominantly commercial development.

Read the policy wording exactly. A target of "10 per cent canopy cover" means something different from "canopy cover equivalent to 10 per cent of site area at 25 years", and the second is the one you can actually evidence.

Measuring the existing canopy defensibly

Start from the tree survey rather than from imagery. A tree survey to BS 5837:2012 records crown spread, usually as radii in four directions, which converts to a canopy area per tree with a stated method. That gives a tree-by-tree figure keyed to numbered trees on a plan, which is what an officer can check.

Aerial imagery and lidar have a place for large sites and for groups, provided you state the source, the capture date and the resolution. Two cautions: summer imagery overstates canopy on deciduous sites relative to a leaf-off survey, and imagery cannot distinguish a declining tree from a healthy one.

Decide three conventions and apply them consistently:

• Whether overhanging canopy from off-site trees counts, and if so whether the off-site tree is also counted in the baseline
• Whether overlapping crowns are counted once as a merged polygon or twice as individual areas
• How hedges, scrub and shrub masses are treated, since they fall outside a tree count while still counting towards a greening score

State each convention in the report. Most disagreements with a tree officer are about method rather than arithmetic.

Projecting canopy at maturity rather than at planting

A nursery standard with a 2 m crown contributes about 3 m² of canopy on the day it is planted. The same species in open ground might reach 10 m spread, which is around 79 m², though only if the rooting environment allows it.

Derive the projection in three steps. Take a published mature crown spread for the species under conditions comparable to the site. Adjust it for the rooting volume actually available, because a tree in a 6 m³ pit will not reach the spread of the same species in open soil. Then state the time horizon: many authorities now ask for canopy at 25 years as well as at maturity, because a 60 year projection flatters the scheme and tells the officer nothing about the next planning period.

Species selection follows from the volume, not the other way round. Where the section cannot accommodate the soil a large-canopy species needs, the honest answer is a smaller species with a defensible projection, or fewer trees in better conditions. Our note on trees and drainage, covering soakaways, sewers and root ingress covers the conflicts that usually limit the space available.

Calculating and presenting net change

One table does most of the work. Keyed to tree numbers, it shows what is lost, what is retained and what is proposed.

Two refinements make it more credible. Include the projected growth of retained trees, since a semi-mature oak keeps growing and ignoring that understates the scheme. And add a mortality allowance, because assuming every tree survives 25 years is not realistic. A stated allowance with a replacement condition is more persuasive than a perfect projection.

Illustrative example: a 0.6 ha infill site with 480 m² of existing canopy, 210 m² lost to the footprint, 18 new trees projected to deliver 640 m² at 25 years, giving about 15 per cent site cover against a policy target of 12 per cent. Figures here are illustrative only.

Urban greening factor scoring alongside canopy

The Urban Greening Factor is a weighted area calculation, not a canopy calculation. Each surface type on the site carries a factor reflecting its greening value, the areas are multiplied by their factors, and the total is divided by the site area to give a score against the target.

Trees contribute through the canopy area they cast, and this is where double counting creeps in. Canopy over a lawn, over an intensive green roof or over permeable paving has to be handled according to the scoring rules rather than added on top of the surface beneath it. Set out the calculation as a table of areas and factors, with the treatment of overlaps explained, and the score falls out of it.

The two metrics can pull apart. A site can meet a canopy target with a few large trees while scoring poorly on greening because the ground is hard surfaced, or score well on greening through extensive green roofs while delivering almost no canopy. Where both apply, test both early.

Where the policy pulls against other requirements

Canopy targets collide with other policy on constrained sites, and the collisions are predictable:

1. Daylight and sunlight to habitable rooms and to gardens, where large canopies reduce received light
2. Biodiversity net gain, where canopy area in the metric is accounted differently from canopy area in a local policy
3. Drainage, where tree pits and attenuation compete for the same volume beneath paving
4. Fire service access, visibility splays at junctions and street lighting spacing
5. Utility easements, which frequently rule out the positions the landscape plan prefers

Name the conflict and resolve it in the submission rather than leaving an officer to find it. A Landscape Strategy that explains why five trees were relocated from the frontage to the rear, and what that does to the canopy figure, reads as considered design. A plan with trees drawn over a gas main reads as a drawing exercise.

The tables a tree officer will accept

Give the officer a document they can check in twenty minutes without reopening the drawings:

• A tree schedule keyed to survey numbers, with existing canopy area per tree and its retention category
• A planting schedule keyed to the landscape plan, with species, stock size, soil volume provided and projected canopy at the stated horizon
• The net change table, with site area and percentages
• A statement of the conventions used for overlaps, off-site canopy and shrub masses
• The source and date of any imagery relied on
• The mortality allowance and the proposed replacement mechanism

Conditions follow from this, and the standard wording on species, stock and establishment is covered in our note on tree planting conditions. Browse the PlanScope report library to see which tree and landscape documents a scheme is likely to need.

This is general guidance rather than 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.