Urban greening factor and canopy cover targets

The urban greening factor is a scoring method that measures how much of a site is covered by vegetation, weighted by the quality of that cover. In London it operates through London Plan policy G5, which sets target scores of 0.4 for predominantly residential development and 0.3 for predominantly commercial development, and the urban greening factor planning requirement now appears in a growing number of local plans outside London too. It is a quantity measure. That is both its usefulness and its limit.

What the factor measures

Each surface type on the site is given a factor between 0 and 1. Semi-natural vegetation retained or established on the ground scores at the top. Sealed surfaces score zero. Everything else sits in between, with deeper substrates, more established planting and greater soil volume scoring higher.

The score is the weighted average across the site. Multiply each area by its factor, add the results, divide by the total site area. A site of 2,000 m² needs the sum of area multiplied by factor to reach 800 to hit a 0.4 target.

That arithmetic drives the design conversation, because it tells you immediately that a small area of high-scoring cover cannot compensate for a large sealed yard. Covering a quarter of a site with rain gardens does less for the score than people expect, and sealing an access road costs more than they expect.

Working the calculation

Take the factors from the adopted policy or supplementary document rather than from memory, because the tables differ between the London Plan and the local plans that have adapted it.

An illustrative worked example, using assumed factors that you should replace with the adopted values for your area:

That gives 580 divided by 2,000, or 0.29, well short of a 0.4 residential target. Moving 200 m² of permeable paving to deeper-substrate roof planting, and upgrading the amenity grass to species-rich meadow, closes most of the gap without reducing the development.

Submit the calculation as a table in exactly this form, with areas cross-referenced to a colour-coded plan. A score asserted without the arithmetic is the most common reason the condition gets drafted tightly.

Surface types and the weightings behind them

The weightings reward three things. Soil depth, because it determines what can grow and how long it survives. Connectivity to the ground, because planting over structure has a shorter life and more constrained root volume. And habitat quality, because species-rich planting supports more than mown grass.

Hence the hierarchy. Retained ground-level vegetation beats new planting. Deep substrate beats shallow. Species-rich beats amenity grass. Green walls score modestly, because their maintenance dependency is high and their failure rate is real.

Permeable paving is where most arguments happen. It scores low because it provides a drainage benefit rather than a greening one, and applicants who have used it to satisfy the drainage strategy often assume it counts twice. It does not. A Drainage Strategy and a greening score measure different things, and the overlap is smaller than it looks.

Tree canopy and how it is counted

Canopy is normally measured as the projected canopy area at a stated point in the tree's life rather than at planting, which is why the specification of the tree and its pit matters as much as the number of trees.

Two conditions usually attach to counting canopy at maturity. The soil volume must be adequate for the species, commonly expressed as a proportion of the projected canopy area multiplied by a rooting depth, and the pits should be connected rather than isolated. The second requirement is the one that fails on podium schemes, where individual planters give a tree no room to develop.

A tree survey for the existing trees and a planting schedule for the new ones should agree with the greening calculation. Where a retained tree is credited at its current canopy, check that the construction will not reduce the root protection area to the point where that canopy cannot be sustained.

Canopy cover targets in local plans work differently from the greening factor. A target expressed as a percentage of site area under canopy is a separate test, and a scheme can pass one and fail the other.

Reconciling the score with parking and drainage

Three tensions come up on nearly every site.

1. Surface parking. Sealed bays score zero and permeable bays score little, so the score pushes parking underground or off site, which has a cost the appraisal has to carry.
2. Drainage. Attenuation basins planted as wet meadow score well, while below-ground tanks score nothing, so the drainage design and the greening score should be developed together rather than sequentially.
3. Fire service access. Hard standing for appliance access is non-negotiable and scores zero, and it is often the single largest sealed area on a constrained urban site.

The resolution is usually vertical. Deep-substrate roof planting over the parts of the building that can carry the load, with the structural implications picked up early, is the only way many urban sites reach a 0.4 score. That is a structural decision, not a landscape one, and it has to be taken before the frame is designed.

Evidence to submit and the conditions that follow

A submission that holds up contains the calculation table, a plan colour-coded by surface type with areas annotated, a roof plan showing substrate depths, a planting schedule with species and sizes, soil volume calculations for trees, and a maintenance and management plan. Set it out in the Landscape Strategy or, where the scheme is larger, in a Green Infrastructure Plan.

Expect conditions requiring the approved substrate depths to be built as drawn, the planting to be carried out in the first planting season after occupation, and replacement of failures over a defined period. On larger schemes, monitoring reports for several years after completion are common.

The practical risk is value engineering. Substrate depth is invisible once built and is the first thing to be reduced on site, which turns a 0.4 score into something lower and puts the development in breach. Photographic evidence of the build-up at installation is the least costly insurance available.

Where the measure falls short

Say plainly what the greening factor does not do. It does not measure biodiversity value, so a site can score well with amenity grass and ornamental shrubs while delivering little for wildlife. It does not measure habitat connectivity. And it does not substitute for the statutory biodiversity metric, which is a different calculation with its own rules, dealt with in a Biodiversity Net Gain Assessment.

Nor does it measure survival. A score delivered on paper and lost to a failed green wall three summers later has achieved nothing, which is why the maintenance plan deserves more attention than it usually gets.

Used well, the factor is a useful early design discipline that forces greening into the structural grid rather than onto the leftover space. Used badly, it is a spreadsheet exercise. Browse all 66 PlanScope report types to see which landscape and ecology documents your scheme is likely to need.

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