Counting trees in an urban greening factor score
Trees earn their greening factor score through projected canopy area, but the factor applied depends on the soil volume you give them. Here is how the calculation works and where it goes wrong.
An urban greening factor trees score is an area calculation, so the first thing to fix is what area a tree actually earns. In the London Plan system the credited area is the projected canopy of the tree at maturity, and the factor applied to that area depends on the soil volume you have given it. Draw a 50-year oak into a 2 m by 1 m pit and the score will not reward you for the canopy you have drawn on the plan.
How a factor score is built
The method is the same wherever it has been adopted. Every surface on the site is assigned to a cover type, each cover type carries a factor between 0 and 1, and the weighted areas are summed and divided by the total site area.
London Plan Policy G5 sets target scores of 0.4 for predominantly residential development and 0.3 for predominantly commercial development, applied to major schemes. Several authorities outside London have adopted their own green space factor in local plans or supplementary guidance, using the same arithmetic with locally set targets and occasionally different cover types.
The score is a design tool with a policy number attached. Officers use it to test whether greening was designed in or added at the end, and a scheme that scrapes the target with green roofs alone usually reads as the latter.
Two practical points about scope. The calculation covers the whole application site, including areas you would rather forget, so access roads, bin stores and substation compounds all dilute the result. And the target applies at determination, which means any value engineering after permission that trims substrate depth or swaps a tree pit for a planter puts the scheme out of line with the approved plans.
What trees are worth against everything else
The weightings reward permanence and rooting conditions. These are the London figures most often applied in practice.
| Surface type | Factor |
|---|---|
| Semi-natural vegetation retained or established on site | 1.0 |
| Standard tree planted in a connected pit with generous soil volume | 0.8 |
| Intensive green roof, settled substrate of 150 mm or more | 0.8 |
| Extensive green roof, settled substrate of 80 mm or more | 0.7 |
| Standard tree in a restricted pit | 0.6 |
| Amenity grassland, regularly mown | 0.4 |
| Permeable paving | 0.1 |
| Sealed surface | 0 |
Two things follow. A tree in a properly connected pit outscores a green roof of the same area, and the gap between a well-rooted tree and a restricted one is the difference between 0.8 and 0.6 across a canopy that may be 50 m² or more. On a constrained site that single decision can move the whole score by two or three hundredths.
And the retained mature tree beats everything. Existing semi-natural vegetation kept on site attracts the top factor, which is the one point where the greening calculation and the Tree Survey prepared to BS 5837:2012 pull in the same direction.
Measuring canopy area without guessing
Use a species-specific mature canopy diameter from a reputable nursery specification or an arboricultural source, then take the circle area. Record the assumption next to each tree in the schedule.
An illustrative example. A semi-mature small-leaved lime specified at 8 m mature canopy diameter gives a projected canopy area of about 50 m². In a connected pit at factor 0.8 it contributes 40 m² of weighted area. In a restricted pit at 0.6 it contributes 30. Ten such trees across a scheme is a difference of 100 m² of weighted area, which on a 4,000 m² site is 0.025 on the final score. The numbers here are illustrative and your species and site will differ.
Do not overlap canopies in the calculation. Where two crowns will meet at maturity, the overlapping area is counted once. Officers check this by eye against the landscape plan, and double counting is the most common arithmetic error in submitted scores.
Retained trees and the soil volume question
Retained trees carry the score and they also carry the risk. A tree shown as retained but standing inside a construction exclusion zone that the drainage layout ignores will not survive, and the score built on it is worthless.
Soil volume is where new planting is won or lost. The rule of thumb in the London guidance is that a tree needs rooting volume related to its mature canopy area, delivered through connected pits, structural soils or root channels under paving rather than an isolated hole in a slab. Get the drainage engineer and the arboriculturist in the same meeting early. Retrofitting soil volume after the services layout is agreed is close to impossible.
Where retention and layout genuinely conflict, say so in the Landscape Strategy and show the alternatives tested. Officers accept a reasoned loss far more readily than a score that quietly assumes a tree will cope.
Where the score fights the layout
Greening factor targets push against plot efficiency, parking and podium structures. Three tensions come up repeatedly.
Basement extents. A basement under the whole footprint caps substrate depth everywhere above it, which pushes every roof and courtyard into the lower factor bands.
Fire tender access. Hardstanding sized for appliance access is a sealed surface at factor 0, and it tends to be located exactly where the landscape plan wanted trees.
Refuse and servicing. Bin stores, tracking and drop-off bays consume ground-level area that carries no factor at all.
None of these is a reason to abandon the target. They are the reason to run the calculation at concept stage, when moving a core or a ramp still costs nothing but drawing time.
What the officer will actually check
Expect the case officer, or a landscape consultee, to open the spreadsheet and test four things: that areas sum to the site area, that canopies are not double counted, that soil volumes match the factor claimed, and that the cover types on the plan match the ones in the table.
Submit the calculation as a schedule tied to a coloured plan, with each area labelled. A score given as a single number with no working is treated as unevidenced, and it usually generates a request for further information at week three.
Keep the schedule live through the design stages. The version submitted at outline rarely survives reserved matters intact, and rebuilding the calculation from scratch at that point tends to reveal that someone has been counting the same courtyard twice. Where the scheme is in a borough with its own greening guidance, read that document before the first calculation rather than after the first refusal, because the cover type list is sometimes extended locally.
Delivery is then secured by condition, typically a landscape and management condition requiring the planting specification, soil volumes and a replacement period. Where long-term maintenance is the concern, the position on tree management secured through a planning agreement is worth reading before you agree wording. Where retained trees affect internal daylight, the trade-off is set out in the note on retained trees and daylight to habitable rooms.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
What urban greening factor score does a scheme need?
London Plan Policy G5 sets target scores of 0.4 for predominantly residential major development and 0.3 for predominantly commercial major development. Authorities outside London that have adopted a green space factor set their own targets in the local plan or supplementary guidance, so check the adopted wording before assuming the London figures apply.
How is a tree's area measured for the calculation?
Use the projected canopy area of the tree at maturity, taken from a species-specific mature canopy diameter and converted to a circle area. Record the assumed diameter alongside each tree in the schedule. Where two crowns will meet at maturity, count the overlapping area once, because double counting is the most common arithmetic error.
Does a green roof score better than a tree?
Not usually. A standard tree in a connected pit with generous soil volume attracts a higher factor than an extensive green roof of the same area, and retained semi-natural vegetation attracts the top factor of all. Green roofs are valuable where ground-level space is committed to access and servicing, so treat them as a complement.
Why does soil volume change the factor?
Because the calculation rewards trees that will reach the canopy size claimed for them. A tree in a connected pit, structural soil or root channel sized to its mature canopy carries the higher factor, while a tree in a restricted pit drops a band. That difference across ten trees can move a site score by a few hundredths.
How is the greening score secured after permission?
Normally by a landscape and management condition requiring the planting specification, soil volumes, an implementation timetable and a replacement period for failed stock. That is why value engineering after permission matters. Trimming substrate depth or swapping a tree pit for a planter puts the built scheme out of line with the approved plans.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.