Urban greening factor: calculating a score that passes
The urban greening factor is a weighted average across the whole red line area, not just the landscaped parts. Here is how the weightings work, where the score is won, and how to present a calculation an officer can check.
An urban greening factor calculation is a weighted average. Measure every surface on the site, multiply each area by the factor the policy assigns to that surface type, add the results and divide by the total site area. London Plan Policy G5 sets the targets that most applicants are chasing: 0.4 for predominantly residential development and 0.3 for predominantly commercial development, applied to major schemes. The arithmetic takes an afternoon. Getting from 0.22 to 0.4 on a tight urban plot takes a landscape strategy and some hard choices about the roof.
Outside London the same method is spreading through local plans, helped by the urban greening factor in Natural England's Green Infrastructure Framework for England. Check whether your authority has adopted a target and at what level, because an aspiration in a supplementary document carries less weight than a policy in an adopted plan.
What the factor actually measures
It measures the quality of greening per square metre of site, not the quantity of planting. A scheme can plant a great deal and score badly if most of the planting is species-poor mown grass, and a scheme can score well with less area if the surfaces chosen do more ecological and drainage work.
Two definitions matter before you start. The site area is the red line area on the application, so a generous red line that includes adopted highway will pull the score down. And the surface areas must sum to the site area, which means every square metre has to be assigned a type, including the footprint of the building itself at a factor of zero unless it carries a vegetated roof.
That second point is where most first attempts go wrong. People calculate the score across the landscaped areas only, get a comfortable number, and then find the officer recalculating it across the whole site.
Surface types and their weightings
The London Plan table assigns a factor to each surface type. The ones that decide most schemes are these:
| Surface type | Factor |
|---|---|
| Semi-natural vegetation retained or established on site | 1.0 |
| Wetland or open water, semi-natural and not chlorinated | 1.0 |
| Intensive green roof or vegetation over structure, settled substrate at least 150 mm | 0.7 |
| Hedges | 0.7 |
| Standard trees in connected pits with soil volume of at least two thirds of the projected canopy area | 0.7 |
| Flower-rich perennial planting | 0.6 |
| Rain gardens and other vegetated drainage elements | 0.6 |
| Extensive green roof, substrate 80 to 150 mm | 0.4 |
| Groundcover planting | 0.4 |
| Amenity grassland, species-poor and regularly mown | 0.3 |
| Extensive green roof, substrate below 80 mm | 0.3 |
| Water features, chlorinated, and unsealed gravel | 0.2 |
| Permeable paving | 0.1 |
| Sealed surfaces, concrete and asphalt | 0.0 |
Vertical greening is treated inconsistently between boroughs, so confirm with the authority how a green wall will be scored rather than assuming a factor. The same applies to any surface your landscape architect has invented a name for. If it is not in the table, agree the classification in writing before the drawings are issued.
Where a green roof earns most
The step from an extensive roof at 0.4 to an intensive one at 0.7 is the single most effective move available on a dense site, and it is a structural decision rather than a landscape one. Settled substrate depth of 150 mm or more means a dead load the frame has to carry, so it belongs in the structural brief at concept stage, not in a planning condition.
The sub-80 mm roof scoring 0.3 is worth noticing. A thin sedum mat specified for cost reasons scores barely better than mown grass and performs worse in drought. If the roof is going to be green at all, the increment from 80 mm to 150 mm usually buys more score per pound than anything else on the drawing.
Roofs are also contested space. Photovoltaic arrays, plant, access zones and amenity terraces all want the same square metres, and a biosolar arrangement with panels raised above a biodiverse substrate is the usual compromise. The arithmetic of that competition is set out in roof area arithmetic: photovoltaics against everything else.
Trees and the area you may credit
Trees score well but only on stated terms. The 0.7 factor for standard trees applies where the pits are connected and the soil volume reaches at least two thirds of the projected canopy area, which on a street tree means a continuous trench rather than a series of isolated boxes.
The area credited is the projected canopy, so the calculation needs a stated canopy diameter per species with a note of the age at which it is assumed. A drawing showing 3-metre canopies for newly planted trees and a calculation using 8-metre canopies is the kind of inconsistency that gets picked up at validation. Pick one basis, state it, and apply it across every tree.
Retained mature trees in a group with understorey planting can sit in the semi-natural vegetation category at the top of the table, which is a strong reason to design around them. Retention also keeps the arboricultural case simple, and the two assessments should share the same tree numbering.
Getting to the target on a dense site
Start from the honest baseline. On an urban infill site with a building covering 55 per cent of the plot, the maximum achievable score is capped by the roof treatment, because the ground is largely gone. The sequence that works:
- Fix the roof build-up first, because it sets the ceiling on what the rest of the site can achieve.
- Convert every area of mown grass to perennial planting or groundcover unless there is a functional reason for lawn.
- Make the drainage work twice by designing attenuation as rain gardens and vegetated swales rather than as buried tanks.
- Replace permeable paving with planting wherever a surface does not need to be trafficked.
- Only then look at podium and terrace areas, which are expensive per point gained.
Step three is where greening and drainage meet, and the overlap between vegetated drainage and carbon is covered in sustainable drainage and carbon. A scheme that designs its attenuation as surface features normally picks up both the greening score and a simpler drainage argument.
Illustrative example: a 0.25 hectare residential site, building footprint 1,300 m², of which 900 m² carries an intensive green roof at 0.7 and 400 m² is plant and access at zero. Ground level provides 400 m² of perennial planting at 0.6, 250 m² of rain garden at 0.6, 120 m² of tree canopy at 0.7 and 530 m² of sealed surface. The weighted total divided by 2,500 m² gives a score in the region of 0.43, clearing the residential target with a small margin. The figures are illustrative only.
Showing the calculation so an officer can check it
The submission needs three things that agree with each other: a drawing with each surface type hatched and keyed, a schedule listing area, factor and weighted area by type, and a statement of the total site area that matches the red line.
Two habits make the check painless. Put the running total at the bottom of the schedule and show the division explicitly, rather than presenting a final figure. And cross-reference each surface type to the landscape drawing number and revision, because landscape drawings change late and a score calculated against revision B is worthless when revision D is approved.
Where the scheme also carries a Sustainability Statement, the greening score belongs there or in the landscape documents, and the same figures should not appear differently in two places. Officers read both. A separate Green Infrastructure Plan is the right home for the wider connectivity and habitat argument that the score alone does not capture.
Maintenance commitments that follow the score
A score is a design promise, and authorities secure it. Expect a condition requiring a landscape and ecological management plan, typically covering an establishment period of five years with replacement planting, and expect it to be checked.
Three commitments worth planning for rather than conceding later: irrigation provision for roof planting, which drought summers have made a live issue; access arrangements for maintaining roofs and walls safely, which affects the design of the parapet and the fall arrest system; and a named party responsible for the planting after the development is handed over, which on a build-to-rent scheme is straightforward and on a scheme sold off in units is not.
The uncomfortable part is that a factor score says nothing about whether the planting survives. A thin extensive roof scoring 0.4 on paper and dying in its second summer has delivered less than species-poor grass. Specify for the microclimate you actually have, and say so in the management plan.
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Frequently asked questions
What urban greening factor score do I need?
London Plan Policy G5 sets targets of 0.4 for predominantly residential development and 0.3 for predominantly commercial development, applied to major schemes. Outside London, check whether the adopted local plan has set a target, since an aspiration in a supplementary document carries less weight than an adopted policy.
Is the score calculated across the whole site or just the landscaping?
The whole site. The divisor is the red line site area, and every square metre has to be assigned a surface type, including the building footprint at a factor of zero unless it carries a vegetated roof. Calculating across landscaped areas only produces a flattering figure that the case officer will recalculate.
How much better is an intensive green roof than an extensive one?
An intensive roof with settled substrate of at least 150 mm scores 0.7, an extensive roof between 80 and 150 mm scores 0.4, and one below 80 mm scores 0.3. The step up to intensive is the most effective single move on a dense site, but it is a structural load decision that belongs in the concept brief.
How is tree canopy area credited?
Standard trees score 0.7 where pits are connected and soil volume reaches at least two thirds of the projected canopy area, which usually means a continuous trench. The area credited is the projected canopy, so state the canopy diameter assumed per species and the age it relates to, and use the same basis on the drawing and in the schedule.
How do green walls score?
Vertical greening is treated inconsistently between boroughs, so agree the classification with the authority in writing before the drawings are issued rather than assuming a factor. The same caution applies to any proprietary surface build-up that does not map onto a category in the policy table.
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.