The urban greening factor in a Green Infrastructure Plan

The urban greening factor is a single number describing how much of a site is covered by vegetated or water surfaces, weighted by how much each surface type contributes ecologically. Multiply the area of each surface by its factor, add the results, divide by the site area, and you have the score. The London Plan sets target scores of 0.4 for predominantly residential development and 0.3 for predominantly commercial development, and a growing number of authorities outside London have adopted their own versions through local plan policy.

It is a design tool disguised as a compliance calculation. Run it once at concept stage and it changes the layout. Run it at validation stage and you are papering over a scheme that will not reach the target.

What the factor measures and where the target comes from

The calculation sits inside a green infrastructure or urban greening submission, alongside the landscape drawings and the planting schedule. Its inputs are areas and its output is a ratio.

The target comes from policy rather than from the calculation. In London, Policy G5 of the London Plan sets the interim targets and applies the requirement to major development. Outside London, look at the adopted local plan and any design or green infrastructure supplementary planning document, because the target, the weightings and the scope of application all vary and some authorities use a different metric entirely.

Confirm three things before starting. Which document sets the target. Whether the target applies to your scheme's scale and use. And what the authority takes as the denominator, which is usually the site area but is sometimes the developable area or the area within the red line excluding highway land. That last question changes the answer by more than most design decisions do.

Surface types and their weightings

The weightings reward permanence, substrate depth and ecological value. The London Plan's table is the most widely used starting point, and it runs on this logic.

Verify every figure against the current table in the adopted policy document before you rely on it, because authorities adapt the list and the values when they adopt their own version.

Two patterns fall straight out of the numbers. Sealed surface contributes nothing, so reducing the footprint of roads, parking and hard landscape lifts the score faster than adding planting. And amenity lawn scores less than half what flower-rich perennial planting does, which is why the calculation tends to push schemes away from mown grass.

Green roofs, walls and the substrate depth question

Substrate depth is where green roof claims fall apart at technical review.

An intensive green roof with a settled substrate depth of at least 150 mm attracts the higher factor, because that depth supports a wider planting palette and survives drought. A thin sedum mat laid over a waterproofing layer is a different product, ecologically and in the weighting. The distinction is settled depth after consolidation, not the depth on the manufacturer's data sheet, and the roof build-up drawing has to show it.

Green roofs also carry structural, drainage and maintenance consequences. The structural engineer needs the saturated weight early, the drainage strategy should account for the attenuation the roof provides, and the maintenance regime has to be real, because an unirrigated extensive roof that dries out in its second summer stops being vegetation at all.

Green walls score moderately and cost heavily. A modular system with irrigation is a mechanical installation with a service life and a replacement cost, and reviewers increasingly ask who maintains it in year fifteen. Climbers rooted in ground-level soil cost less, grow slower and last longer, and they usually make a better case in the submission than a panel system does.

Trees and the canopy area credited

Trees are credited by the area of their canopy, and the factor depends on the rooting volume provided.

A standard tree planted in a connected pit with a soil volume proportionate to the projected canopy of the mature tree attracts the higher factor. A tree in an undersized pit attracts less, and the reason is practical rather than punitive: a tree with insufficient rooting volume never reaches the canopy area you claimed. Where trees sit over a podium or a basement, the soil depth and drainage detail decide whether the claimed species is deliverable at all.

Use the projected canopy of the mature tree at a realistic age, state the species and the assumed canopy diameter in the schedule, and do not double count the area beneath a tree that is also claimed as another surface type. Double counting is the single most common error a reviewer finds.

Trade-offs against amenity space and layout

The factor competes with other requirements for the same square metres, and the conflicts are predictable.

Children's play space usually needs a level, durable surface. Fire appliance access needs hardstanding capable of carrying an appliance, which is the same conflict the piece on the fire appliance access section of a fire statement works through from the other direction. Refuse collection needs a drag distance and a standing area. Accessible parking bays need firm level surfaces. Each of those pushes towards sealed surface, which scores zero.

The resolutions that work are structural rather than cosmetic. Stacking parking below a podium and greening the deck. Using reinforced grass for fire appliance standing where the appliance loading and the maintenance regime allow it. Concentrating hard surface into a single well-designed route rather than spreading it. Specifying flower-rich perennial planting instead of lawn wherever mowing access is not needed.

Run the calculation against two or three layout options while the drawings are still easy to change.

Recording the calculation on a plan

A reviewer needs to check your arithmetic against a drawing, so present it so they can.

1. A colour-coded surface cover plan, with each surface type shown as a distinct hatch and keyed to the table.
2. A schedule listing each surface type, its measured area, the factor applied and the weighted area.
3. The site area used as the denominator, with a note on what is included and excluded.
4. The resulting score to two decimal places, against the target.
5. A tree schedule with species, mature canopy diameter, canopy area claimed and rooting volume provided.
6. Roof build-up sections showing settled substrate depths.

Give the areas in square metres to the nearest whole unit and make them add up to the site area. A surface cover plan whose areas do not total the site area is the fastest way to lose a reviewer's confidence in every other number in the document.

Securing delivery and maintenance by condition

A score achieved on paper and lost on site is the recurring failure, and authorities have learned to condition against it.

Expect conditions requiring the landscape scheme to be implemented in the first planting season following occupation, requiring replacement of anything that dies or is removed within five years, requiring a landscape and ecology management plan covering a longer period, and requiring green roof build-ups to be approved before installation. On larger schemes a management company and a funded maintenance regime may be secured through the section 106 agreement instead.

Write the maintenance plan as if you will be held to it, because you will. Irrigation for green walls, mowing regimes for wildflower areas that need a late summer cut and collection, and inspection of green roof drainage outlets all belong in it with frequencies attached. A landscape strategy that specifies planting without specifying who waters it in year one is an incomplete document, in the same way that the lighting section of a landscape strategy is incomplete without a maintenance and control regime.

Browse the 66 report types on PlanScope to see how a green infrastructure submission sits alongside the drainage, ecology and landscape evidence a major application carries.

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