Urban Greening Factor: hitting the target score

The Urban Greening Factor is a weighted area calculation, and on a constrained London site it is won or lost in the first week of design. The Mayor recommends a target score of 0.4 for predominantly residential development and 0.3 for predominantly commercial development, measured across the whole site area. Chasing an Urban Greening Factor score after the footprint, the podium level and the drainage strategy are fixed means adding sedum to whatever roof is left, which is the most expensive and least effective way to get there.

Get the surface weightings in front of the design team at concept stage and the score usually looks after itself.

What the factor measures

The method is simple arithmetic. Each surface on the site is assigned to a type, each type carries a factor between 0 and 1, the areas are multiplied by their factors and summed, and the total is divided by the site area.

Two consequences follow from that denominator. The whole site counts, including the building footprint, so a tight site with high coverage starts from a low base and has to recover the score at roof and podium level. And the red line boundary matters, because enlarging the application site to take in an area of poor quality surface drags the score down.

The factor sits in London Plan policy G5 on urban greening, which asks major development to treat urban greening as a fundamental element of site and building design. Boroughs are expected to develop their own factor through local plan policy, so check the adopted borough plan as well as the London Plan.

Surface types and their weightings

The weightings reward depth of substrate, soil volume and ecological value, and they punish sealed surfaces.

Take the full list and the exact factors from Table 8.2 of the London Plan, because the weightings are prescriptive and the table distinguishes between cases this summary compresses. The point of showing them here is the shape of the incentive rather than the arithmetic.

Note what that shape tells you. Permeable paving scores almost nothing, so paving the courtyard in a permeable block is a drainage measure rather than a greening measure. Amenity lawn scores poorly for the maintenance it demands. A single well-detailed tree pit with proper soil volume can outperform a large area of thin sedum.

The residential and commercial targets

The recommended target scores are 0.4 for predominantly residential schemes and 0.3 for predominantly commercial ones. They are targets rather than absolute thresholds, so a scheme that falls short is not automatically refused, but it has to explain why and what it has done instead.

Mixed use schemes need a decision on which target applies, based on the predominant use. Where the split is close, state the floorspace figures and the reasoning rather than selecting the easier number and hoping nobody checks.

The honest position is that 0.4 on a dense inner London site with a high plot ratio is hard, and officers know it is hard. What does not work is a calculation that reaches the target by generous classification of surfaces that are actually thin sedum, chlorinated water or paving.

Where podium and roof greening counts

Vegetation over structure counts, provided the substrate depth meets the requirement for the category claimed. That makes substrate depth a structural and drainage decision, not a landscape one, and it has to be resolved while the podium slab is being designed.

What to settle early:

• Substrate depth by area, shown on sections, with the category claimed for each zone.
• Imposed loading at saturated weight, agreed with the structural engineer.
• Drainage layer, root barrier and waterproofing build-up, including the overall depth consequence for level thresholds.
• Irrigation strategy, because deep substrate on an exposed roof dries out and a failed roof garden scores nothing.
• Maintenance access, including how plant and materials reach the podium.

Tree soil volume is the detail that most often collapses on site. The higher factor for standard trees depends on a soil volume related to the mature canopy, which means connected pits or continuous trenches rather than isolated pots. A row of trees in individual 2 m³ pits does not attract the higher factor and will not produce the canopy the drawing shows.

Trade-offs with amenity and drainage

The same square metres are wanted by several disciplines, and the Urban Greening Factor is one claim among many.

Children's play space has its own requirement and its own quality standards, and a play area surfaced in wet-pour contributes nothing to the greening score. Amenity space for flats competes for the same courtyard. Attenuation volume wants to sit under the podium where tree pits want to be. Fire service access needs a hard surface in a particular place. And daylight to the lower flats, which is where BRE 209 guidance on dense London sites bites, constrains how much the courtyard can be enclosed.

One overlap works in your favour. Rain gardens and other vegetated drainage features score well and deliver attenuation at the same time, so a scheme that integrates the Drainage Strategy with the landscape design rather than layering one on the other gets paid twice for the same area.

Resolving these claims is what a design-led approach to capacity is for, and optimising site capacity under policy D3 is the right frame for the argument.

Evidencing the score on drawings

Officers verify the calculation against the drawings, so the two have to match exactly.

1. A dedicated greening factor plan, colour coded by surface type, with every square metre of the site allocated to a category.
2. An area schedule listing each category, the measured area, the factor claimed and the product.
3. The calculation, with the site area stated and the resulting score to two decimal places.
4. Sections showing substrate depths for each roof and podium category claimed.
5. Tree pit details with soil volumes, and a plan showing connected pits or trenches.
6. A landscape maintenance and management plan, because the score assumes the vegetation survives.

Common reasons the calculation is challenged: areas that do not add up to the site area, sedum claimed as an extensive green roof without the substrate depth to support it, trees claimed at the higher factor without soil volumes, and balcony planters counted as vegetation.

Pull the landscape case together in a Landscape Strategy and, on larger schemes, a Green Infrastructure Plan that shows how the greening connects beyond the site boundary.

Where boroughs set a higher bar

Several boroughs have adopted their own urban greening policy with a higher target, a borough-specific factor table, or additional requirements on tree canopy cover and species. Those sit in the adopted local plan and, being part of the development plan, carry full weight in the decision.

Check three things for the borough: the adopted target, whether a borough factor table replaces or supplements the London Plan table, and whether the validation checklist asks for the calculation in a particular format. Boroughs that publish a template want it used.

This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.

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