Urban greening and net gain on the same London site

A major application in London has to satisfy urban greening and biodiversity net gain at the same time, and the two are measured in units that do not convert into each other. The urban greening factor scores the surfaces you propose. Biodiversity net gain compares the habitat on the site before work starts with the habitat that will exist afterwards, in biodiversity units, and asks for a 10 per cent uplift secured for 30 years. A planting plan tuned to one of them can fail the other.

Most schemes come unstuck on the second test, because it has a baseline and the greening factor does not.

What the urban greening factor measures

Policy G5 of the London Plan 2021 expects major development to treat urban greening as a fundamental element of site and building design, with target scores of 0.4 for predominantly residential schemes and 0.3 for predominantly commercial ones. The calculation is a weighted sum. Every surface on the site gets a factor, that factor is multiplied by the area of the surface, the products are added together and the total is divided by the site area.

The weighting runs from zero for sealed surfaces up to 1.0 for semi-natural vegetation. Check the published table in the current London Plan guidance before you rely on any single number, because boroughs mark the schedule line by line against the landscape drawing.

Nothing in that calculation looks backwards. A tarmac yard relaid as mown grass moves from zero to 0.4 and the score improves. That is the whole of the test.

What biodiversity net gain measures

Mandatory net gain sits in Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, and applied to major development from 12 February 2024 and to small sites from 2 April 2024. Most permissions now carry a deemed condition requiring the local planning authority to approve a biodiversity gain plan before development starts.

The measurement tool is the statutory biodiversity metric. Habitat parcels on the site are classified by type, scored for distinctiveness and condition, adjusted for strategic significance, and converted into biodiversity units. The post-development figure has to beat the pre-development baseline by at least 10 per cent, and the habitats that deliver the gain have to be managed and monitored for 30 years.

Two consequences follow for a London site. The baseline is whatever was there on the relevant date, so clearing a site early is both an offence risk and a scoring disaster. And the metric does not care how much of your podium is planted. It cares what habitat type that planting becomes and what condition it can realistically reach.

A Biodiversity Net Gain Assessment has to be prepared by a competent person under the Environment Act 2021 regime. An AI-assisted draft is a starting point for that person, never a replacement for the metric calculation or the habitat survey behind it.

Where one design answers both

Several moves score well twice, and they are the ones worth protecting when the budget tightens.

Standard trees in connected pits with real soil volume earn a high greening factor and, planted in sufficient numbers in a sensible matrix, contribute to the urban tree line in the metric. Species-rich perennial planting with varied structure does better than bedding in both systems. Deep-substrate roofs with a varied build-up and some bare ground score strongly for greening and read as a better habitat type than a sedum mat. Rain gardens do triple duty against surface water, greening and habitat.

The common thread is depth of growing medium and variety of structure. Thin, uniform, tidy planting is where the two tests start to diverge.

Where they pull apart

Mown amenity grass is the clearest example. It picks up a respectable greening factor for its area while sitting at the bottom of the distinctiveness scale in the metric, and if it is replacing something scrubbier you will be buying units back somewhere else.

Green walls are the second. They add to the greening score and earn very little in biodiversity units for the area involved, because the metric has limited room for vertical systems.

Green roofs are the third. They can carry a large share of a dense scheme's greening score, and for the same area they return modest biodiversity units compared with ground-level habitat of equivalent size.

And then there is the baseline itself. A neglected yard with ruderal vegetation and self-seeded scrub may carry more units than the planted scheme that replaces it, while scoring close to zero on greening as it stands. That site will pass one test easily and need off-site units or statutory credits for the other. Worth modelling early, before the capacity study hardens, which is a judgement that sits alongside the design-led approach to site capacity rather than after it.

Sequencing the two assessments

Order matters more than effort here.

1. Commission the habitat survey and ecological appraisal in the botanical season, before any clearance, and fix the baseline.
2. Run a first metric calculation against the emerging layout rather than the final one.
3. Score the greening factor on the same drawing, using the same areas, so the two schedules reconcile.
4. Test one alternative that moves area between roof, podium and ground level, and compare both outputs.
5. Lock the landscape drawing, then write the supporting statements from it.
6. Prepare the gain plan content for discharge of the condition after permission, not as an afterthought on site.

Running the metric late is the common failure. By then the podium levels are set, the soil depths are set, and the only lever left is buying off-site units.

Who is competent to prepare each

The greening factor calculation is usually prepared by the landscape designer and sits naturally inside a Landscape Strategy. There is no statutory competence rule attached to it, but the schedule has to be traceable to measured areas on a drawing, and a borough officer will recalculate it.

Net gain is different. The statutory metric expects a competent person, and anything touching protected species requires a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017. If you are drafting in-house and want the work checked before submission, the professional directory is the route to a reviewer who can sign it off.

Presenting both in one submission

One landscape drawing, two schedules, one set of areas. That is the submission that survives an officer's review.

Keep the area take-off in a single table and reference it from both the greening calculation and the metric input sheet, so a reviewer can see that the 340 m² of podium planting in one document is the same 340 m² in the other. Where the two tests disagree, say so in the text and explain the choice you made. Officers are used to the tension. What they object to is a pair of documents that quietly use different numbers for the same courtyard.

An illustrative case makes the point. On a 0.4 ha former depot with a nine-storey residential block, the designer reaches 0.42 on greening through a deep podium, an extensive roof and street trees, while the metric shows a small shortfall against the baseline scrub. The fix is not more roof area. It is a strip of species-rich grassland along the boundary and a revised management plan, which barely moves the greening score and closes the unit gap.

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