Brownfield and BNG: what the data shows

There has been a lot of debate recently about Biodiversity Net Gain (BNG) and brownfield development, prompted by the government’s consultation on whether these schemes should get a special exemption.

Verna’s Mycelia software holds by far the country’s largest dataset on BNG, so we thought it would be worth asking a simple question: what does the data actually tell us about how brownfield and BNG work together in practice? The results surprised us.

We analysed data from Mycelia’s database of over 12,000 developments, drawn from more than 100 Local Planning Authorities (LPAs) across roughly half of England. The data included 1,688 brownfield developments, of which 96% were under 2.5 hectares in size.

Here are 5 things we found:

1. Brownfield achieves net gain more easily than other development types

The median net gain on brownfield sites is 46%, more than treble the 13% median across other development types.

For every Biodiversity Unit (BU) lost on a brownfield site, 1.5 BUs are delivered back on-site.

Most brownfield schemes voluntarily exceed the 10% statutory minimum, and by a wide margin.

2. Brownfield schemes rarely need the off-site market

Only 4.6% of brownfield schemes used off-site BUs, compared with 10.1% of other developments – less than half as often.

That rate held steady across sizes of brownfield sites, meaning that smaller brownfield developments are no more or less likely to need off-site units.

3. The biodiversity value of brownfield sites comes from a small part of their footprint

Across the brownfield developments we looked at, 76% of the total area was sealed or artificial surface carrying no biodiversity value.

The majority of a brownfield site’s biodiversity loss is typically concentrated into just 13% of its footprint, and mostly accounted for by four habitat types: Open Mosaic Habitat (OMH), vacant or derelict land, bare ground, and urban trees.

4. Open Mosaic Habitat is very rare

OMH is one of the harder habitats to assess and to compensate for, so it can present real challenges for developments where it occurs.

However, it is extremely rare. Only 0.6% of brownfield sites in our database have confirmed OMH. Even including those sites where Mycelia’s ecological risk analysis suggests a possibility of unrecorded OMH, the maximum number of brownfield developments potentially affected is less than 2%.

Extrapolating our data to the national picture, OMH-affected brownfield sites are likely to number fewer than 50 per year across the whole of England.

5. Creative urban features do real work

About 15% of the brownfield schemes used creative urban features: green roofs, biodiverse sustainable drainage, bioswales, rain gardens and green walls.

Where they appear, these features deliver a median of around a quarter of the scheme’s net biodiversity uplift.

So… do brownfield developments face particular challenges from BNG?

Our data shows that generally brownfield schemes find it easier and cheaper to deliver BNG. Most brownfield developments voluntarily go significantly beyond 10% BNG, and fewer than 1 in 20 brownfield schemes pay for off-site Biodiversity Units.

In general, brownfield developments are complying straightforwardly with BNG, and creating benefits for people and nature.

The real challenges are likely to be where Open Mosaic Habitat is present, but this is rare: likely no more than 50 sites per year across the country.

If the ecology and policy community wants to ease brownfield development whilst enhancing nature and communities, identifying and supporting these few sites is likely to deliver net benefits.

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