High Court offers guidance on 'outdated' versions of Natural England's Biodiversity Metric

The recent case of Save Bristol Gardens Alliance v Bristol City Council [2025] EWHC 3191 (Admin) provides guidance on the Court’s approach to the use of ‘outdated’ versions of Natural England’s Biodiversity Metric in planning applications. As discussed in previous articles, the Biodiversity Metric is used by developers to assess and demonstrate their site’s compliance with the 10% biodiversity net gain requirement under legislation.

The Claimant challenged the grant of planning permission for the redevelopment of Bristol Zoo Gardens (the Site) on three grounds, however, here we focus solely on the first. Namely, that:

‘The Defendant acted unlawfully in adopting the planning officer’s advice and recommendation that the development’s contribution to biodiversity net gain should be measured by applying Natural England’s ‘Biodiversity Metric 3.0′ published on 7 July 2021’.

The chronology of events here is critical:

• The Landmark Practice (Landmark) were commissioned to assess the biodiversity net gain of the proposed Site. Landmark carried out their initial site survey on 21 July 2021;
• At this time, the latest iteration of Natural England’s Biodiversity Metric was version 3.0 (Version 3.0);
• Natural England (NE) published version 3.1 (Version 3.1) of the Biodiversity Metric in April 2022; and
• Notwithstanding the publication of Version 3.1, Landmark published their biodiversity net gain report (the BNG Report) for the Site based on Version 3.0 on 29 October 2022.

The BNG Report

Based on Version 3.0, the BNG Report concluded that onsite gain would stand at ‘4.53 habitat (area) units, which equates to 39.86% net gain, and a gain of 1.96 hedgerow (linear) units, representing 376.35% net gain‘.

In the BNG Report, Landmark acknowledged that the Biodiversity Metric had been updated since their initial site survey, but referred to Natural England guidance set out in Biodiversity Metric 3.1:

‘Users of the previous Biodiversity Metric 3.0 should continue to use that metric (unless required to do otherwise by their client or consenting body) for the duration of the project it is being used for as they may find that certain biodiversity unit values metric 3.1 generates will differ from those generated by Biodiversity Metric 3.0.’

The Bristol Tree Forum’s (BTF) objections to the BNG Report

BTF submitted that Biodiversity Metric 3.0 was unworkable for the purpose of calculating urban tree habitat. The issue had been remedied by Biodiversity Metric 3.1, and this latter metric showed the Site would in fact deliver a biodiversity loss of 22%. BTF concluded that Version 3.0 could still be used in respect of certain calculations within the BNG Report, but Version 3.1 should be used in relation to Urban tree habitats.

On 24 March 2023, Natural England published Biodiversity Metric 4.0 (Version 4.0). BTF stated this latest metric had ‘revolutionised the way urban trees are valued, making it clearer than ever that they are a very important habitat’. They put forward that the Site, under Version 4.0, would result in a net biodiversity loss of 12.52%.

In response, Landmark submitted a further technical note saying it was unreasonable and disproportionate to undertake updates on each release of subsequent versions of the metric. They pointed out that DEFRA themselves felt Version 3.0 was robust enough to release, and be used, to inform development across the country.

What did the Court have to determine?

The Court had to determine whether the planning officer had misled the Committee in any material way in submitting that the BNG Report was reliable, notwithstanding BTF’s objections.

The Court concluded the Committee had not been misled, and that the planning officer had exercised his judgement rationally. Mr Justice Mould set out the following reasoning in his judgment:

• NE’s advice is that the Biodiversity Metric should be used ‘at all stages of a project or scheme, from site selection and options assessment through to detailed design’. The Claimant argued that the first stage of the project must be taken from submission of planning permission (which would have meant a later version of the Biodiversity Metric being used by Landmark). The Court did not accept the Claimant’s argument. The Court stated:

‘considerable lead time…  [is] required… to work up the project from its inception to its state of readiness for submission to the local planning authority under a planning application. NE’s advice in paragraph 1.11 of the metric 3.0 User Guide is founded on that practical reality’. In short, Landmark were right to use and consider Biodiversity Metric 3.0 at the date of the site survey.

• NE’s written advice in such instances is that users should use the metric they began with unless requested to do so by their client or the consenting body. No such request was forthcoming, as Landmark had reflected in their technical note in response to BTF.
• Had BTF’s objections been irrefutable or accepted as correct by the defendant’s nature conservation officer, the decision to treat the BNG Report as reliable would have been difficult. However, on the evidence, BTF’s argument was not irrefutable. Landmark had already refuted it reasonably on the basis of NE’s guidance.
• NE’s guidance on the difference between Versions 3.0 and 3.1 did not state that the contested Table 7-2 in Version 3.0 (that dealt with urban trees) was unworkable or unusable – this was simply BTF’s argument. NE did not suggest that those using Version 3.0 should refresh their work by interpolating calculations.
• It is also worth pointing out that Landmark had re-run the biodiversity calculation applying a more precautionary assumption about the longevity of urban trees on the Site. Landmark acknowledged the proposed gain accordingly dropped from 39.86% to 36%.

Should you wish to discuss any of the issues raised in this article, please contact Edward Wilson.

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