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28 February 20225 minute read

Biodiversity Net Gain – How is it measured?

What is Biodiversity Net Gain and what is it for?

As set out in our previous articles on Biodiversity Net Gain (BNG), under the Planning Act 2008 and the Town and Country Planning Act 1990, the Environment Act 2021 was enshrined in UK legislation in November 2021. In broad terms, BNG can be defined as environmental improvements designed to leave an area in a measurably better natural condition than it was pre-development. The Act introduces a series of measures, policies and targets intended to secure environmental protection and enhancement at the planning stage of developments.

One of the key changes introduced by the Act is a requirement for developers to demonstrate that new proposals will result in a BNG of at least 10%, secured for a period of at least 30 years. This requirement is due to come into force in November 2023 for development under the 1990 Act, and November 2025 for Nationally Significant Infrastructure Projects under the Planning Act 2008. On 11 January 2022, the government published a consultation on how it is expected to work in practice.

How is it measured?

One of the key questions the consultation seeks to address is how the Biodiversity gains (and losses) caused by development proposals will be measured. The current suggestion is that, subject to any additional consultation or updates, Natural England’s Biodiversity Metric 3.0 is likely to be used.

The Biodiversity Metric 3.0 was introduced in July 2021 and, according to Natural England, provides a means of measuring and accounting for natural losses or gains resulting from development or changes in land management. Specifically, this metric calculates the existing baseline value of a site and predicts its future Biodiversity value taking account of any contributions provided by the creation or enhancement of offsite habitats.

It is envisaged that mandatory assessment of BNG in accordance with Biodiversity Metric 3.0 will inform and improve planning, design, land management and decision-making for applicable developments. As BNG is being applied to schemes brought forward under both the Town and Country Planning 1990 and the Planning Act 2008 with the same objectives, it is broadly assumed (although by no means considered a certainty) that the relevant processes and policies will be consistently applied, including the use of Biodiversity Metric 3.0.

How the metric works

To assess quality, Biodiversity Metric 3.0 scores habitats based on their type, condition and location. Essentially, habitats that are scarce, in superb condition or hold strategic significance (for nature) are likely to score highly on the metric. Biodiversity Metric 3.0 also accounts for any difficulties experienced when creating or enhancing habitat.

The metric and its outputs should be interpreted, alongside ecological expertise and common sense, as an element of the evidence that informs plans and decisions. The user guide published by Natural England alongside the metric sets out a number of key principles and rules that should be applied to all habitat assessments. This guide also makes clear that Biodiversity Metric 3.0 is not intended to operate as a total solution for Biodiversity decisions but, properly applied, its principles and rules are designed to help to ensure the scores generated are a reliable and quantifiable proxy for the relative Biodiversity worth of a habitat or site.

Biodiversity Metric 3.0 also provides that particular consideration should be given to areas of irreplaceable habitats, ancient woodlands, woodland cover and hedgerows, as it is often inappropriate to assess these in the same context as other habitats.

What do assessments involve?

Habitat assessments must be carried out by competent persons who can identify the positive and negative indicator species for the relevant habitat at the time of year the survey is being undertaken. The assessment process will depend on the specific habitat but, in practice, it’s likely to involve a site visit in most circumstances. However, in some cases, particularly where a site contains multiple distinct habitats, it may also be appropriate to carry out a desk study to ensure as much information as possible is available in relation to the type, size, condition and significance of each environment.

This information obtained can then be entered into the Biodiversity Metric 3.0 calculation tool along with proposals for the creation or enhancement of replacement habitat. Based on this information, the calculation tool then provides headline results and detailed results, outputs and graphics.


Concerns have been raised that Biodiversity Metric 3.0 can result in the undervaluation of certain types of important habitat. One particular issue relates to the categorisation of scrubland dominated by brambles, ragwort and thistles as “degraded land” without distinguishing between low-value scrubland and ecologically rare scrub sites. It has also been noted that sand quarries and field margins are also undervalued by Biodiversity Metric 3.0, despite them being extremely valuable for wildlife. Such areas may of course have other benefits, such as geological interest or amenity value.

Some conservationists and academics have suggested that Biodiversity Metric 3.0 is open to abuse, allowing a wide variety of wildlife habitat to be described as “degraded,” meaning that compensation will not be required for its loss. It’s worth remembering that the use of Biodiversity Metric 3.0 is subject to further consultation and further changes or updates may be announced before the BNG requirements come into force. So it’s possible these perceived shortcomings will be addressed as a result of responses received to the current consultation.

However, even with these issues, it appears Biodiversity Metric 3.0 is the most accepted means of objectively assessing Biodiversity losses and gains to ensure the recovery of the natural environment.

If you have any questions about the potential implications of the Biodiversity Net Gain regime for your business, please contact Marc Sorrentino, an associate in our Planning and Land Use team / Infrastructure, Construction and Transport Sector.