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Measuring Biodiversity Net Gain

In the second in a series on Biodiversity Net Gain, Marc Sorrentino and Trevor Ivory consider the issue of metrics.

What is it and what is it for?

The Environment Act 2021 introduced a series of measures, policies and targets intended to secure the protection and enhancement of the environment as part of the planning process.  

Specifically, the Act requires developers to demonstrate that new proposals will result in a Biodiversity Net Gain of at least 10%, secured for a period of at least 30 years. Broadly, Biodiversity Net Gain means environmental improvements which are designed to leave an area in a measurably better natural condition than it was pre-development.  

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These requirements are due to come into force in November 2023 for development under the Town and Country Planning Act 1990, and in November 2025 for Nationally Significant Infrastructure Projects under the Planning Act 2008.  On 11th January 2022, the Government published a consultation on how it is expected to work in practice.

How is it measured

The consultation explores how biodiversity gains (and losses) caused by development proposals should be measured. The current suggestion is that Natural England’s Biodiversity Metric 3.0 is likely to be the metric relied upon. 

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 hoped that mandatory assessment of Biodiversity Net Gain in accordance with the principles of Biodiversity Metric 3.0 will help inform and improve planning, design, land management and decision-making. As Biodiversity Net Gain is being applied to schemes brought forward under both the Town and Country Planning 1990 and the Planning Act 2008, the current assumption is that Biodiversity Metric 3.0 applied with relative consistency.

How the Metric works

To assess relative quality, Biodiversity Metric 3.0 scores habitats based on their type, condition and location. Essentially, scarce, in superbly conditioned and/or hold strategically significant (for nature) habitats 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 are intended to be interpreted, alongside ecological expertise and common sense, as an element of the evidence that informs plans and decisions. Natural England guidance sets out some key principles and rules which should be applied to all habitat assessments. It also makes clear that Biodiversity Metric 3.0 is not intended to operate as a total solution to biodiversity decisions, but its principles and rules are designed to help to ensure that scores are a reliable and measurable proxy for the relative biodiversity worth of a habitat or site.

Biodiversity Metric 3.0 also provides that areas of irreplaceable habitats, ancient woodlands, woodland cover and hedgerows should be given specific consideration because assessing these in the same context as other habitats may be inappropriate.

What do assessments involve?

Habitat assessments must be carried out by competent persons able to identify positive and negative indicator species for the relevant habitat at the time of year the survey is being undertaken. The assessment process will be dependent upon the specific habitat but, in practice, is likely to revolve around a site visit. However, in some cases, particularly where a site contains several distinct habitats, a desk study may also be helpful to ensure that as much information about the type, size, condition and significance of each environment is collected.

The information obtained and the developer’s proposals for the creation or enhancement of replacement habitat can then be inputted into the Biodiversity Metric 3.0 calculation tool. Based on the submitted information, the calculation tool will provide results, outputs and graphics. 


Initial concerns have been raised that Biodiversity Metric 3.0 could result in the undervaluation of certain types of important habitat. One example is the categorisation of scrubland dominated by brambles, ragwort and thistles as “degraded land”; the metric allegedly fails to distinguish between low-value scrubland and ecologically rare scrub sites. It has also been noted that sand quarries and field margins may also be undervalued by Biodiversity Metric 3.0 despite being extremely valuable habitats for wildlife.

Some conservationists and academics have also suggested that Biodiversity Metric 3.0 is open to abuse, allowing a wide variety of wildlife habitat to be categorised as ‘degraded’ resulting in no requirement to compensate its loss. 

It is worth bearing in mind that the use of Biodiversity Metric 3.0 is subject to further consultation. Further changes or updates may be announced prior to the requirements coming into force and it remains possible that the perceived shortcomings will be addressed in future.

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

Marc Sorrentino is an Associate and Trevor Ivory is a partner at DLA Piper.

See also: Biodiversity net gain and the Environment Act 2021 - Amy Truman and Henry Jeffreys examine the elements of the Environment Act 2021 which secure the provision of biodiversity net gain.

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