Testing the sensitivity of the Defra Biodiversity Net Gain Metric 2.0 Tool

Biodiversity Net Gain (BNG) is currently a hotly debated topic. For the second in our series of blogs exploring Biodiversity Net Gain, Hannah Hepworth discusses the Defra Biodiversity Net Gain Metric 2.0 Tool and how sensitive it is to subjective inputs.

Before joining JBA Consulting in October, Hannah undertook research into Defra’s BNG Metric 2.0 Tool including how sensitive the results are to the user of the metric, as part of her Master Thesis .

Given that the Draft 2020 Environment Bill will potentially include a mandatory requirement of 10% BNG for all new developments, and the Biodiversity Net Gain Metric 2.0 was released in December 2019 to assist Local Planning Authorities and developers to calculate BNG, it is important to understand various aspects of its application.

The research was undertaken through two case studies:

  • one for a site that had the potential to achieve a very high BNG percentage
  • another for a site that only had the potential to just achieve the minimum BNG percentage.

Different scenarios were run for both case studies to determine the effects of each component of the metric on the overall BNG percentage achieved. The research shows the importance of getting the BNG assumptions correct – and checking the inputs and assumptions made.

A small error or incorrect assumption regarding a single parameter can potentially cause a big difference in the overall result – and therefore the outcome for a scheme.

Hannah commented:

My research produced some results that were expected – but also some that were not so obvious. Where the case study was only predicted to achieve a small BNG, one incorrect input into the metric was found to have a significant effect on the overall result, causing the case study to either be just under or over the minimum BNG percentage of 10%.

For the case study that was predicted to achieve a large BNG value, it was less likely that one incorrect input would jeopardise the BNG percentage, giving a result below the 10% target. However, in this case study an incorrect entry would have been more likely to have a significant effect on the result. In the case studies reviewed, an incorrect post-development condition input of ‘fairly good’ to ‘moderate’ in the low BNG case study, caused a decrease of 2.84% in BNG, while for the high BNG case study, under the same conditions, the BNG value decreased by 31.72%.

Want to know more?

We can advise how best to approach Biodiversity Net Gain and undertake reviews of assumptions made, to make sure the metric has been properly and fairly applied. Our teams are in a position to help both developers and Local Authorities ensure project calculations are robust and any proposals claiming BNG are rightfully entitled to claim them.

Email Laura Thomas or Rachael Brady for more information about Biodiversity Net Gain. You can also learn more about our Ecology services on our webpage,

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