A biodiversity metric is just a tool used by ecologists to measure changes in biodiversity. Crucially, metrics are only concerned with habitats and do not take protected (or other) species into consideration.

The Defra Metric works by assigning every habitat on a site a ‘unit value’ according to its relative importance for biodiversity. The unit value encapsulates the habitats biodiversity value, by considering a number of factors, such as the type of habitat, its condition and connectivity to other habitats. In this way, a very small area of a particularly valuable habitat, such as ancient woodland, can be equated to a much larger area of a less valuable habitat such as an arable field – both can be included under the label of a ‘habitat unit’. Similar provisions apply to hedgerows and other linear features which might occur on site – small lengths of high quality, connected linear features may well equate to much longer stretches of poor quality, unconnected linear features.
Using a Metric enables meaningful comparison between the existing value of a site and what will be delivered through development or management.
                                     Baseline biodiversity units = Distinctiveness x Condition x Significance x
                                                         Connectivity x Area in hectares (or length in km)

A biodiversity score is calculated based on credits assigned to each of these components and then multiplied by the area or length (if it is a linear corridor) of the site. The predicted gain is then calculated in the same way, adding-in new biodiversity elements and new components relating to risks associated to the development, regarding:

• Spatial risk: distance of offset from site
• Temporal risk: time for habitats to reach target condition
• Delivery risk: difficulty of habitat creation

These elements are applied to calculate the onsite (and offsite) post-development score:
                                          Post-development biodiversity units = Distinctiveness x Condition x
                                    Significance x Connectivity x Area in hectares (or length in km) / Spatial x
                                                                              Temporal x Delivery risks

The Biodiversity Net Gain is then a fairly simple calculation:
                       Biodiversity units created = Post development biodiversity units – Baseline biodiversity units

Defra highlights that the proposed Biodiversity Net Gain (BNG) metric is just one part of the biodiversity puzzle. 

Existing legislation protecting key species, habitats and designated sites remain. 

                                                                This means BNG is not a licence to trash a site – 

                                      If the Mitigation hierarchy is not followed the Local Authority can refuse planning permission.

The metric also does not include species composition, habitat structure or ecological functionality. These all need to be assessed to gain a full picture of the biodiversity contribution in a site, albeit, these are somewhat subjective.

Local SPD's might stipulate the inclusion of particular nest boxes / roost boxes etc, as part of their biodiversity requirements, these however do not accrue value within the Defra Metric, which is not entirely logical as a nest box built into a building, will offer a safe and secure nesting location, almost certainly safer than a nest in a newly planted hedgerow.

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