Supplementary Planning Document: Biodiversity Net Gain

2.1. A key goal of biodiversity net gain is to achieve that gain within the development site. This includes avoiding any on-site loss, mitigating any loss if it cannot be avoided, remediating any lost or damaged biodiversity on-site and as a last resort, compensating for any on-site loss off-site.

This list illustrates this hierarchy for the prioritisation of achieving net gain on-site:

2.2. The latest government metric must be used by applicants as was available in the most recent survey season prior to application unless an alternative is agreed by the council prior to application submission. The calculation is derived by use of a Biodiversity Metric set by the government. This is a spreadsheet-based tool; and can be used in conjunction with a qualitative ecological assessment. The metric is used to calculate the units of biodiversity gained or lost as a result of development on a site, and that which can be gained on a potential off-set site.

The government’s Biodiversity Metric is subject to a series of 8 Principles and 6 Rules which are summarised below and are listed in full in Appendix 2. The government’s metric guidance and the prevailing law should be consulted to establish if there is an exemption from the biodiversity net gain assessment for the proposed development.

Principles of the biodiversity metric:

1. The metric does not change the protection afforded to biodiversity
2. Biodiversity metric calculations can inform decision-making where application of the mitigation hierarchy and good practice principles conclude that compensation for habitat losses is justified
3. The metric’s biodiversity units are only a proxy for biodiversity and should be treated as relative values
4. The metric focuses on typical habitats and widespread species; important or protected habitats and features should be given broader consideration
5. The metric design aims to encourage enhancement, not transformation, of the natural environment
6. The metric is designed to inform decisions, not to override expert opinion
7. Compensation habitats should seek, where practical, to be local to the impact
8. The metric does not enforce a mandatory minimum 1:1 habitat size ratio for losses and compensation but consideration should be given to maintaining habitat extent and habitat parcels of sufficient size for ecological function

Rules of the biodiversity metric:

1. Where the metric is used to measure change, biodiversity unit values need to be calculated prior to the intervention and post-intervention for all parcels of land or linear features
2. Compensation for habitat losses can be provided by creating new habitats, or by restoring or enhancing existing habitats. Measures to enhance existing habitats must provide a significant and demonstrable uplift in distinctiveness and/or condition to record additional biodiversity units
3. 'Trading down’ must be avoided. Losses of habitat are to be compensated for on a ‘like for like’ or ‘like for better’ basis. New or restored habitats should aim to achieve a higher distinctiveness and/or condition than those lost. Losses of irreplaceable or very high distinctiveness habitat cannot adequately be accounted for through the metric
4. Biodiversity unit values generated by biodiversity metric 3.0 are unique to this metric and cannot be compared to unit outputs from version 2.0, the original Defra metric or any other biodiversity metric. Furthermore, the 3 types of biodiversity units generated by this metric (for area, hedgerow and river habitats) are unique and cannot be summed
5. It is not the area/length of habitat created that determines whether ecological equivalence or better has been achieved but the net change in biodiversity units. Risks associated with creating or enhancing habitats mean that it may be necessary to create or enhanced a larger area of habitat than that lost, to fully compensate for impacts on biodiversity
6. Deviations from the published methodology of biodiversity metric 3.0 need to be ecologically justified and agreed with relevant decision makers. While the methodology is expected to be suitable in the majority of circumstances it is recognised that there may be exceptions. Any local or project-specific adaptations of the metric must be transparent and fully justified

2.3. All development proposals that require biodiversity net gain can use the government’s metric to support their biodiversity impact assessment. This will indicate whether the resultant development is likely to be positive (gain), negative (loss) or neutral in its impacts on biodiversity.

2.4. The requirements for biodiversity net gain do not replace or undermine existing habitat and species protection for protected sites or irreplaceable habitats, or for existing requirements for ecological assessments and species surveys. Decisions relating to habitats or species subject to statutory protection under national legislation remain subject to those requirements. Similarly, impacts to irreplaceable habitats shall be considered outside the biodiversity net gain system.