2. How is Biodiversity Net Gain Measured?
Biodiversity Net Gain is measured using a free biodiversity accounting tool published in Microsoft Excel format, called the DEFRA Statutory Biodiversity Metric. It is a statutory requirement to use this version of the Metric or the Small Sites Metric, no other versions will be accepted.
The metric uses habitats and ‘biodiversity units’ as a proxy for biodiversity value. These ‘units’ provide a quantifiable way to measure the biodiversity value of the planning application site both at the pre- and post-development stages of the proposal; the measurable difference in biodiversity units indicates either a net loss or net gain in biodiversity.
There are three types of biodiversity units, which are calculated in three separate ‘modules’ of the metric: area units, hedgerow units and watercourse units. A 10% gain must be achieved separately for each unit type present on site.
Metric calculations must follow the methodology in the Statutory Biodiversity Metric/Small Sites Metric User Guides and be informed by Statutory Condition Assessment Sheets.
Major developments
The Statutory Biodiversity Metric must be used to measure BNG. This should be completed by a ‘competent person’, suitably trained, qualified and experienced; generally considered to be an ecologist.
Minor developments
The Small Sites Metric (SSM) can be used for minor planning applications and should be completed by a ‘competent person’. This person does not need to be an ecologist but should be a person capable in identifying habitats present on the site before the development and identifying the management requirements for habitats which will be created or enhanced within the landscape design. The competent person must carry out the habitat survey and assessment and be able to confidently identify the habitats likely to occur in a given geographic location at the time of year the survey is undertaken.
Not all small sites are eligible to use the SSM. The SSM cannot be used if the development site includes the following:
- baseline habitats on site are not included within the SSM (habitats of high distinctiveness and above)
- habitats on site are priority habitats (except for some hedgerows and arable filed margins)
- proposed habitats are not included within the SSM (habitats of high distinctiveness and above)
- the proposals result in watercourse encroachment
- there are statutory protected sites or habitats present on site
- there are European protected species present, e.g., bats
The applicant should note that they may also choose to use the Statutory Biodiversity Metric irrespective of the application type, habitats or species present and that there may be advantages in doing so in respect to lower baseline (pre-development) unit scores compared with the Small Sites Metric.
How to assess ‘Strategic Significance’
Strategic significance is one of the habitat characteristics which must be recorded within the Statutory Metric and the Small Sites Metric to calculate the biodiversity value of a site. It describes the local significance of the habitat based on its location and the habitat type.
The strategic significance of a habitat type will, in due course, be defined on a local basis through Local Nature Recovery Strategies (LNRSs). The Devon County LNRS is currently being developed and is expected to be completed in the Spring of 2025.
In the meantime, Devon County Council guidance (see below) should be consulted in accordance with the Statutory Biodiversity Metric User Guide to assist in allocating strategic significance on a site-specific basis. Other local plan documents may also provide justification for allocating a medium or high strategic significance, e.g., habitats located within the Clyst Valley Regional Park.
When applying a high or medium significance for a habitat type you should record the justification/name of the plan specified in the metric user comment section.
High strategic significance:
- Habitats within the areas identified on the Rebuilding Devon’s Nature Map i.e., within a Strategic Nature Area* or river corridor – currently identified as within 20m either side of a larger river or 10m either side of a smaller water course (consultant to justify 20 or 10m within the comments section of the Metric)
- Habitats which provide foraging opportunities within a Sustenance Zone around a greater or lesser horseshoe bat, Bechstein’s bat, grey long-eared bat maternity roost or supports cirl buntings within 250m breeding zones*
- Habitats which provide ecological extensions, buffers or links to designated sites and/or Priority Habitats
- Habitats which fall within the biodiversity map (showing ecological networks, stepping stones and local wildlife rich habitats) in the relevant Local Plan, where this exists
* mapped on the Devon County Council Environment Viewer
Medium strategic significance:
- Habitats which clearly contribute to the ecological functionality within a landscape e.g., buffering priority habitats, providing connectivity, supporting cirl buntings in cirl 2km zones or providing bat flight lines. Justification must be given in the comments section of the Biodiversity Metric
Low strategic significance:
- Not in any of the areas listed above
What is Significant Onsite Biodiversity Net Gain?
Defined as areas of habitat enhancement which contribute significantly to the proposed development’s BNG, relative to the biodiversity value before development.
Enhancement types could include:
- habitats of medium or higher distinctiveness
- habitats of low distinctiveness which create a large number of biodiversity units relative to the biodiversity value of the site before development
- habitat creation or enhancement where distinctiveness is increased relative to the distinctiveness of the habitat before development
- areas of habitat creation or enhancement which are significant in area relative to the size of the development
- enhancements to habitat condition, for example from poor or moderate to good
Significant onsite gains must be secured by a legal agreement or a planning condition. This will be determined on a site-by-site basis depending on the type of enhancement achieved.
Significant onsite habitat enhancements are likely to form an integral part of the development, such that the applicant would be expected to include the following information with their original application:
- landscape proposal plans
- draft Habitat Management and Monitoring Plan
- statutory biodiversity metric/small sites metric calculations and condition assessment forms showing both pre and post development biodiversity units
The LPA will be responsible for monitoring the site to determine whether the landowner is meeting their obligations.