GuideBiodiversity Net Gain

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.

Paragraph 9 of Schedule 7A of the Town and Country Planning Act 1990 requires that where an applicant relies upon a significant increase in onsite habitat biodiversity value, the “significant habitat enhancement” must be subject to a planning condition, section 106 agreement, or conservation covenant requiring that habitat enhancement to be maintained for at least 30 years after the development is completed.

Criteria used to define significant onsite enhancement

The following criteria set out what are considered significant onsite enhancements in Devon, and hence the LPA would expect such enhancements to be maintained and monitored (with monitoring reports submitted periodically to the LPA) for at least 30 years after the development is completed.

Criteria 1: Onsite creation, enhancement, or retention, of habitats assigned medium distinctiveness or higher in the Statutory Biodiversity Metric, will be considered as significant.

 Rationale: Medium or higher distinctiveness habitats, by their nature as higher value, are likely to contribute a larger number of biodiversity units towards the delivery of the biodiversity net gain objective. It is therefore important that their creation or enhancement is fully secured to ensure the biodiversity net gain objective is met. 

Criteria 2: The onsite creation or improvement in condition of low distinctiveness habitats (excluding units delivered from by vegetated gardens), will be considered significant, where:

• the combined number of units delivered is equal to or greater than 0.5

Rationale: The goal of Biodiversity Net Gain is to deliver measurable gains in biodiversity as a result of development. Low distinctiveness habitats can represent an important part of a development’s Biodiversity Gain Plan.

How will significant onsite enhancements be secured?

On sites where medium, high or very high distinctiveness habitats are involved, 30 year maintenance and monitoring will usually be secured using a Section 106 Agreement. Exceptions may be made to this where only medium distinctiveness habitats are involved and securing for 30-years by condition alone is considered appropriate, these will typically be:

• Very small sites where the baseline consisted only of very low or low distinctiveness habitats.
• Where the target condition is no greater than moderate and the value of the medium biodiversity unit’s post-development is below 0.5.
• Where the target condition is no greater than moderate and the habitat would typically meet target condition through standard management practices due to their location or where suitably protected e.g. other neutral grassland along road verges, habitats appropriately fenced from damaging activities, small numbers of individual urban trees etc.

In cases where onsite enhancements involve only low distinctiveness habitats, then, for habitats listed below, if the target condition is good, a s106 will usually be used to secure the 30 year management and monitoring. In all other instances a condition will normally be the chosen mechanism.

• Grassland - modified grassland
• Lakes - ornamental lake or pond
• Urban - bioswale
• Urban - facade bound green wall
• Urban - ground based ground wall
• Urban - intensive green roof
• Urban - rain garden
• Urban - sustainable drainage system (SuDS)
• Woodland and forest - other coniferous woodland

Rationale: Where a target condition of good is proposed, habitat management will normally be required in order to achieve the proposed condition. Advice from the LPA on changes to the management regime may be required and ecological expertise needed to assess monitoring reports. In contrast, when poor condition is the target for a low distinctiveness habitat, monitoring will not require consideration of condition assessment criteria and changes to ecological management plans. As such monitoring intervals may be less frequent and reviewing monitoring work will not require staff with ecological expertise.

Monitoring Requirements within a Habitat Management and Monitoring Plan

Habitat Management and Monitoring Plans should be proportionate to complexity of the habitats being created. The scope of monitoring needed will vary on a case-by-case basis depending on what is being proposed.

Where significant onsite enhancements are secured via a condition, monitoring is likely to be less frequent and focus on demonstrating at the start of the 30 year period that all the proposed habitats on the site have been created as was planned. Subsequent monitoring reports would mainly be required to show that the habitats are still present onsite and in the expected condition.