Deliver more for nature with BNG

Deliver more for nature with BNG 752 565 Greengage Environmental

What is biodiversity?

Biodiversity is the measure of all living things, from plants and animals to fungi and microscopic bacteria, alongside the natural systems that support them.

It forms the basis of many essential environmental processes, regulating our world and enriching our lives. Biodiversity is not a desirable addition to modern life, but an indispensable partner in upholding environmental health, underpinning our economy, culture and wellbeing. ​

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What is biodiversity net gain?

Biodiversity net gain (BNG) makes sure development has a measurably positive impact (‘net gain’) on biodiversity. BNG requires developers to provide an increase in appropriate natural habitat and ecological features over and above that being affected, in such a way it is hoped that the current loss of biodiversity through development will be halted and ecological networks can be restored at a landscape scale.

Why do developers/local planning authorities (LPAs)/land managers need to be aware of BNG?

In England, a minimum 10% BNG target becomes mandatory for the majority of developments on 12th February 2024. This is under Schedule 7A of the Town and Country Planning Act 1990 (as inserted within Section 14 of the Environment Act 2021).

BNG will became mandatory for ‘small sites’ from April 2024 and for Nationally Significant Infrastructure Projects (NSIPs) from November 2025.

BNG assessments

In a nutshell, a biodiversity net gain assessment compares the biodiversity value of a site before and after development and calculates how much new or enhanced habitat, and of what type(s), is required to deliver sufficient biodiversity net gain.

  • Baseline habitat information is collected using UK Habitat Classification (UKHab) methodology.​
  • The (baseline and proposed) habitat types, condition, distinctiveness, strategic significance values are categorised and allocated within the statutory biodiversity metric calculator tool.
  • This provides a proxy for the baseline biodiversity value ​of the site before development (referred to as Biodiversity Units).
  • Biodiversity Units are split into three categories: Habitat Units (HU) representing area-based habitats, Hedgerow Units (HeU) representing linear-based terrestrial habitats and Watercourse Units (WU) representing linear-based aquatic habitats.
  • The baseline (before) value is compared with the proposed (after) value across the three categories to determine the percentage net gain in biodiversity value.
  • Mitigation hierarchy​ is applied within the site design (i.e. avoid, mitigate, compensate).
  • Trading rules ​are applied (i.e. requirements to ensure habitats of the same broad type or the same/higher distinctiveness are delivered are part of the BNG process, so that high value habitats cannot simply be replaced with low value habitats)​.
  • BNG Good Practice Principles are applied during the process.
  • BNG is as well as, not instead of, protected species considerations and surveys.

Applying the mitigation hierarchy

A development design and the BNG process should aim to maximise BNG delivery on-site, following the below approach. Where all options have been explored but sufficient BNG cannot be delivered fully on site, registered off-site measures, also following the mitigation hierarchy can then be implemented.

The following mitigation hierarchy approach should be applied:

  • Avoidance: avoid creating significant impacts from the outset, through measures such as careful placement of development e.g. in order to avoid impacts on habitats of highest value.
  • Mitigation: reduce the duration, intensity and/or extent of impacts (including direct, indirect and cumulative) that cannot be completely avoided, as far as is practically feasible. This may include retention or enhancement of habitats.
  • Compensation: Off-site measures, to deliver BNG elsewhere working to a spatial hierarchy (i.e. within the locality of the development site before looking at options further afield), if 10% BNG cannot be delivered fully on-site. This may take the form of delivering enhancements or habitat creation directly on other land within the same ownership, or on third party land or indirectly via a financial contribution to a third party such as a Habitat Bank. Failing this, statutory biodiversity credits are available from the government in certain circumstances but only as a last resort.

N.B. Created habitats (both on and off-site) will have to be maintained for at least 30 years, with their protection secured through a planning obligation or conservation covenant.


Biodiversity net gain is captured in the planning process under the general biodiversity gain condition which is a pre-commencement condition. Once planning permission has been granted, a biodiversity gain plan must be submitted and approved by the planning authority before commencement of the development.

Biodiversity gain plan

Planning permission includes a general biodiversity gain condition, so development cannot start until the LPA approves the biodiversity gain plan and biodiversity metric tool calculation.

A biodiversity gain plan documents how a development will achieve biodiversity net gain. It should include:

  • Completed metric tool calculation
  • Pre-development and post-development plans (showing the location of on-site habitat, the direction of north and drawn to an identified scale)

Developers must also provide: 

  • A compensation plan if the development affects irreplaceable habitats.
  • Biodiversity net gain register reference numbers if they’re buying off-site units.
  • Proof of purchase if they’re buying statutory biodiversity credits.
  • A habitat management and monitoring plan for off-site or significant on-site gains.

The LPA has 8 weeks to approve or refuse your biodiversity gain plan. If they approve it, you will have discharged (fulfilled) your general biodiversity gain condition.

Key take home messages

  • Statutory Biodiversity Metric must be used for applicable development sites after the 10% BNG target becomes mandatory (Feb 2024).
  • Engage with an ecologist early (ideally at the viability stage).
  • Explore the constraints and opportunities early on to avoid/minimise impacts on high value habitats​.
  • Don’t degrade pre-baseline survey – highest habitat value is otherwise applied by default.
  • Be aware that woodlands, wetlands and (greenish) brownfield sites have high baseline values.
  • BNG is as well as, not instead of, species surveys.
  • Mitigation hierarchy is key. ​
  • Consider the construction phase and site operation​- when planning habitat retention.
  • Offsite compensation has a 30 year commitment (Habitat Management and Monitoring Plan).
  • More than the metric – BNG assessments need to consider: trading rules, BNG good practice principles and the mitigation hierarchy.
  • While BNG is a condition, not taking it into consideration at the viability/design stages can have costly consequences.

Greengage can assist you throughout the biodiversity net gain assessment process to meet the statutory target of 10% biodiversity net gain, or secure off-site compensation and build resilience within developments to thrive in the future. Please contact Faye Durkin

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