The Solent Forum

Working in partnership for the future

Biodiversity & Marine Net Gain

Biodiversity Net Gain (BNG) is an approach to development that leaves biodiversity in a better state than before. Where a development has an impact on biodiversity it encourages developers to provide an increase in appropriate natural habitat and ecological features over and above that being affected. It is hoped that the current loss of biodiversity through development will be halted and ecological networks can be restored. It will apply from November 2023 for developments in the Town and Country Planning Act 1990 and to small sites from April 2024.

The Environment Act 2021 provides for BNG by way of amendments to the Town and Country Planning Act 1990 and the Planning Act 2008. Development on land and in intertidal locations (down to MLWM) will be mandated to deliver BNG. The Act states development must achieve at least a 10% net gain in value for biodiversity – a requirement that habitats for wildlife must be left in a measurably better state than before the development. Developers must submit a ‘biodiversity net gain plan’ alongside usual planning application documents. The local authority must assess whether the 10% net gain requirement is met in order to approve it. If net gain is not achievable on-site, the plan will need to include off-site habitat enhancements, in line with the mitigation hierarchy; the local authority must be satisfied that this is secured through a planning obligation or conservation covenant. There is a separate system that applies to marine development (Marine Net Gain (MNG)) which is currently under development.

Marine Net Gain

MNG will apply to developments, or infrastructure forming part of development, beyond the Mean Low Water Mark. Many marine developments will however ultimately land onshore and will also have terrestrial and intertidal elements, they may be subject to both BNG and MNG in the future. Defra have committed to ensuring that MNG is as coherent and consistent with land-based BNG as possible. In 2022 Defra consulted on the principles of marine net gain.

There is a broad call for meaningful and strategic aspirations for the marine and intertidal space rather than just a metric. In 2021, ABPmer was commissioned by the Offshore Wind Energy and Change (OWEC) Marine Net Gain Task & Finish Group, which is working closely with Defra’s Offshore Wind Enabling Actions Programme (OWEAP), to conduct a survey amongst key stakeholders and develop recommendations for strategic targets for Marine Net. This work has led to an assumptions paper that describes a proposed approach looking at where intertidal/marine are failing and where there is a need to foster recovery.


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