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What is Biodiversity Net Gain?

Sweco authors: Dr Martin Brammah, Richard Webber-Salmon, Lorna McDonald

Published on: 09/01/2023

In keeping with the ambitions of the UK Government’s 25 Year Plan to Improve the Environment, Biodiversity Net Gain, commonly referred to as BNG, is an approach to development that seeks to “leave the environment in a better state than we found it”.

The Practical Guide to BNG produced by CIEEM, IEMA and CIRIA defines BNG as “development that leaves biodiversity in a better state than before, and an approach where developers work with local governments, wildlife groups, landowners and other stakeholders in order to support their priorities for nature conservation”.

Essentially, the BNG process involves quantifying the biodiversity value of a development site pre- and post-development to assess in a robust, consistent and transparent way, the impact on biodiversity that a proposed development will have. Armed with this knowledge developers can then identify what steps to take to ensure that they put back more biodiversity than has been lost during the development process.

From a legal perspective, Scotland and Wales are still developing their approach to BNG, but in England, under the adopted Environment Act 2021, developers will have to deliver a mandatory minimum 10% BNG from every development which receives Town and Country Planning Act 1990 planning permission from an as-yet-unconfirmed date, likely to be November 2023. A similar requirement will come into force for Planning Act 2008 consents (Nationally Significant Infrastructure Projects) from 2025.

In England, the biodiversity value of sites is calculated using the Biodiversity Metric developed by Department for Environment, Food & Rural Affairs (Defra), which assigns a number to each parcel of habitat type found on site, based on three measures of habitat value (this is sometimes referred to as ‘biodiversity quality’). These are distinctiveness (i.e. how rare the habitat is nationally); condition (from ‘Poor’ to ‘Good’); and strategic significance (i.e. whether or not the habitat is aligned to local planning policy).

Risk factors are applied to any habitats that are to be enhanced or created post-development, to account for how difficult they are to create, how long it will take them to achieve the required condition (to deliver sufficient biodiversity units for net gain) and – in the case of newly created habitats – how far from the site they are to be created. These are described at Temporal multipliers and Difficulty multipliers in the Biodiversity Metric.

In simple terms, the three steps of a Biodiversity Metric calculation are as follows:

Step 1 – Calculate Pre-development Biodiversity Units

Area of habitat parcel x Habitat value = Pre-development Biodiversity Units

Step 2 – Calculate Post-development Biodiversity Units

Area of habitat parcel x Habitat value x Risk factors = Post-development Biodiversity Units

Step 3 – Calculate Biodiversity Outcome

Pre-development Biodiversity Units – Post-development Units = Biodiversity Outcome

Where the outcome is negative, the development will result in biodiversity net loss; where the outcome is zero, the development will result in no net loss in biodiversity; and where the outcome is positive, the development will result in biodiversity net gain.

Developments that will result in net loss or no net loss will be required to deliver additional biodiversity units in order to achieve the minimum 10% BNG. This will either require changes to the scheme (to deliver more biodiversity units on-site); creation/enhancement of habitats off-site; or purchasing biodiversity credits from a third party (who will take responsibility of habitat creation and future management). Note that penalties may apply where biodiversity credits are created / enhanced / purchased in a different locality to the site.

To ensure that habitats achieve the necessary condition, developers must demonstrate how the habitats created or enhanced to deliver BNG will be maintained for a minimum of 30 years. The House of Lords has agreed an amendment to the Environment Bill which will allow the government to review and increase the duration for which BNG sites must be secured.


When will biodiversity net gain be mandatory?

Although the delivery of a minimum 10% BNG is not yet mandatory in England, many Local Authorities have already introduced new planning policies that require demonstration of a minimum 10% net gain for planning permission to be granted. As such, many developers have spent the past year working with Ecologists including Sweco’s consultants, Landscape Architects and landowners to meet BNG requirements as part of their planning submissions.

How to achieve biodiversity net gain

The fundamental aim of BNG is, as above, to protect and enhance biodiversity (the variety of life on Earth), not only for its intrinsic value, but also for its enormous value to the economy and our wellbeing. In the words of Sir Robert Watson (Tyndall Centre for Climate Change Research), “Everything that enables us to live comes from nature”.

But how do we achieve BNG in planning and design? An excellent starting point is to apply the 10 Good Practice Principles found the CIEEM, IEMA and CIRIA practical guide:

  • Apply the mitigation hierarchy – Do everything you can to first avoid and then minimise impacts on biodiversity. Only compensate for losses that cannot be avoided as a last resort, and aim to do this in a way that delivers the most benefit for nature conservation (in some cases better gains will be achieved off-site).
  • Avoid losing biodiversity that cannot be offset elsewhere – Avoid impacts on irreplaceable biodiversity, as these cannot be offset to achieve either no net loss or net gain.
  • Be inclusive and equitable – Engage stakeholders (particularly ecologists!) early, and involve them in designing, implementing, monitoring and evaluating the approach to net gain.
  • Address risk – Habitat risk is addressed in the Defra Biodiversity Metric in terms of time (i.e. when new habitats are created; and when they will achieve desired condition) and difficulty (i.e. how hard they are to create), however it is important to be aware of other risks, for example the availability of local off-site land for habitat creation, or the availability of suitable biodiversity credits for purchase.
  • Make a measurable net gain contribution – After applying the mitigation hierarchy, make sure you achieve a quantifiable net gain that contributes directly towards nature conservation priorities.
  • Achieve the best outcomes for biodiversity – This starts with obtaining as accurate a baseline as possible(see principle 3 above), to make sure you know exactly what habitats you have on site. This will inform what the best outcome for biodiversity will look like. In some cases this will mean replacing what is lost, like for like. In others there may be an opportunity to create different habitats that deliver greater benefits for nature conservation, or that contribute to conservation priorities at local, regional and national levels. In some cases, delivery of BNG might result in improved connectivity, with on-site habitats providing an important corridor between adjacent habitats off site, to allow better movement of wildlife in the landscape.
  • Be additional – Achieve nature conservation outcomes that demonstrably exceed existing obligations.
  • Create a net gain legacy – This will be achieved through implementation of the requisite 30-year management plan.
  • Optimise sustainability – Prioritise BNG and, where possible, optimise the wider social and environmental benefits.
  • Be transparent – Communicate all net gain activities in a transparent and timely manner, sharing the learning with all stakeholders.