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A focus on Biodiversity Net Gain

by Matt Downs

By Richard Tomlinson, Chartered Landscape Architect and Associate Landscape Manager at The Environment Partnership (TEP) Ltd.

What is Biodiversity Net Gain (BNG)? 
You may have come across the term in recent months as the government moves towards implementing a mandatory minimum 10% increase of biodiversity on all new developments by January 2024 – small scale developments and Nationally Significant Infrastructure Projects will follow later. But what does this actually mean and how is it implemented?  

The standard definition from UK Government states that “Biodiversity net gain (BNG) is a way to contribute to the recovery of nature while developing land. It is making sure the habitat for wildlife is in a better state than it was before development”.

The process, however, is like a lot of things, fairly simple to explain but rather more difficult to pull off. Of course with some developments it will be easier to achieve a net gain in biodiversity than on others. 

Above: Ancient woodland – credit TEP.

Understanding Biodiversity Baseline
The first point of call is to understand the site’s biodiversity baseline value. This is achieved by completing a structured and formal assessment using a standard tool. Natural England, along with the Department for Environment, Food & Rural Affairs (DEFRA), developed and created the Biodiversity Metric (current version is 4.0). The metric tool takes a habitat-based approach to measure a site’s biodiversity value. First a detailed site appraisal, marking the condition of existing habitat against pre-determined condition assessment criteria (UKHab), which assigns a grade of poor, moderate or good condition. The site assessment information is then uploaded into the DEFRA Metric, which then further appraises the habitat’s strategic significance within the landscape, to arrive at a total habitat value for the site. These are scored as baseline biodiversity units, which is the currency of the metric.

Figure 1 (above): Baseline habitat and pre-intervention score.

BNG proposals
The proposed development is then overlaid on to the baseline survey, which reveals the potential impacts on the baseline habitat and identifies opportunities for enhancement of retained features. For example, a retained habitat in poor condition could be enhanced to moderate condition through specific management interventions. There are a number of guiding principles and best practice documents that provide a structured approach to developing a BNG strategy. British Standard 8683.2021 sets the requirements for the BNG assessment process, and the Chartered Institute of Ecology and Environmental Management (CIEEM) has developed guiding principles for designing, implementing, maintaining and monitoring a site’s Biodiversity. 

The landscape and habitats that will be implemented post development are entered into the metric. They must demonstrate a minimum 10% net gain, along with satisfying predetermined trading rules to ensure lost habitats are replaced ‘like for like or like for better’. Following confirmation that 10% BNG is achievable, a detailed 30 year landscape and habitat management plan is produced. This states how the post development habitats will be enhanced, managed, and monitored; demonstrating the best possible chance of achieving the committed biodiversity units. 

What is the value of a Biodiversity unit? 
This would depend on what we mean by ‘value’. BNG units are assigned a value weighting depending on the habitat’s significance. For example; if the habitat is a rare or nationally significant habitat it will score a greater metric value, and therefore will require a greater level of mitigation or compensation if proposed to be lost. Some habitats are considered so valuable that they cannot be measured using the standard format, these are classed under National Planning Policy as ‘irreplaceable habitats’ i.e. Ancient Woodland. That being said, if the development scheme does include loss of irreplaceable habitat, it could only be permitted under very limited circumstances and a bespoke compensation scheme will be required. 

A monetary value can also be assigned to each biodiversity unit (BU). These can be traded on the open market and is relevant if the development cannot achieve the minimum requirement of 10% BNG within the development red line boundary. There is no hard and fast rule for the financial value assigned to a BU, however, the industry in general is beginning to set the rates. For example; the typical cost for 1 BU of other neutral grassland in moderate condition is £30k. In addition to a developer being able to purchase BU’s on the open market, the government has released statutory units. These are more expensive to purchase and require a 2 for 1 ratio, meaning they are doubly expensive compared to the open nature market. Prices for Statutory BU’s range between £42k for low distinctiveness habitat, up to £650k for high distinctiveness habitats. 

The demand for off-site biodiversity units has established the creation of ‘habitat banks’. This is where landowners and/or developers set aside areas and undertake habitat enhancements for the purpose of bringing the generated biodiversity units to the market to trade, or to offset local development that cannot meet the required 10% BNG within the redline boundary. It’s important to note that a development should always seek to achieve the required biodiversity within its own redline. To further this the metric includes a spatial risk multiplier, reducing the value of habitats that are used as offsite compensation where these are located outside the Local Planning Authority, or National Character Area. 

Above: Wildflowers on Roof – credit: TEP.

Delivering and achieving BNG
Mitigation hierarchy, establishment, adaptive management, and a robust monitoring strategy to identify potential issues in good time are all essential aspects of delivering BNG. Early collaboration between the landowner, design team, ecologists, landscape architects and land managers is essential in securing a positive outcome. Establishing the premise that the proposed development can deliver an increase in biodiversity is one thing, but this will need to be realised by skilled landscape operatives and contracts managers, who will be required to establish and maintain a wide variety of both broad and niche habitat types; whilst working closely with ecologists to monitor the success of the scheme over time. 

Some key points about BNG
• BNG is about measuring the quantity and condition of habitats on site pre and post development

• DEFRA metric uses Biodiversity units as its main currency / units for some habitats are valued higher than others

• BNG ensures an increase in habitat units post development by either on site or off site provision

• Developers should try to achieve BNG within the development red line boundary 

• Biodiversity units can be purchased, and statutory units are available from the government, but these typically cost more than units bought on the open nature market.

References available on request.

About the author:
Richard Tomlinson is a Chartered Landscape Architect and Associate Landscape Manager at The Environment Partnership (TEP) Ltd. TEP have produced a number of helpful guides for developers, landowners and management companies wanting to further understand BNG – follow @TEP_Ltd on LinkedIn, Twitter (X), Instagram and Facebook, and check for updates on our website www.tep.uk.com.

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