Understanding the Calculation of Biodiversity Net Gain (BNG) Credits: A Step-by-Step Breakdown

As environmental conservation becomes an increasingly important focus in development, the introduction of Biodiversity Net Gain (BNG) regulations has created a structured approach to ensuring that development projects contribute positively to biodiversity. One key element of this system is the calculation of BNG credits, which are measured using the Biodiversity Metric tool developed by Natural England. This tool quantifies the biodiversity value of land into biodiversity units, which developers can then use to meet regulatory requirements or trade in biodiversity credits.

Here’s a detailed step-by-step guide to how these biodiversity credits are calculated and how they work in practice:

Step 1: Habitat Type and Area

The first step in calculating BNG credits is to map and categorise the habitat types on the development site. These habitats can range from grassland, woodland, and wetlands to more specific types like heathland or urban green spaces. Each type of habitat has its own ecological value and is a critical factor in the metric's calculation.

• Measuring the area: The size of each habitat type, measured in hectares, is a fundamental input. Larger areas typically support more biodiversity, so the area calculation plays a key role in determining the number of biodiversity units the site possesses.

For example, a 5-hectare woodland will generally have a higher biodiversity score than a smaller plot of the same habitat.

Step 2: Habitat Condition

Next, each habitat is assessed for its condition, which is categorized from poor to excellent. The condition reflects the quality of the habitat and its ability to support wildlife. the "condition" of your habitat is determined in the opinion of the specific surveyor used and this could unfortunately leave room for disaster. Some surveyors may grant greater "conditions" to land based on their relationship to the owner. The things the surveyors will have to take into account are:

• Condition factors: Aspects such as vegetation structure, species diversity, and management practices are used to determine the condition of each habitat. A healthy, well-maintained habitat will earn a higher biodiversity unit score compared to one that is degraded or poorly managed.

  
  

For instance, a dense, mature woodland with a rich variety of species will score higher than a fragmented, poorly maintained patch of trees.

Step 3: Distinctiveness

In addition to area and condition, the distinctiveness of each habitat is rated. Distinctiveness refers to how ecologically rare or important a habitat type is. This step ensures that rare and high-value habitats receive higher biodiversity unit scores.

• High-distinctiveness habitats: Examples include ancient woodland or species-rich grasslands, which are both uncommon and critical for biodiversity. These habitats will earn significantly more biodiversity units per hectare than low-distinctiveness habitats like improved grassland, which may be more common and less ecologically significant.

  
  

Step 4: Connectivity

The connectivity of the habitat is another important factor. This measures how well the habitat connects with other natural areas or ecosystems. Well-connected habitats are more valuable because they allow species to move freely between areas, increasing genetic diversity and resilience.

• Better connectivity: A woodland that is linked to other forests or wetlands through wildlife corridors will score higher than an isolated woodland. This reflects the enhanced biodiversity value that comes from well-connected ecosystems.

  
  

Step 5: Time to Target Condition and Difficulty of Creation/Enhancement

When creating or enhancing a habitat, the time it takes for the habitat to reach its target condition and the difficulty of achieving this condition are considered. Some habitats, such as woodland, may take decades to mature, while others may establish more quickly.

• Time factor: For example, if a developer is restoring a wetland, it may take years for the habitat to fully develop into its target condition. The BNG units are adjusted to reflect this time lag.

  
  

• Difficulty factor: If a habitat is particularly challenging to create or restore, the BNG units may be adjusted further to account for the potential uncertainty of achieving the desired ecological outcome.

Step 6: Calculate Total Units (Pre- and Post-Development)

Once all these factors have been assessed, the pre-development BNG units are calculated by combining the area, condition, distinctiveness, and connectivity scores for each habitat type on the site. This provides a baseline BNG value.

Similarly, the post-development BNG units are calculated by taking into account any losses of habitat due to development, as well as any enhancements or newly created habitats. These post-development BNG units must meet or exceed the pre-development units by at least 10% to satisfy the BNG requirement.

Step 7: Achieving Net Gain

The goal of the BNG regulations is to ensure that post-development biodiversity exceeds pre-development biodiversity by at least 10%. If a developer fails to meet this threshold through on-site improvements, they must offset the shortfall by purchasing off-site biodiversity units from other landowners or by buying statutory BNG credits from the Government.

For example, if a project starts with 50 biodiversity units and enhancements increase this to 55 units, the developer would need to acquire 5 more units to reach the 10% net gain requirement. They could purchase these units from another site or, as a last resort, through the Government’s statutory credit scheme.

Example BNG credit Calculation

To bring this process to life, let’s consider an example:

• Initial Site: You have a 10-hectare site made up of degraded grassland and low-quality woodland. After using the Biodiversity Metric tool, you assess the pre-development baseline score to be 50 biodiversity units.

  
  

• Enhancement Efforts: After enhancing the grassland and restoring the woodland, the post-development score improves to 60 units. Since this is a 20% improvement, it meets the 10% net gain requirement.

  
  

If the improvements only raised the post-development score to 55 units, you would need to acquire 5 additional BNG units through off-site purchases or the Government credit scheme to meet the mandatory net gain.

Conclusion: A Structured Approach for Measurable Gains

The detailed process for calculating BNG credits ensures that biodiversity improvements are measurable, consistent, and grounded in science. By using the Biodiversity Metric tool, developers and landowners can quantify the ecological value of their sites and contribute to environmental sustainability while meeting regulatory requirements.

This approach ensures that biodiversity net gain is not just a box-ticking exercise, but a genuine contribution to restoring and enhancing ecosystems across England. It provides a transparent and standardised method for ensuring that development and nature conservation can go hand in hand, offering a blueprint for sustainable land management in the future.