BIODIVERSITY NET GAIN BNG: UNDERSTANDING THE METRIC

Understanding the Biodiversity Metric and Biodiversity Net Gain

In England, development projects must now demonstrate measurable ecological improvement through Biodiversity Net Gain (BNG). This requirement was introduced through the Environment Act 2021 and requires most planning applications to deliver at least a 10% net gain in biodiversity.

To calculate this improvement, practitioners use the Statutory Biodiversity Metric developed by Natural England. The metric provides a structured method for measuring biodiversity losses and gains associated with development.

Understanding how the metric works is essential for landscape architects, ecologists and planners involved in the design and assessment of development proposals.

What is the Biodiversity Metric?

The Biodiversity Net Gain Metric is a standardised accounting tool used to audit and quantify biodiversity losses and gains.

Rather than measuring every species directly, the tool uses habitats as a proxy for biodiversity value. These habitats are translated into biodiversity units, which allow comparisons between existing ecological conditions and proposed interventions.

The metric compares:

• Baseline conditions (existing habitats before development)

• Post-intervention conditions (habitats created or enhanced through the project)

The difference between the two determines whether the project achieves the required net gain in biodiversity units.

The methodology follows the ecological mitigation hierarchy, which prioritises:

1. Avoiding impacts

2. Minimising impacts

3. Restoring habitats on site

4. Compensating through off-site biodiversity units where necessary

   
   

What Are Biodiversity Units?

Biodiversity units are the standard measure used within the metric to quantify habitat value.

There are three unit types:

• Area habitats (such as grassland, woodland or wetlands)

• Hedgerows

• Watercourses

Each unit type must independently achieve the required 10% net gain and cannot be traded or converted between categories.

The metric considers both:

• On-site habitat change

• Off-site habitat creation or enhancement

  
  

Key Principles of the Biodiversity Net Gain Metric

The use of the Biodiversity Net Gain Metric is governed by several important principles.

Firstly, the metric must be completed by a competent professional with appropriate ecological expertise.

Secondly, existing biodiversity protections still apply. Protected species legislation, ecological surveys and required licences remain essential elements of the planning process.

The metric should also be used in accordance with professional codes and best practice guidance. It is not intended to replace ecological expertise but rather to support informed decision-making.

Importantly, biodiversity units are a proxy measure for ecological value. While they provide a useful comparison tool, they cannot capture the full complexity of ecological systems.

Finally, habitat creation and enhancement proposals must be realistic, deliverable and locally appropriate, ensuring that biodiversity improvements can be successfully achieved over the long term.

Distinctiveness of Habitat Types

Within the metric, habitats are assigned a distinctiveness category reflecting their ecological importance.

Habitats are classified into five broad bands:

• Very High Distinctiveness – internationally scarce or highly threatened habitats

• High Distinctiveness – priority habitats requiring conservation action

• Medium Distinctiveness – semi-natural habitats with significant biodiversity benefits

• Low Distinctiveness – habitats with relatively limited biodiversity value

• Very Low Distinctiveness – habitats with little ecological value such as hardstanding

Priority habitats defined under the Natural Environment and Rural Communities Act 2006 are generally categorised within the higher distinctiveness bands.

These categories influence the number of biodiversity units assigned to each habitat parcel.

Habitat Condition Assessment

In addition to habitat type, the metric evaluates the ecological condition of habitats.

Condition assessments measure how closely a habitat approaches its ecological optimum. The metric includes more than 20 standardised condition assessment sheets covering different habitat types.

Habitats are typically classified as:

• Poor

• Moderate

• Good

These assessments must be undertaken carefully, as the condition score directly affects the biodiversity unit calculation.

Where a habitat parcel contains areas of differing condition, it should be split into smaller parcels to ensure accurate assessment.

Strategic Significance

Another important factor within the metric is strategic significance, which reflects the ecological importance of habitats in relation to local conservation priorities.

Strategic significance is often determined using Local Nature Recovery Strategies (LNRS), which identify priority locations for habitat restoration and enhancement.

Where an LNRS has not yet been published, planning authorities may refer to other local documents such as:

• Local Plans

• Biodiversity Action Plans

• Protected site management strategies

These strategies help ensure that habitat creation contributes to wider ecological networks.

Watercourses and Encroachment

Watercourses are assessed separately within the biodiversity metric and include additional multipliers.

Encroachment within riparian zones can reduce ecological function and therefore affect biodiversity unit calculations. Examples of encroachment include:

• buildings and structures

• engineered riverbanks

• footpaths or agricultural activity within riparian zones

• weirs, jetties or artificial channel modifications

Such interventions can significantly reduce habitat quality and biodiversity value if not carefully designed.

The Importance of the Biodiversity Net Gain Metric

The Biodiversity Metric represents a major shift in how ecological outcomes are evaluated within the planning system.

By introducing measurable biodiversity targets, the system aims to ensure that development contributes positively to nature recovery.

For landscape architects and environmental professionals, the metric provides a structured framework for integrating ecology, landscape design and planning policy.

Used effectively, it can help deliver landscapes that are not only functional and attractive, but also ecologically resilient and biodiversity rich.

Landscape Institute Event.