Defra Statutory Biodiversity Metric or Small Sites Metric: What’s your best approach?

There’s no getting around the UK Environment Act 2021 mandate: Developers must plan to leave biodiversity at their project’s end in better shape than it was at their project’s start.

But there is a choice to be made regarding which tool to use to develop a project planning application for the Local Planning Authority, to deliver the required 10% BNG.

Will you use Defra Statutory Biodiversity Metric or Small Sites Metric?

Knowing which one to use — and how ecologists can support your project — is key to delivering a compliant and sustainable development.

Let’s break down the differences between these two metrics, their ideal applications, and how ecologists can help ensure you’re on the right path.

What are the Defra Statutory Biodiversity Metric and Small Sites Metric?

Both metrics are part of the government’s effort to streamline the BNG process, ensuring that new developments not only avoid unnecessary harm to biodiversity but also actively enhance it. Here’s a closer look at each:

Defra Statutory Biodiversity Metric 

The Defra Statutory Biodiversity Metric has also taken on additional names, due to its status as the fourth iteration of this tool (and updates have continued). It is regularly referred to using shortened monikers: BNG metric 4.0, Defra metric 4.0, or Biodiversity Metric 4.0. But regardless of naming, Defra Statutory Biodiversity Metric was selected by the UK Government as the primary tool for assessing biodiversity losses and gains in most developments. It was designed to offer a robust, consistent, and standardized way of measuring the biodiversity value of a site before, during, and after development.

Defra Statutory Biodiversity Metric:

• Is suitable for large-scale and complex developments.
• Measures biodiversity across various habitat types, including grasslands, woodlands, rivers, and wetlands.
• Incorporates both the ecological distinctiveness of habitats and their condition to calculate biodiversity value.
• Adapts to project area, whether it is in an urban location or rural.

Small Sites Metric

This tool helps developers keep biodiversity planning on track without the intricacies of the standard Statutory Biodiversity Metric tool. Small Sites Metric is a user-friendly tool that streamlines and simplifies the Biodiversity Net Gain planning process for smaller developments. It eases the burden on small-scale developers and local authorities without compromising biodiversity goals.

Small Sites Metric:

• Is less comprehensive and complex than the full Defra metric, focusing on the most relevant data for smaller projects.
• Provides easy BNG planning for small sites, typically fewer than 10 homes or sites smaller than 1 hectare.
• Is based on pre-defined habitats and typical conditions, removing the need for complex habitat surveys.
• Reduces time pressures on smaller projects.

How developers can select the right metric

As a developer, the choice between the Defra Statutory Biodiversity Metric (i.e., BNG metric 4.0, Defra metric 4.0, Biodiversity Metric 4.0) and the Small Sites Metric comes down to the scale and complexity of your project. Here are a few guidelines:

• Start with size: If your project exceeds 10 homes or spans more than 1 hectare, detailed assessment that the Defra Statutory Biodiversity Metric provides is generally required. this metric provides. For smaller projects, the Small Sites Metric is more appropriate.
• Consider ecological complexity: Developments that involve complex habitats or are located near protected sites should use the Defra Statutory Biodiversity Metric to ensure a thorough evaluation. If your project is likely to affect a variety of habitats or protected areas, the statutory metric ensures you capture the ecological intricacies that will inform robust mitigation and enhancement strategies. Simpler sites with straightforward ecological impacts, such as brownfield or urban infill sites with limited ecological value, are well suited to the Small Sites Metric.
• Determine budget and time concerns: For small-scale projects, the Small Sites Metric reduces the need for detailed surveys, helping you save time and money while still meeting biodiversity targets. It’s tailored for situations where a quick, standardized approach is both time- and cost-effective. This tool also helps smaller developers and local authorities meet statutory requirements without the need for intensive surveys or assessments.

Choosing the right metric early on in your project can prevent delays and ensure smooth compliance with planning authorities. Consider these use cases for each metric:

Sample use cases for Defra Statutory Biodiversity Metric

• Large residential or commercial developments.
• Infrastructure projects such as roads or railways.
• Sites within or near ecologically sensitive areas.
• Projects impacting multiple habitat types or extensive land.

Sample use cases for Small Sites Metric

• Minor residential projects (fewer than 10 homes).
• Commercial developments on small plots.
• Projects with limited habitat complexity.
• Sites in non-sensitive ecological areas.

How Ecologists Can Help

Whether you’re using the Defra Statutory Metric or the Small Sites Metric, working with a qualified ecologist can streamline the process and ensure that your biodiversity assessment is accurate and compliant.

When applying the Defra Statutory Metric, ecologists play a crucial role in assessing the ecological baseline of the site, calculating biodiversity units, and advising on appropriate enhancement measures. Their expertise ensures that every habitat on site is accurately identified and assessed according to its distinctiveness and condition.

For large site developers working with Defra Statutory Biodiversity Metric, ecologists can:

• Conduct comprehensive Habitat Surveys, gathering detailed data on the biodiversity value of your site.
• Interpret metric results, helping you understand the implications of your biodiversity score and guiding mitigation strategies.
• Prepare biodiversity enhancement plans, designing habitat improvements or offsite compensation strategies to meet your BNG obligations.

For small site developers using the Small Sites Metric, ecologists can:

• Simplify the process further by applying the pre-defined habitat categories and conditions of the Small Sites Metric.
• Remove the need for in-depth surveys, while still ensuring compliance with BNG requirements.

In general, for the BNG planning process, ecologists can help support either metric tool by:

• Advising if the Small Sites Metric is appropriate for your project or if you need to upgrade to the full Defra Statutory Biodiversity Metric.
• Handling the metric application to ensure your project is compliant with local authority requirements.
• Ensuring that your project is submitted with all the necessary biodiversity documentation to facilitate planning approval.

Selecting the right biodiversity metric — whether the Defra Statutory Biodiversity Metric or the Small Sites Metric — is crucial to delivering a compliant, efficient, and sustainable development. While the former is designed for larger and more complex projects, the latter provides a streamlined solution for smaller sites.

In either case, working closely with an experienced ecologist ensures that your biodiversity assessments are accurate and that your project aligns with BNG requirements, ultimately leading to better outcomes for both nature and development.

Choosing the right metric and having ecological expertise on hand will help you navigate the BNG process smoothly, avoid costly delays, and enhance the environmental credentials of your project.

See how AiDash BNGAI™ can help.