Biodiversity Net Gain Metric 4.0 – What you need to know

Biodiversity net gain metric 4.0 is a metric that measures the difference in biodiversity between a development site before and after development, with the aim of ensuring that there is a net gain in biodiversity. The BNG metric 4.0 is a specific calculation used to quantify the net gain in biodiversity units for a development project. The biodiversity metric 4.0 came into effect in march 2023 and is the updated version of the biodiversity metric 3.1.

BNG metric 4.0 is a ratio of the area of habitat created or enhanced on-site to the area of habitat that was lost due to development. This calculation takes into account both the quantity and quality of the habitat. The goal of BNG metric 4.0 is to achieve a net gain in biodiversity by ensuring that the quantity and quality of habitat on the site is increased as a result of the development.

The update to the biodiversity net gain metric includes updated versions of the calculation tool, user guides and condition assessment sheets to name a few.

In the UK, BNG metric 4.0 has been incorporated into the planning system as a requirement for certain types of development projects under the Town and Country Planning Act 1990. It is part of a wider approach to sustainable development that aims to ensure that development projects have a positive impact on the environment and contribute to the protection and enhancement of biodiversity.

Biodiversity net gain planning condition

Under the Environment Act 2021 and through the application of the national planning policy framework (nppf), anyone looking to obtain planning permission through a planning application (with a few exceptions) must deliver at least 10% biodiversity net gain. This will become law in November 2023.

This net gain will be measured using the biodiversity net gain metric 4.0 and all types habitats must be maintained for at least 30 years. The record published by Natural England regarding the metric states, ‘Natural England will be recommending to the Secretary of State that the Biodiversity Metric 4.0 forms the basis of the statutory biodiversity metric used to underpin future mandatory biodiversity net gain as set out in the Environment Act 2021’.

Implementing BNG

If you’re uncertain about the potential impact of mandatory BNG on your planning application, it’s advisable to contact your local planning authority for detailed discussions on your plans and their alignment with BNG. If your project is likely to be impacted, we suggest that you get in touch with a skilled ecologist at the earliest opportunity.

Our ecologists can assess whether your development plans meet the necessary standards and recommend any measures that may be required to comply with the biodiversity standards. With our team’s extensive experience in completing BNG, we can offer guidance throughout the planning process. Our experienced ecologists can also visit your development site to create and develop any necessary biodiversity net gain plans, required BNG reports and offer technical consultations.

We conduct a habitat classification assessment, and the data obtained is used in a metric to measure the natural environment’s changes pre- and post-development. This metric is currently known as Defra Metric 4.0. Landscape planting plans and management plans are employed to determine post-development biodiversity measures.

Calculating Biodiversity Net Gain

Biodiversity net gain (BNG) is calculated by comparing the biodiversity value of a site before development to the value after development, focusing on losses and gains of biodiversity.

To calculate BNG, the following steps can be taken:

  1. Identify the baseline biodiversity value of the site before development. This can be done by conducting a habitat survey to identify the different habitats and species present on the site and their relative importance in terms of biodiversity.
  2. Determine the biodiversity value of the site after development. This involves assessing the quality and quantity of the habitats that will be created or enhanced as a result of the development. The value of the habitats can be assessed using a standardized biodiversity metric 4.0 calculation, which takes into account factors such as habitat quality, rarity of species, and connectivity with other habitats. For small development sites, metric users can utilise the small site metric for biodiversity, which is a simplified version of the 4.0 metric.
  3. Calculate the net gain in biodiversity. Measuring biodiversity net gain is done by subtracting the baseline biodiversity value from the biodiversity value after development. If the biodiversity value after development is higher than the baseline value, the net gain will be positive.
  4. Implement measures to enhance biodiversity on site if necessary to achieve a positive net gain. This may involve incorporating features such as green roofs, planting native vegetation, and creating wildlife corridors to improve habitat quality and connectivity.

It’s important to note that the biodiversity net gain metric 4.0 is a complex process that requires input from ecologists, planners, and other experts. It’s recommended that you work with a qualified professional to ensure that your calculations are accurate to achieve biodiversity gains.

How can Collington Winter assist?

Our team of ecologists are experts in the field of biodiversity net gain planning law and can keep you up to date with new and changing legislation. Our Ecologists are experienced in using the biodiversity net gain metric 4.0 calculation tool, so you can be assured your biodiversity plans are accurate.

Our team’s experience and qualifications in BNG means we can support our clients pre and post development in order to reach the required mandatory biodiversity net gain bracket that is becoming law in late 2023.

If you require a consultation on the biodiversity metric system, or to find out more about our other services, please contact our Ecology Director, Olivia Collington, via email at

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