What is a Biodiversity Net Gain Assessment?

What Does a Biodiversity Net Gain Assessment Involve?

A Biodiversity Net Gain (BNG) assessment evaluates the ecological value of a site before development and compares it to the predicted value after development.

This process focuses on:

• Identifying existing habitat within the site
• Measuring biodiversity value using recognised metric tools
• Assessing how development will affect site habitats
• Designing measures to achieve biodiversity improvements

The outcome is a clear, measurable calculation showing whether a development will result in a net gain.

This assessment is typically carried out by a qualified ecologist and is required for both major developments and many small sites, depending on the scope of the project.

 

Why Are BNG Assessments Required?

BNG assessments support the implementation of national policy introduced through the Environment Act 2021, which aims to ensure development contributes positively to the natural environment.

Rather than simply protecting biodiversity, the policy encourages measurable improvements. Local planning authorities rely on these assessments to confirm that development proposals align with local nature recovery strategies and broader environmental objectives.

By requiring a structured assessment, the planning system ensures that biodiversity considerations are built into development design from the outset.

 

Step-by-Step: How a BNG Assessment Works

To fully understand what a Biodiversity Net Gain assessment is, we find it helps to break down the process into key stages.

1. Define the Site and Red Line Boundary

The assessment begins by identifying the development site, usually defined by the red line boundary submitted with the planning application.

This boundary determines which habitats are included in the baseline calculation and ensures the assessment reflects the full extent of the proposed development.

2. Conduct Habitat Surveys

An ecologist will carry out field surveys to record site habitats and assess their condition. This may include grassland, woodland, hedgerows, water features, and other habitat types.

The survey data forms the foundation of the assessment and must accurately reflect current site conditions.

3. Calculate Baseline Biodiversity Value

Using recognised tools provided by Natural England, the ecologist calculates the biodiversity value of the site in units.

These calculations consider habitat type, condition, size, connectivity, and strategic significance within the wider landscape.

This establishes the baseline against which future improvements are measured.

4. Assess Post-Development Value

The next stage involves modelling how the development will affect biodiversity. This includes both habitat loss and proposed habitat creation or enhancement.

Design proposals are used to calculate the expected biodiversity value after development has been completed.

5. Identify Required Biodiversity Uplift

The difference between baseline and post-development values determines whether the project achieves the required uplift.

If the development does not meet the required increase, additional measures must be identified to close the gap.

 

Delivering Biodiversity Net Gain

Achieving biodiversity improvements requires a combination of design, ecological input, and long-term planning.

In many cases, biodiversity gains are delivered through:

• Habitat creation within the development site
• Enhancement of existing habitat features
• Off-site biodiversity projects secured by developers
• Use of the biodiversity gain sites register where appropriate

Where these options are not sufficient, developers may need to consider statutory biodiversity credits as a last resort.

 

The Role of Biodiversity Gain Plans

A key output of the assessment process is the preparation of biodiversity gain plans, which set out how biodiversity improvements will be achieved.

These plans typically include:

• Baseline and post-development biodiversity calculations
• Details of proposed habitat enhancements
• Location and design of ecological features
• Long-term management proposals

These documents are submitted to planning authorities and form part of the decision-making process.

 

Long-Term Management and Monitoring

A central principle of BNG is that biodiversity improvements must be maintained over time.

To support this, developers must prepare a habitat management and monitoring plan (HMMP). This document explains how habitats will be managed for at least 30 years following development.

 

Who Needs a Biodiversity Net Gain Assessment?

A BNG assessment is required for most developments that fall within the scope of current planning policy.

This typically includes:

• Residential and commercial developments
• Infrastructure projects
• Changes in land use affecting existing habitat
• Developments affecting greenfield or semi-natural land

Both developers and land managers may be involved in delivering biodiversity improvements, particularly where off-site habitat creation is required.

 

Why Early Ecological Input Matters

Understanding what a Biodiversity Net Gain assessment is highlights the importance of engaging ecological consultants at an early stage.

As always, Collington Winter advises early input to allow for biodiversity considerations to be incorporated into site design, reducing the need for costly changes later in the process.

 

Frequently Asked Questions

What is a Biodiversity Net Gain assessment used for?

A Biodiversity Net Gain assessment is used to measure the ecological value of a site before and after development. It helps demonstrate whether a proposal will deliver the required improvement in biodiversity as part of the planning process.

Is a BNG assessment required for small sites?

Yes, many small sites are now subject to BNG requirements, although simplified approaches may apply. The exact requirements will depend on the scale and nature of the development and should be confirmed with the local planning authority.

What happens if a development does not achieve biodiversity net gain?

If a development cannot achieve the required uplift on-site, additional measures such as off-site habitat creation may be needed. In some cases, developers may need to purchase statutory biodiversity credits to meet the shortfall.

Who prepares a Biodiversity Net Gain assessment?

A qualified ecologist typically prepares the assessment, including habitat surveys, biodiversity calculations, and supporting documentation. This ensures the information submitted with the planning application is accurate and compliant.

How long do biodiversity improvements need to be maintained?

Biodiversity improvements must generally be maintained for a minimum of 30 years. This requirement is usually secured through planning conditions or legal agreements.

 

How Can Collington Winter Support You?

Biodiversity Net Gain assessments are now a key requirement for many development projects, and understanding how to approach them is essential for securing planning permission.

Collington Winter Environmental provides expert ecological consultancy services to support BNG compliance. Our team carries out habitat surveys, prepares biodiversity assessments, and produces the documentation required for planning applications.

We work closely with developers and land managers to design practical solutions that deliver measurable biodiversity improvements while supporting project objectives.

If you need guidance on what a Biodiversity Net Gain assessment is or require support with your project, contact Collington Winter Environmental at info@collingtonwinter.co.uk or by filling out our enquiry form on this page for expert advice.