Biodiversity net gain (BNG) is now a mandatory requirement for most development projects in England. Under the Environment Act 2021, developers must demonstrate at least a 10% increase in biodiversity value compared to the baseline biodiversity value of the development site.
Understanding how to calculate net gain and meet BNG requirements is now a critical part of the planning process. From biodiversity metric calculations to biodiversity gain plans and off-site units, developers must navigate a detailed and highly structured framework before planning permission can be secured.
This guide explains how biodiversity net gain is measured, how biodiversity units are calculated, and the practical steps required to achieve BNG compliance.
What Is Biodiversity Net Gain?
Biodiversity net gain BNG is a planning requirement designed to ensure development leaves the natural environment in a measurably better state than before development took place.
The process uses a standardised approach to measure biodiversity loss and enhancement using standardised biodiversity units. These calculations are carried out using the statutory biodiversity metric developed by Natural England.
Official policy guidance is set out within the UK Government’s biodiversity net gain guidance, which explains how developers must calculate biodiversity net gain and demonstrate compliance with BNG requirements.
Understanding the Biodiversity Metric
The biodiversity metric measures biodiversity value based on habitat type, habitat condition, habitat size and strategic significance. The statutory biodiversity metric tool applies these factors to calculate the number of biodiversity units present before and after development.
The metric tool assesses existing habitats within the red line boundary of the development site, including area habitats and linear habitats such as hedgerows, woodland, watercourses and rivers.
The statutory biodiversity metric is available through Natural England’s biodiversity metric framework, which includes the metric tool, calculation guidance and technical methodology.
For small developments and qualifying small sites, a simplified version known as the small sites metric may be used. This simplified approach is intended to reduce complexity for lower-risk schemes while still supporting biodiversity gain objectives.
Step 1: Establish the Baseline Biodiversity Value
The first stage in any biodiversity net gain assessment is establishing the baseline biodiversity value of the development site.
A competent person, typically a professional ecologist, will assess the site and identify all habitat parcels within the red line boundary. This includes evaluating habitat type, ecological condition and habitat size in metres squared or hectares.
The biodiversity metric then converts this information into biodiversity units, representing the ecological value of the site before development begins.
This baseline biodiversity is critical because all future calculations are measured against it.
Step 2: Assess Post-Development Biodiversity Value
Once the baseline biodiversity value has been established, the proposed development is assessed to calculate the post-development biodiversity value.
This includes evaluating proposed actions such as habitat enhancement, habitat creation and onsite delivery of new habitats. Developers must demonstrate how the scheme will support biodiversity and deliver measurable biodiversity gain.
The metric tool applies risk multipliers based on habitat difficulty, delivery timeframe and location. This means habitats created in the future are typically worth less than existing habitats lost during development.
These calculations determine how many biodiversity units are retained, enhanced or lost as part of the development.
Step 3: Apply the Biodiversity Gain Hierarchy
The biodiversity gain hierarchy sits alongside the mitigation hierarchy within planning policy. Developers are expected to first avoid biodiversity loss wherever possible before considering compensation measures.
This means site design should prioritise retaining existing habitats and achieving on-site biodiversity enhancements before relying on off-site units.
Where onsite delivery cannot fully achieve BNG, developers may need to explore alternative strategies such as habitat banks or off site BNG arrangements.
Step 4: Secure Off-Site Biodiversity Units if Required
If biodiversity units lost through development cannot be replaced on site, developers must secure off-site biodiversity units from a registered biodiversity gain site. For simple development projects, this is undertaken after planning consent is achieved. For large or complex developments, or developments affecting sensitive habitats, approximate off-site provision of biodiversity units may need to be demonstrated as part of the pre-planning submission details, with the finer details firmed up during the condition discharge stage.
These sites are managed by land managers and are legally secured for at least 30 years through conservation covenants or planning obligations with a responsible body or local authorities.
All off-site biodiversity gains must be registered with Natural England on the biodiversity gain site register before units can be allocated to a development project.
Strategic alignment with Local Nature Recovery Strategies and local habitat map priorities can also influence biodiversity metric calculations and unit value.
Step 5: Prepare the Biodiversity Gain Plan
The biodiversity gain plan is a formal document submitted to the local planning authority as part of the planning process.
It must demonstrate:
- The baseline biodiversity value of the site
- The post-development biodiversity value
- How will biodiversity gain be achieved
- Whether off-site units or statutory biodiversity credits are required
- How will habitat management and monitoring be secured
Without an approved biodiversity gain plan, development cannot lawfully commence.
What happens if BNG cannot Be Achieved?
If developers cannot achieve sufficient biodiversity gain through on-site or off-site measures, statutory biodiversity credits may be purchased from the UK Government as a last resort.
These statutory credits are intentionally priced above private market rates to encourage habitat creation and biodiversity enhancement through the private BNG market.
However, local planning authorities will expect developers to demonstrate that all other alternative strategies have been fully explored before relying on statutory biodiversity credits.
Special Considerations for Irreplaceable Habitat
Irreplaceable habitat is treated differently under the statutory metric framework. Habitats such as ancient woodland cannot simply be replaced through biodiversity units or habitat creation elsewhere.
In these cases, planning policy places greater emphasis on avoidance through careful site design and early ecological advice.
Why Early Ecological Advice Matters
Developers are strongly encouraged to seek ecological advice as early as possible in the planning process.
Early biodiversity net gain assessment can identify potential constraints, reduce delays and improve site design. It also helps developers understand how many units may be required and whether off-site solutions will be necessary.
This is particularly important for development projects involving complex habitats, significant biodiversity value or constrained development sites.
The Importance of Accurate Calculations
Accurate biodiversity metric calculations are essential to ensure planning compliance and avoid delays to planning permission.
Errors in habitat classification, condition assessment or unit calculations can result in incorrect biodiversity gain assessments and potential challenges from local planning authorities.
Using a competent person with experience in the statutory biodiversity metric and BNG policy framework is therefore critical.
Expert Support for Biodiversity Net Gain
Calculating biodiversity net gain and meeting BNG requirements involves ecological assessment, metric calculations, planning strategy and long-term habitat management.
As a specialist provider of biodiversity net gain services, Civity supports developers, land managers and local authorities through every stage of the BNG process. From biodiversity gain assessments and habitat creation strategies to off-site BNG solutions and biodiversity gain plans, their team helps ensure projects remain compliant, efficient and aligned with planning policy.
Conclusion
Understanding how to calculate net gain and meet BNG requirements is now essential for securing planning permission in England.
By accurately measuring biodiversity value, applying the biodiversity gain hierarchy and integrating biodiversity into site design from the outset, developers can achieve compliant and sustainable outcomes that support both development and nature recovery.
