Since February 2024, most developments in England have been subject to mandatory Biodiversity Net Gain (BNG) under the Environment Act 2021. For developers navigating planning permission, one of the most common questions is:
What is the process for purchasing BNG units to meet planning requirements?
Where a scheme cannot fully deliver biodiversity net gain on site, developers must secure legally compliant off-site biodiversity units before their Biodiversity Gain Plan can be approved by the local planning authority (LPA).
This article explains the regulatory framework, the practical steps involved in purchasing BNG units, and how to ensure compliance with BNG regulations.
Understanding Biodiversity Net Gain Requirements
Under the current biodiversity net gain requirements, developers must demonstrate a minimum 10% uplift in biodiversity value.
The process begins with a baseline assessment of the development site using the statutory biodiversity metric. This establishes:
- The pre-development biodiversity value
- The predicted post-development biodiversity value
- The resulting gain or loss in biodiversity units
The metric calculation quantifies habitat type, ecological quality, condition and strategic significance. It ensures that biodiversity improvements are measurable and transparent.
BNG prioritises enhancing biodiversity through a structured framework known as the mitigation hierarchy:
- Avoid harm
- Minimise impacts
- Restore on site
- Compensate via off-site gains (only when necessary)
Only after exhausting opportunities for on-site enhancements should developers consider purchasing offsite BNG units.
Step 1: Maximise On-Site Gains
The first obligation is to deliver as much uplift as possible within the red line boundary of the development site.
Common BNG measures include:
- Retaining and enhancing existing on-site habitat
- Creating new habitats such as woodland, wetland or species-rich grassland
- Upgrading habitats such as modified grassland
- Integrating sustainable drainage features
For major developments and many smaller schemes, achieving significant on-site gains reduces exposure to off-site costs and aligns with sustainable development principles.
If, after applying the metric tool, there remains a shortfall in net gain units, off-site BNG must be secured.
Step 2: Identify the Required Off-Site Units
The metric output will specify:
- Number of biodiversity units required
- Habitat type (e.g. grassland, woodland, wetland)
- Whether any units are subject to trading rules
- Strategic significance requirements
Importantly, BNG regulations incorporate a biodiversity gain hierarchy and a spatial element known as the BNG proximity principle.
The BNG Proximity Principle and Spatial Multiplier
Under the BNG spatial multiplier, units located further from the development site attract a higher unit requirement.
In simple terms:
- Units within the same local planning authority area are most favourable
- Units in an adjacent LPA may be acceptable
- Units further afield require more biodiversity units to compensate
This reflects the policy objective of maintaining ecological integrity within local habitats and protecting the natural environment at a landscape scale.
The proximity principle means developers should prioritise:
- Onsite BNG
- Off-site units within the same LPA/NCA
- Units in an adjacent LPA/NCA
- Government statutory biodiversity credits (as a last resort)
Step 3: Source Suitable Units from Registered Habitat Banks
Offsite BNG units must come from land that is:
- Legally secured via a Conservation Covenant or Section 106 agreement
- Registered on the Biodiversity Gain Site Register
- Supported by a long-term monitoring plan
- Overseen by a designated responsible body or local authority
- Compliant with Natural England requirements
These sites are often referred to as habitat banks or local habitat banks.
Before purchasing BNG units, developers should verify:
- The site is formally listed on the Biodiversity Gain Site Register
- The units are available and unallocated
- The habitat type matches the trading rules
- The units meet proximity and spatial multiplier requirements
- The off-site habitat is secured for at least 30 years
Failure to confirm registration risks delays in local planning approval.
Step 4: Enter into a Legal Allocation Agreement
When developers buy BNG units, they are securing:
- A quantified number of biodiversity units
- From a specific biodiversity gain site
- Legally secured for long-term habitat creation and management
The agreement will typically:
- Allocate a defined number of net gain units
- Reference the gain site register entry
- Confirm compliance with statutory biodiversity rules
- Provide documentation for submission with the Biodiversity Gain Plan
At this stage, units are formally linked to the development project.
Step 5: Submit the Biodiversity Gain Plan
Before development can commence, the developer must submit a compliant Biodiversity Gain Plan to the local planning authority.
The plan must demonstrate:
- Pre-development biodiversity baseline
- Post-development biodiversity calculation
- On-site gains achieved
- Off-site gains secured
- Evidence of legal security
- Monitoring and reporting arrangements
The LPA will assess whether the plan satisfies statutory biodiversity net requirements before discharging the condition.
What If Off-Site Units Are Not Available?
If suitable off-site biodiversity units cannot be sourced locally, developers may purchase statutory biodiversity credits from the government via Natural England.
However:
- Statutory credits are intentionally priced higher than market units
- They are designed as a last resort
- The biodiversity gain hierarchy must be demonstrated first
For nationally significant infrastructure projects, separate but aligned statutory biodiversity requirements apply, though the core principles remain consistent.
Key Compliance Risks to Avoid
When purchasing BNG units, developers should ensure:
- The biodiversity metric calculation is accurate and signed off
- Units are sourced from a legally secured gain site
- Spatial multiplier implications are accounted for
- The biodiversity gain hierarchy is clearly evidenced
- The monitoring plan aligns with statutory expectations
- Allocation agreements are robust and auditable
Local authorities are scrutinising biodiversity gain plans carefully, and incomplete documentation may delay planning permission.
At Civity
At Civity, we support developers through the structured process of purchasing BNG units to meet planning requirements.
We:
- Review statutory biodiversity metric outputs
- Confirm unit shortfalls and trading rule implications
- Identify suitable units within the relevant local planning authority area as first priority
- Allocate offsite BNG units from registered habitat banks
- Provide all necessary documentation to support Biodiversity Gain Plan submission
- Liaise with your project ecologist where required
We do not carry out on-site ecological surveys or calculate the biodiversity metric for development land. Our role is to facilitate compliant, transparent allocation of legally secured off site units from third-party land managers.
Delivering Biodiversity Net Gain with Certainty
Purchasing BNG units is not simply a financial transaction. It is a regulatory step embedded within the planning system to ensure measurable biodiversity improvements.
To summarise, the process involves:
- Completing a statutory biodiversity baseline assessment
- Maximising onsite BNG
- Identifying any shortfall in biodiversity units
- Applying the BNG proximity principle
- Securing suitable units from the Biodiversity Gain Site Register
- Submitting a compliant Biodiversity Gain Plan
When structured correctly, off site BNG supports sustainable development while maintaining ecological integrity across local habitats.
As biodiversity net gain becomes a permanent feature of planning and development plans, early engagement and careful coordination are essential to avoid delays and ensure full compliance with BNG regulations.
Disclaimer
This article is intended for general guidance only and does not constitute legal, ecological, financial, or planning advice. Biodiversity Net Gain requirements are governed by the Environment Act 2021, associated regulations, and guidance issued by DEFRA and Natural England, which may be updated over time. Developers should seek independent professional advice specific to their development site, biodiversity metric calculations, and local planning authority requirements before purchasing BNG units or statutory biodiversity credits.
