Biodiversity Net Gain (BNG) is now a mandatory requirement for most development projects in England. Introduced under the Environment Act 2021 and embedded within Schedule 7A of the Town and Country Planning Act, it requires developers to deliver at least a 10% increase in biodiversity value compared to the baseline condition of a development site.
While the principle is straightforward, delivery in practice involves a structured process of ecological assessment, metric calculation, planning compliance, legal agreement, and long-term habitat management. This article explains how BNG operates in practice, from initial baseline measurement through to delivery and monitoring.
The Legal Framework Behind Biodiversity Net Gain
Mandatory BNG is secured through the planning system. A biodiversity gain condition is attached to planning permission, requiring the submission and approval of a Biodiversity Gain Plan before development can lawfully commence.
This framework is supported by:
- Schedule 7A of the Town and Country Planning Act
- DEFRA guidance and the statutory Biodiversity Metric
- The mitigation hierarchy, which prioritises avoidance, on-site delivery, and then off-site compensation
Local Planning Authorities (LPAs) are responsible for reviewing Biodiversity Gain Plans and ensuring that proposals meet statutory requirements. Increasingly, LPAs are scrutinising both the quality of metric inputs and the realism of proposed habitat delivery.
Step 1: Measuring Baseline Biodiversity Value
The first stage in delivering BNG is establishing the baseline biodiversity value of the development site.
This is undertaken by a qualified ecologist, who surveys existing habitats within the red line boundary and classifies them using the UK Habitat Classification system. These habitats are then assessed using the DEFRA Biodiversity Metric.
The metric calculates biodiversity units based on:
- Habitat type and distinctiveness
- Condition
- Area
- Strategic significance and location
This baseline provides the benchmark against which post-development biodiversity value is measured.
For smaller development sites, a simplified small sites metric may be used, although most major developments require full metric calculations.
Step 2: Designing On-Site Biodiversity Net Gain
Developers must first seek to achieve BNG on-site, in line with the mitigation hierarchy.
This involves enhancing or creating habitats within the development boundary. Examples include:
- Creating species-rich grassland
- Planting trees and native scrub
- Incorporating green roofs or sustainable drainage systems (SuDS)
- Improving the condition of retained habitats
LPAs expect developers to demonstrate that on-site opportunities have been reasonably maximised before relying on off-site solutions.
The Biodiversity Metric is used to model proposed habitat changes and calculate how many biodiversity units will be delivered on-site.
Step 3: Addressing Shortfalls Through Off-Site Biodiversity Units
Where sufficient gains cannot be achieved on-site, developers must secure off-site biodiversity units.
These units are generated from land that is managed specifically for habitat creation or enhancement, commonly referred to as biodiversity gain sites. To be valid, these sites must:
- Be secured through a legal agreement (Section 106 agreement or Conservation Covenant)
- Be registered on Natural England’s Biodiversity Gain Site Register
- Commit to habitat creation and management for a minimum of 30 years
Off-site units must be allocated to a specific development and recorded within the Biodiversity Gain Plan.
The DEFRA Biodiversity Metric applies spatial risk multipliers, meaning that units located closer to the development site, or within the same local planning authority area, are generally more valuable in meeting requirements.
Step 4: Using Statutory Biodiversity Credits as a Last Resort
If developers cannot deliver sufficient on-site or off-site biodiversity gains, they may purchase statutory biodiversity credits from the government.
These credits are deliberately priced at a premium and are intended to act as a last resort. Their use must be justified within the Biodiversity Gain Plan and is typically discouraged where market-based solutions are available.
Step 5: Submitting and Securing Approval of the Biodiversity Gain Plan
Once planning permission is achieved, the pre-commencement requirement to achieve biodiversity net gain is satisfied by detailing the approach taken (see steps 1-4 above) with a biodiversity gain plan, submitted to the relevant local planning authority. The Biodiversity Gain Plan is a statutory requirement and a central component of BNG compliance.
It must set out:
- Baseline biodiversity value
- Proposed on-site habitat enhancements
- Any off-site biodiversity units secured
- Use of statutory credits (if applicable)
The plan must demonstrate how the development will achieve at least a 10% net gain in accordance with the DEFRA Biodiversity Metric and the mitigation hierarchy.
Development cannot lawfully begin until the Biodiversity Gain Plan has been approved by the Local Planning Authority.
Step 6: Legal Agreements and Long-Term Management
All off-site biodiversity gains, and some on-site enhancements, must be secured through legally binding agreements. These typically take the form of:
- Section 106 agreements with the Local Planning Authority, or
- Conservation Covenants with a Responsible Body
A Habitat Management and Monitoring Plan (HMMP) is required to set out how habitats will be created, managed, and monitored over time.
These agreements ensure that biodiversity gains are delivered and maintained for a minimum of 30 years, with clear responsibilities for monitoring and reporting.
Practical Considerations in Delivering BNG
In practice, the delivery of BNG is often influenced by legal, commercial, and programme constraints.
Key considerations include:
- Registration and legal agreements can be a critical path, particularly for off-site units
- Developers increasingly require pre-registered, policy-compliant units to avoid delays
- The availability of suitable off-site units can vary significantly by location
- LPAs are placing greater emphasis on deliverability and long-term management certainty
As the market matures, there is a growing distinction between:
- Registered and legally secured biodiversity units, which are readily usable
Early-stage or speculative sites, which may not meet planning requirements
Special Considerations for Infrastructure and Irreplaceable Habitats
Nationally Significant Infrastructure Projects (NSIPs) follow a separate consenting regime, although similar principles apply.
Irreplaceable habitats, such as ancient woodland, are treated differently within the Biodiversity Metric. In these cases, compensation is not considered adequate, and avoidance remains the primary objective.
Delivering Biodiversity Net Gain in Practice
In practice, BNG is a structured and measurable process that integrates ecological assessment, planning, legal compliance, and long-term land management.
Successful delivery requires coordination between developers, ecologists, landowners, and local authorities to ensure that biodiversity gains are realistic, measurable, and secured for the long term.
As the BNG market continues to develop, off-site biodiversity units and formally registered gain sites are becoming an increasingly important part of how developments achieve compliance.
At Civity
At Civity, we facilitate compliant biodiversity net gain by connecting developers with pre-registered off-site biodiversity units from our landbank.
We work with landowners to bring forward biodiversity gain sites, including structuring legal agreements, supporting site registration, and ensuring alignment with DEFRA metric requirements.
For developers, we:
- Review submitted biodiversity metrics
- Identify the number and type of units required
- Allocate suitable off-site units in line with statutory trading rules
- Support completion of relevant sections of the Biodiversity Gain Plan, working alongside the project ecologist where required
Summary
Biodiversity Net Gain is now an embedded part of the planning system in England, requiring developers to demonstrate measurable and lasting improvements to biodiversity.
Its delivery relies on a combination of accurate ecological assessment, robust metric application, legally secured habitat creation, and long-term management.
As implementation progresses, the focus is increasingly on certainty, compliance, and deliverability ensuring that biodiversity gains are not only calculated, but successfully realised over a 30-year timeframe.
Disclaimer:
This article is provided for general informational purposes and reflects the biodiversity net gain framework in England at the time of writing. It does not constitute legal, ecological or planning advice. Developers and landowners should seek professional advice tailored to their specific circumstances and refer to current government and Natural England guidance to ensure compliance.
