Biodiversity Net Gain (BNG): What Developers Need to Know
Since February 2024, most planning permissions in England must deliver at least 10% biodiversity net gain. Here's everything you need to know about this mandatory requirement and how it affects your development.
What is Biodiversity Net Gain?
Biodiversity Net Gain (BNG) is a planning requirement introduced by the Environment Act 2021 that mandates developments to deliver a measurable improvement in biodiversity value. The goal is to ensure that habitats for wildlife are left in a demonstrably better state after development than they were before.
In practical terms, this means most planning permissions are now granted with a condition requiring the development to achieve at least a 10% increase in biodiversity value compared to the pre-development baseline. This represents a fundamental shift in how ecological considerations are integrated into the planning system.
When Does BNG Apply?
The mandatory BNG requirement has been phased in:
- 12 February 2024: BNG became mandatory for most major developments (10+ dwellings or sites over 1 hectare)
- 2 April 2024: Extended to small sites (under 10 dwellings or sites under 1 hectare)
- May 2026: Proposed start date for Nationally Significant Infrastructure Projects (NSIPs)
Exemptions from BNG
Certain development types are exempt from mandatory BNG requirements:
- Permitted development (where no planning permission is required)
- Householder applications (extensions, loft conversions, etc.)
- Change of use applications with no external works
- Self-build and custom housebuilding (single dwelling)
- Developments affecting habitat of less than 25 square metres or 5 metres of linear habitat (de minimis exemption)
- Biodiversity gain sites (land registered for habitat banking)
Understanding the Biodiversity Metric
The 10% gain is measured using the statutory Biodiversity Metric 4.0 (or later versions), a standardized calculation tool developed by Natural England. The metric assigns biodiversity units to habitats based on:
- Habitat type: Different habitats have different baseline values (e.g., ancient woodland scores higher than amenity grassland)
- Size: The area or length of the habitat
- Condition: The ecological quality of the habitat (good, moderate, or poor)
- Strategic significance: Whether the habitat aligns with local nature recovery strategies
A competent ecologist must complete a baseline assessment of the site before development and calculate how the proposed development will affect biodiversity units. The post-development habitat creation or enhancement must then demonstrate at least a 10% net gain.
How to Achieve Biodiversity Net Gain
Developers have three options to meet the 10% requirement, applied in a strict hierarchy (the "mitigation hierarchy"):
1. On-Site Habitat Creation (Preferred)
The priority is to deliver biodiversity gains on the development site itself. This might include:
- Creating wildflower meadows or species-rich grassland
- Planting native hedgerows and trees
- Installing green roofs or living walls
- Creating ponds, wetlands, or sustainable drainage systems (SuDS) with ecological value
- Enhancing existing habitats through better management
2. Off-Site Biodiversity Gains
If on-site delivery isn't possible or sufficient, developers can secure off-site gains through:
- Purchasing biodiversity units from registered habitat banks
- Entering agreements with landowners to create or enhance habitats elsewhere
- Contributing to local authority biodiversity schemes
Off-site gains must be secured through a legal agreement (Section 106 or conservation covenant) and registered on the national biodiversity gain sites register.
3. Statutory Biodiversity Credits (Last Resort)
As a last resort, developers can purchase statutory biodiversity credits from the government. These credits fund habitat creation elsewhere, but they are deliberately priced to be more expensive than the alternatives to incentivize on-site and local off-site delivery.
Need an Ecological Assessment?
BNG assessments require competent ecological expertise. We work with trusted ecological consultants who can complete baseline surveys, biodiversity metric calculations, and habitat management plans to support your planning application.
If your site includes protected trees, our partner Ryder Tree Consultancy can provide arboricultural surveys and tree protection plans alongside your ecological assessments.
The 30-Year Maintenance Requirement
A critical aspect of BNG is that habitat gains must be secured and maintained for a minimum of 30 years. This is enforced through:
- Habitat Management and Monitoring Plans (HMMPs): Detailed plans for how habitats will be created, managed, and monitored over 30 years
- Legal agreements: Section 106 agreements or conservation covenants that bind current and future landowners
- Registration: All biodiversity gain sites must be registered on the national register
Developers should factor ongoing management costs into project viability assessments. Failure to maintain habitats can result in enforcement action and potential prosecution.
What Do You Need to Submit?
For planning applications subject to BNG, you'll typically need:
- Biodiversity Gain Information: A completed biodiversity metric calculation showing the baseline and post-development biodiversity values
- Biodiversity Gain Plan: Details of how the 10% gain will be achieved and maintained
- Habitat Management and Monitoring Plan: A 30-year plan for habitat creation and maintenance
- Ecological survey reports: Phase 1 habitat survey as a minimum, potentially Phase 2 surveys for protected species
The Biodiversity Gain Plan must be approved by the local planning authority before development can commence. This is typically secured through a pre-commencement condition.
Practical Tips for Developers
- 1Commission surveys early: Ecological surveys are seasonal. Plan ahead to avoid delays.
- 2Integrate BNG into design: Consider biodiversity from the outset, not as an afterthought.
- 3Avoid high-value habitats: If possible, locate development away from existing high-quality habitats to minimize the baseline loss.
- 4Check local requirements: Some local planning authorities have adopted higher BNG targets (e.g., 20%) through local plan policies.
- 5Budget for 30 years: Factor in ongoing habitat management costs when assessing project viability.
Impact on Development Viability
BNG introduces additional costs that developers must factor into their appraisals:
- Ecological survey and assessment fees
- On-site habitat creation and landscaping costs
- Off-site biodiversity unit purchases (if required)
- 30-year habitat management and monitoring costs
- Legal and registration fees
However, well-designed green infrastructure can also add value by enhancing the attractiveness of developments to buyers and meeting growing demand for sustainable homes.