Biodiversity Net Gain became mandatory for most planning applications in England on 12 February 2024. Over two years in, many ecologists are still navigating the practical realities of delivering BNG alongside traditional ecological assessments.
This guide covers what BNG means in practice for working ecologists - the legal requirements, the metric, exemptions, common pitfalls, and what's changing in 2026.
What Is Biodiversity Net Gain?
Biodiversity Net Gain (BNG) is a legal requirement under Schedule 7A of the Town and Country Planning Act 1990, inserted by the Environment Act 2021. It requires most new developments in England to deliver a measurable 10% improvement in biodiversity compared to the pre-development baseline.
In plain English: if your development destroys or degrades habitats, you must create or enhance enough habitat to end up with at least 10% more biodiversity than you started with. This gain must be maintained for at least 30 years.
The Statutory Biodiversity Metric
Biodiversity value is measured using the Statutory Biodiversity Metric, published by Defra and Natural England. The metric calculates biodiversity units based on three habitat modules:
- Area habitats - measured in hectares, producing Area Biodiversity Units (ABUs)
- Hedgerows - measured in kilometres, producing Hedgerow Biodiversity Units (HBUs)
- Watercourses - measured in kilometres, producing Watercourse Biodiversity Units (WBUs)
Each habitat is scored based on:
- Habitat type and distinctiveness - how ecologically valuable the habitat type is (from Very Low to Very High)
- Condition - how good the habitat quality is (Poor, Fairly Poor, Moderate, Fairly Good, Good)
- Strategic significance - whether the habitat is within a Local Nature Recovery Strategy area or Nature Recovery Network
- Area or length - the physical extent of the habitat
The metric multiplies these factors together to produce a biodiversity unit score. The post-development score must be at least 10% higher than the baseline score.
Who Needs to Deliver BNG?
BNG applies to most planning applications granted permission in England, including:
- Major residential developments
- Commercial and industrial developments
- Infrastructure projects
- Minor developments (since April 2024)
Exemptions
Not every development needs BNG. Current exemptions include:
- Householder applications (extensions, loft conversions, garden buildings)
- Permitted development
- Self-build and custom-build housing (limited exemption)
- Developments affecting biodiversity of negligible value
- Small scale developments on previously developed land in some circumstances
The government announced in December 2025 that an area-based exemption for smaller sites (up to 0.2 hectares) will be introduced, along with a targeted exemption for residential brownfield development. Implementation details are expected in 2026.
BNG for Nationally Significant Infrastructure Projects (NSIPs) becomes mandatory from May 2026.
What Ecologists Need to Deliver
For a planning application subject to BNG, the ecologist's role typically includes:
Baseline Assessment
- A UKHab habitat survey of the site to classify and map all habitats within the red line boundary
- Condition assessment of each habitat parcel against the metric's condition criteria
- Recording hedgerow lengths and types
- Recording watercourse lengths and types
- Completing the baseline tab of the statutory biodiversity metric tool
Post-Development Design
- Working with the project team to design the habitat layout for the completed development
- Specifying habitat types, areas, and target conditions for created and enhanced habitats
- Completing the post-development tab of the metric tool
- Ensuring the 10% net gain target is met across all three modules (area, hedgerow, watercourse)
Biodiversity Gain Plan
- Preparing or contributing to the Biodiversity Gain Plan that must be submitted to and approved by the Local Planning Authority
- The gain plan must include the completed metric, habitat management plans, and evidence of any off-site or credit arrangements
- The LPA must approve the gain plan before development can start
Trading Rules
The metric includes trading rules that restrict what habitats can replace what:
- Very High distinctiveness habitats lost must be compensated with the same habitat type
- High distinctiveness habitats lost must be compensated with the same broad habitat type or higher
- You cannot trade down - replacing woodland with amenity grassland will not satisfy the metric even if the unit numbers work out
The metric tool flags trading rule violations automatically. Check the trading summary tabs carefully before finalising your calculation.
On-site vs Off-site vs Credits
BNG follows a mitigation hierarchy:
- Avoid habitat loss through design changes
- Minimise unavoidable losses
- On-site compensation - create or enhance habitat within the development site
- Off-site compensation - create or enhance habitat on a registered gain site elsewhere
- Statutory credits - purchase credits from Natural England as a last resort
On-site delivery is preferred by planning policy. Off-site units can be purchased from registered gain sites on Natural England's Gain Site Register. Statutory credits are significantly more expensive and are intended as a last resort only.
Common Pitfalls
Underestimating the baseline
A common mistake is surveying the site after vegetation has been cleared or degraded, resulting in a low baseline that makes the 10% target easy to hit. Planning authorities are alert to this. If a site has been deliberately degraded, the LPA can require the baseline to be calculated from the pre-degradation habitat condition.
Forgetting hedgerows and watercourses
Each module must independently achieve 10% net gain. A development that achieves 150% gain on area habitats but loses 30% on hedgerows will fail the metric. Always survey and calculate all three modules.
Unrealistic target conditions
Setting ambitious target conditions (e.g. Good condition grassland within 5 years of creation) may satisfy the metric calculation but will be challenged by the LPA. Habitat creation takes time. Be realistic about what conditions are achievable and over what timescale.
Not considering BNG early enough
BNG should inform the site layout and design from the earliest stages. Retrofitting habitat creation into a finalised masterplan is difficult and expensive. Engage an ecologist at the pre-application stage to advise on how BNG can be integrated into the development design.
BNG and Existing Ecological Requirements
BNG does not replace existing protections for designated sites or protected species. If your site has bats, GCN, or is near a SSSI, you still need the appropriate surveys, licences, and mitigation. BNG is an additional requirement on top of these.
However, habitat created for species mitigation (e.g. a GCN pond created as part of a District Level Licensing scheme) may contribute towards BNG targets in some circumstances. Discuss with your LPA and NE caseworker how mitigation and BNG interact for your specific project.
Start With a Desktop Study
Before you can design a BNG strategy, you need to understand the ecological context of your site. What designations are nearby? What species have been recorded? What is the GCN risk zone? This context informs both your PEA and your BNG approach.
EcoCheck gives you that baseline context instantly - statutory designations, protected species records, GCN risk zones, Ancient Woodland, and Priority Habitats for any GB location. Use it to scope your site before commissioning the full UKHab survey and metric calculation.
Patrick O'Connor is a Freelance Ecologist at Kinterra Consulting and the developer of EcoCheck - an instant ecological desktop assessment tool for any GB location. Try it free for 3 days at ecocheck.co.