Under the Environment Act 2021, the UK government has introduced Biodiversity Net Gain (BNG) as a legal requirement for most development projects in England. This policy ensures that all developments leave the natural environment in a measurably better state than before development, requiring at least a 10% net gain in biodiversity units. But what exactly are BNG units, how are they calculated, and what role do they play in the planning system?
This article provides a clear explanation of BNG units, the statutory biodiversity metric, and how landowners and developers can navigate the new rules with confidence and clarity.
What Are Biodiversity Units?
Biodiversity units are a standardised measure of the biodiversity value of land, as calculated using the Statutory Biodiversity Metric. They are used to quantify both biodiversity losses from development and biodiversity gains from habitat creation or enhancement.
Each biodiversity unit reflects the overall ecological quality of a habitat, calculated based on four core factors:
- Habitat type and distinctiveness – defined using the UK Habitat Classification system
- Habitat condition – assessed as poor, moderate, or good
- Habitat size – measured in hectares (for area habitats) or metres (for linear habitats such as hedgerows and watercourses)
- Location – including whether the habitat aligns with a Local Nature Recovery Strategy and attracts a strategic significance multiplier
For proposed habitats, the value of biodiversity units is also influenced by three additional factors: difficulty, delay and distance. These act as negative adjustment multipliers, meaning proposed or compensatory habitats are typically worth less than existing habitats at the point impacts occur.
- Difficulty – how challenging the habitat is to create successfully
- Delay – how long the compensatory habitat will take to establish and reach its target condition after impacts occur
- Distance – how far the offset habitat is located from the development site
In practical terms, these adjustments mean you cannot simply remove a habitat in one location and replace the same area elsewhere on a like-for-like basis. Numerically, the metric is designed to incentivise avoidance, retention and onsite enhancement over offsite replacement.
BNG units are categorised into:
- Area habitat units (e.g. grassland, scrub, woodland)
- Hedgerow units
- Watercourse units
These categories must be calculated and tracked separately using the statutory biodiversity metric tool.
How Are Biodiversity Units Measured?
To calculate how many biodiversity units are present on a development site, a competent person (typically a qualified ecologist) must complete the statutory metric or the Small Sites Metric for minor applications.
The metric is used to assess the existing habitats on site and to model any proposed changes. It then calculates the overall biodiversity value before and after development, providing a figure in biodiversity units and the % change in value. A development must deliver at least 10% more units than it started with – either on-site, through off-site biodiversity units, or by purchasing statutory credits as a last resort. This forms the foundation of the biodiversity gain plan required as part of the planning process.
What Does the Metric Consider?
The statutory biodiversity metric tool scores habitats based on:
- Distinctiveness – how rare or ecologically valuable the habitat is
- Condition – the quality of the habitat
- Size – the area or length of the habitat
- Strategic significance – i.e., is identified in a Local Nature Recovery Strategy
Together, these factors produce a numerical score representing the biodiversity value of each habitat parcel. The more ecologically valuable the habitat, the more units it holds.
For example, a parcel of underperforming arable land might initially score only 2–4 units per hectare. If that land is restored to species-rich grassland or traditional orchard, it could generate 12–20 units per hectare depending on condition and location.
Why Do BNG Units Matter for Developers?
Under the new mandatory BNG requirements, developers must:
- Complete a BNG plan to discharge their BNG condition, prior to commencement
- Provide evidence using the statutory biodiversity metric
- Show how their development work will lead to at least 10% net gain
- Follow the mitigation hierarchy: avoid impacts first, then mitigate, then compensate
If a development causes a net loss, developers must buy off-site units from a registered habitat bank or, as a final option, purchase statutory credits from the UK government.
Local Planning Authorities (LPAs) will assess plans using the metric to ensure a consistent way of interpreting the value of a site before and after development.
Why Do BNG Units Matter for Landowners?
For landowners and land managers, BNG units represent a new revenue opportunity: you can sell biodiversity units to developers who need to offset biodiversity loss.
Units are generated by improving or creating natural habitats—for example:
- Converting degraded pasture to species-rich grassland
- Planting and managing native hedgerows
- Restoring wetlands or watercourses
- Establishing wildflower meadows
Once a site is registered on the Biodiversity Gain Site Register and legally secured (via a Section 106 agreement or Conservation Covenant), units can be sold on the open market to fulfil developer obligations.
The amount a site can generate depends on factors like:
- How many units are deliverable
- The habitat type and target condition
- Strategic significance in the local area
- Your intended land management decisions over a 30-year period
At Civity, we partner with landowners to navigate this process from initial scoping assessment through to site registration and unit sales, ensuring each project meets all biodiversity net gain requirements.
Off-Site Units and the Role of Habitat Banks
Where developers cannot meet the 10% gain on site, they must secure off-site biodiversity units from third-party landowners. These off-site units must:
- Be formally registered
- Be delivered under a legal agreement
- Be maintained for at least 30 years
This is where habitat banks come in: they provide pre-approved habitat enhancement schemes that developers can use to meet their legal obligations.
How Many Units Do Developers Need?
The number of units required depends on the baseline ecological value of the development site and the impact of the planned works. A small site with few existing habitats may require only fractional units to achieve net gain. A large development affecting valuable habitats may require many more units, particularly if high-distinctiveness habitats are involved.
The exact figure is determined through the biodiversity metric, and the strategy for achieving net gain must be included in the biodiversity gain plan submitted to the local planning authority.
In Summary
BNG units are central to delivering the government’s new approach to support biodiversity through the planning system. They provide a consistent way to measure biodiversity, track net gain, and support long-term habitat improvements across the country.
Whether you’re a developer seeking compliance or a land manager exploring ways to improve your land while generating income, understanding how biodiversity units work is now essential. With the right guidance, these new rules offer a practical route to achieving environmental enhancement at scale.
At Civity…
We facilitate compliant and transparent Biodiversity Net Gain transactions between developers and landowners. We support developers by reviewing submitted biodiversity metrics, confirming unit requirements in line with trading rules, and brokering the allocation and sale of BNG units from registered gain sites within our landbank. We do not carry out on-site ecological surveys on developed land or register developer-owned land, but we work closely with your appointed ecologist to support completion of the Biodiversity Gain Plan.
For landowners, we provide a full-service route to bringing land forward as a registered biodiversity gain site, from initial scoping through to habitat delivery, long-term management, and monitoring over the 31-year agreement term, with a focus on making nature recovery viable and rewarding over the long term.
