What Are BNG Units?

Everything we see is measured in units, whether that's feet and inches for height or length, milligrams and kilograms for weight, and one to 11 for sound. The same goes for biodiversity net gain (BNG), a condition of planning permission for most developments in England since February 2024.¹

Although there are several exemptions (found here), biodiversity net gain requires developers to leave the environment in a better state than they found it, by a minimum increase of 10%.

To calculate that percentage, land affected by development projects must be assessed and valued, and the measure used to do that is called a biodiversity unit.²

This guide will explain what biodiversity units are, how they are calculated, and their key role in the compliance with biodiversity net gain for developers.

BNG Units Explained

Biodiversity units are the currency of the biodiversity net gain BNG market.

In the simplest terms, a biodiversity unit is a standardised measure of a habitat parcel (defined as a "linked area of habitat of the same distinctiveness, condition and strategic significance"),³ calculated using either the statutory biodiversity metric tool⁴ or, for developments not defined as "major" and which meet specific criteria, the small sites metric tool.⁵

Both convert real-world ecological data about a habitat into a measurable biodiversity unit that can be compared and traded.

Calculating BNG Units in a Habitat Parcel

Whether a developer uses the statutory biodiversity metric tool or the small sites metric tool, they both work the same way. They divide habitat into three distinct types of biodiversity units:

• Area habitat units
• Hedgerow units
• Watercourse units

Each of these is calculated in a separate 'module' of the biodiversity metric.⁶

For each habitat parcel, the metric analyses data gathered during ecological surveys and BNG assessments covering four factors:

• A habitat parcel's size, measured in hectares for area-based habitats and kilometres for linear habitats such as lines of trees, hedgerows and watercourses.
• Its distinctiveness, ie: how ecologically valuable the habitat category is on a nationally recognised scale.
• Its condition, including how well the habitat is currently functioning.
• Its strategic significance, including the habitat's location relative to ecological networks and any existing Local Nature Recovery Strategies (LNRS).

Each element is given a score reflecting each unit's biodiversity value, ranging from low to high. The scores across the entire development are combined to give a comprehensive, baseline biodiversity value before development.

The same process is then repeated for the post-development outcome. This time, any existing habitats retained are calculated, as well as lost habitats, those that are created or enhanced on-site, and any off-site units or statutory credits needed to make up any BNG shortfall.⁷

The second figure is subtracted from the first and the sum is presented in both units and as a percentage,⁸ so a developer can see immediately whether their planned design has achieved the minimum 10% BNG uplift or not.

Post-Development Risk Multipliers

Three other criteria can affect the value of biodiversity units as calculated by the statutory biodiversity metric tool or the small sites metric tools:

• Their difficulty of creation or enhancement
• The time from the start of the creation or enhancement to the target condition
• How far away biodiversity units are from the original development (known as spatial risk)⁹

How Many BNG Units Does a Site Generate?

It's impossible to put a general figure on the number of biodiversity units on a single piece of land, because it will depend on the types and condition of the habitats it holds.

Cultivated farmland will likely achieve a low distinctiveness score, while top-quality lowland mixed deciduous woodland will have significantly greater biodiversity unit value. Likewise, converting arable land to species-rich grassland, native woodland, or wetland can unlock substantially greater unit values than simply improving an existing, lower-value habitat.

This variation is why habitat type matters so much in the BNG market - both for developers looking to source off-site units, and for land managers exploring the potential for habitat creation on their own land. In either case, our expert team can help.

On-Site BNG Units

Before they start to think about biodiversity net gain units, developers should apply the mitigation hierarchy to their design. Part of the National Planning Policy Framework,¹⁰ it requires developers to follow a strict pathway at the start of their projects:

• Avoid impacting habitat
• Mitigate any effects on habitat
• Restore any impact
• Offset any adverse effects

Developers who have followed the mitigation hierarchy and whose projects are subject to mandatory BNG must then follow the biodiversity gain hierarchy, as laid down in the Town and Country Planning Act 1990, inserted by the Environment Act 2021.¹¹

It prioritises achieving on-site gains within the site's red-line boundary wherever possible.¹² That could mean creating or enhancing habitats to generate the legally required, minimum 10% biodiversity net gain within or alongside the development.

In 2026, the regulations were amended to enable sites deemed "not major development" to pursue on- and off-site BNG units at the same time.¹³

Where on-site gains alone cannot achieve the minimum 10% target, developers can turn to off-site units purchased from registered habitat banks.

Buying Off-Site Biodiversity Gain Units

Off-site biodiversity units are generated by land managers or owners, who create or enhance natural habitats on their own land.

They are listed on the official biodiversity gain sites register, and secured via legal agreement - either a section 106 agreement or a conservation covenant - with a responsible body for a minimum of 30 years.

A habitat management and monitoring plan must be in place to confirm the habitats are reaching and maintaining their objectives. Local authorities will enforce these obligations throughout the duration of the scheme.¹⁴

Trading rules set out in the statutory biodiversity metric tool and the small sites metric tools mandate that any enhanced or created biodiversity units must be of the same type and quality of the habitat that was lost or damaged.¹⁵

We can help ensure your on- and off-site biodiversity units meet all your BNG needs.

Statutory Biodiversity Credits - the Last Resort

The final stage of the biodiversity gain hierarchy kicks in if neither on site gains nor off site units from the private market can fully meet the mandatory 10% minimum uplift in BNG.

Regarded as a last resort, statutory biodiversity credits are sold by the UK government, and are priced deliberately higher than off-site BNG units¹⁶ and include an automatic spatial risk multiplier that doubles the amount of units needed, to dissuade developers from skipping the earlier steps in the biodiversity gain hierarchy.

Before developers can buy statutory biodiversity credits, they must provide evidence that all other avenues were exhausted. The statutory biodiversity metric tool and small sites metric tools will calculate the amount of statutory biodiversity credits needed for a project.¹⁷

Two statutory biodiversity credits are typically needed for every one biodiversity unit required - reflecting the spatial risk multiplier that applies to this last-resort route - meaning the effective cost can be significantly higher than the headline per-credit price might suggest.

We Can Help With All Aspects of BNG Units

If you're a developer and struggling to understand your BNG unit requirements, or want to source off-site biodiversity units, we can help.

Our team can offer jargon-free guidance on the best solution for your project, while our online marketplace makes it straightforward to buy BNG units quickly, with all legal documentation included.

If you're a landowner thinking about generating biodiversity units from your land, talk to us. Our insights and expertise can help identify what's best for you and your land.

Email us at sales@greenshank-environmental.com or fill in this contact form and we'll be in touch as soon as possible.