BNG Units: Buying and Using Biodiversity Units

BNG units: understanding what your development needs

BNG units are the numerical measure used to assess habitat value within England's biodiversity net gain framework. Understanding biodiversity units is essential before buying biodiversity units or comparing offers for off-site BNG units. The units your development needs depend on its habitat losses, proposed gains and the statutory metric's rules, not simply the size of the site. Statutory biodiversity credits are a different mechanism, available as a last resort where appropriate on-site and off-site delivery cannot meet the requirement.

Collington Winter Environmental helps applicants establish a defensible shortfall and understand which delivery options may address it. A purchase should follow the ecological assessment, not lead it. Buying units without first checking habitat categories, trading rules, location and registration arrangements can leave a development with an expense that does not resolve its biodiversity gain obligation.

What are biodiversity units?

Biodiversity units translate specified habitat characteristics into a measurable value. They are not a count of individual animals or plants, nor a universal measure of every ecological effect. The statutory biodiversity metric uses habitat as a proxy for biodiversity and provides a consistent framework for comparing the baseline with the proposed outcome. Protected species requirements and designated site safeguards remain separate considerations.

The statutory framework generally requires at least a 10% gain for development within scope in England. Introduced through the Environment Act 2021 and Schedule 7A of the Town and Country Planning Act 1990, mandatory BNG commenced for major development from 12 February 2024 and for small sites from 2 April 2024. Exemptions and transitional arrangements must be checked before assuming that a particular scheme needs units.

Different unit types are not interchangeable

The metric treats area habitats, hedgerows and watercourses separately. A surplus of area-habitat units does not automatically compensate for a hedgerow or watercourse shortfall. Within each relevant module, the trading rules also affect what habitat gains can address particular losses. A simple offer of a total number of units therefore provides too little information to establish suitability.

This is why an assessment should explain both the percentage result and whether the trading rules are satisfied. A development may show an apparently positive overall figure while still having an unresolved habitat-specific issue. Before discussing a purchase, identify the exact type of shortfall and the characteristics of the units that can legitimately resolve it.

How BNG units are calculated

Establish the baseline habitat value

For area habitats, the baseline calculation considers habitat area, distinctiveness, condition and strategic significance. Hedgerow and watercourse modules use their applicable inputs and rules. These values should be based on a competent survey and accurate mapping, with parcel references that connect the calculation to the habitat plan. A spreadsheet is only as dependable as the evidence used to populate it.

The baseline date also matters. Relevant earlier habitat degradation can affect the value that must be used, so clearing a site before assessment does not necessarily remove the original habitat from the calculation. Explain site management history to your ecologist and retain useful earlier survey information. A transparent starting point helps prevent later disputes about the scale of the obligation.

Model the proposed habitat outcome

The post-development calculation identifies habitats that will be retained, enhanced or created. Gains are adjusted for factors such as time to reach target condition and the difficulty of delivering the habitat. These risk adjustments recognise that a newly proposed habitat does not immediately provide the same value as an established habitat that is lost.

Consequently, replacing the same area of a habitat does not necessarily replace the same number of units. Proposed conditions also need to be realistic for the location and management regime. Where a design relies on an ambitious target, ask what establishment works, ongoing management and monitoring are needed to achieve it. Our biodiversity metric calculations service helps test these assumptions.

Identify the residual shortfall

Compare the baseline and proposed values using the metric's rules and the relevant target. The result identifies whether on-site delivery is sufficient and where a residual deficit remains. Do not calculate a purchase requirement by simply subtracting two headline figures without checking how the metric treats each habitat category and any proposed off-site contribution.

A unit requirement can change when the development layout changes. Roads, drainage infrastructure, utilities and revised landscaping can all affect the available habitat area or achievable condition. Confirm the design version underlying any purchasing advice and agree how subsequent revisions will be reviewed before committing to units that may no longer match the final scheme.

Buying biodiversity units: the practical process

Confirm what you need before approaching suppliers

Start with a checked metric and a clear description of the shortfall. This should identify the relevant unit module, habitat requirements and any trading restrictions. Consider whether further on-site avoidance or enhancement can reduce the shortfall in accordance with the biodiversity gain hierarchy. Off-site delivery is an important option, but it should sit within a reasoned overall strategy.

When requesting an offer, provide enough information for the supplier to identify a suitable allocation. A broad request for the cheapest units can miss the constraints that determine whether the offer will count. Ask for details of the supplying habitat, its location, the legal arrangements and its registration status, rather than relying on a generic description of biodiversity benefit.

Apply the spatial risk multiplier

Off-site habitat location affects its contribution through the spatial risk multiplier. Broadly, delivery closer to the development within the relevant local planning authority or National Character Area receives more favourable treatment than provision further away. The exact contribution must be calculated using the statutory metric rather than estimated from distance alone.

This means that a supplier's advertised unit quantity is not necessarily the quantity that will count towards your development's target. Compare offers after applying the correct multiplier and trading rules. A lower headline cost for a distant site may not represent a comparable offer if more units are needed to provide the same effective contribution.

Check legal security, registration and allocation

Off-site biodiversity gains must be secured through the appropriate legal mechanism, generally a planning obligation or conservation covenant, with habitat delivery and maintenance for at least 30 years. Natural England operates the biodiversity gain site register. Registration and allocation connect the secured habitat gains to the development that intends to rely on them.

Understand the distinction between reserving units, completing a commercial transaction, registering a gain site and recording an allocation. These are related but different steps. The biodiversity gain plan needs appropriate evidence of the registered off-site contribution. Coordinate supplier arrangements, legal advice and the planning programme so that a provisional purchase does not become an obstacle to approval.

How habitat banks supply off-site BNG units

A habitat bank is land managed to create or enhance habitat that can provide biodiversity gains for allocation to development. Some banks undertake work ahead of demand; others have delivery programmes linked to their secured commitments. The relevant question is not merely whether a site is described as a habitat bank, but whether its actual habitats, units and delivery arrangements meet your development's needs.

A credible review considers the baseline, proposed gains, management plan, responsible parties, legal security and monitoring arrangements. Where several developments use the same bank, the allocations must be clear so that the same gain is not counted twice. Read our habitat banking guide for an introduction to how landowners and developers participate in this delivery route.

Long-term management is central to the value being purchased. Habitat targets must be backed by practical action and a mechanism for responding if delivery falls behind expectations. Buyers should understand what the supplier is committing to provide and obtain appropriate advice on contract terms. Ecological suitability and commercial protections both matter, but they answer different questions.

Statutory biodiversity credits: a last resort

Statutory biodiversity credits are purchased from government when the gain objective cannot be met through appropriate on-site and off-site delivery. They are not the same product as off-site biodiversity units from a habitat bank. The applicant must explain why credits are needed and follow the applicable purchasing and gain-plan requirements, including the relevant metric treatment.

Do not assume that credits can be substituted casually for any unsuitable off-site offer. Their role is deliberately last resort within the biodiversity gain hierarchy. Equally, do not assume that every habitat loss can be resolved through a financial purchase. Trading restrictions, irreplaceable habitat provisions and other ecological protections can require a different or more specific response.

Why there is no universal biodiversity unit price

Unit costs depend on habitat type, location, delivery circumstances, legal arrangements and market availability. A meaningful comparison must identify what the offer includes and how many effective units the development receives after the metric rules are applied. There is no reliable universal price that can be quoted for every development or habitat category.

Budgeting should also consider professional assessment, legal work, registration arrangements and management responsibilities where relevant. Ask who pays for each part of the process and what happens if the required allocation changes. A transparent project-specific assessment is more useful than a broad price assumption that excludes essential delivery or compliance steps.

Why choose Collington Winter

Collington Winter Environmental is based in Bolton and works throughout England. Managing Director Olivia Collington is a Chartered Environmentalist with the IES and has experience with the DEFRA Statutory Biodiversity Metric and habitat bank offsetting. We help connect unit requirements with the habitats, planning evidence and delivery arrangements behind them.

For wider guidance, explore what biodiversity net gain is, when assessments are required and whether BNG is mandatory. To discuss BNG units or an off-site shortfall, call 01204 939 608 or use our contact page.