What is biodiversity net gain, and why does it matter for developers?

Biodiversity net gain (BNG) is an approach to development that aims to leave habitats in a measurably better state than before the works began. It involves assessing existing habitats, calculating how development will change their value and delivering sufficient habitat creation or enhancement to achieve a net improvement.

In England, most development subject to mandatory BNG must deliver at least a 10% gain. This is measured using a statutory calculation, rather than simply planting 10% more trees or setting aside 10% of the land.

For developers, BNG affects site selection, design, planning evidence, budgets and ongoing responsibilities. Understanding it early helps establish how a scheme can meet its obligations without relying on changes made at the last moment.

Why is biodiversity net gain needed?

Development can remove habitats, reduce their quality and interrupt connections between wildlife populations. Individual losses may appear modest, but repeated losses across an area can leave fewer places for wildlife to feed, breed and move.

BNG seeks to address this by requiring a measurable improvement alongside development. The focus is not simply on adding attractive planting: it is on creating or enhancing habitats with ecological value.

For example, replacing species-rich grassland with a regularly mown lawn would not provide an equivalent outcome just because both areas look green. Habitat type, condition and the time needed to establish replacements matter.

BNG also encourages developers to consider biodiversity alongside drainage, access, public space and green infrastructure, rather than treating ecology as an isolated requirement.

When did mandatory BNG begin?

Mandatory BNG began for most relevant major planning applications in England on 12 February 2024. It extended to relevant small developments from 2 April 2024.

However, exemptions and transitional arrangements matter. From 6 August 2026, an exemption applies to qualifying developments on sites of 0.2 hectares or less, provided they do not negatively impact onsite priority habitat. Application dates and special rules for variations to existing permissions can affect eligibility.

Other exemptions also exist, so developers should check the current position rather than assume every application requires BNG. An exemption from the statutory BNG requirement does not remove other obligations relating to protected species, habitats or planning policy.

These rules concern England. Projects elsewhere in the UK need advice appropriate to their jurisdiction.

How is biodiversity net gain calculated?

The biodiversity metric translates habitat information into biodiversity units. It takes account of factors including habitat type, area or length, condition and strategic significance.

An assessment establishes the value of existing site habitats before development, known as the baseline. The proposed scheme is then assessed to identify retained habitats, losses, enhancements and new habitat creation.

The calculation also accounts for delivery risks, including the time needed for new habitats to reach their intended condition. A newly planted habitat cannot automatically replace an established habitat of higher value.

Area habitats, hedgerows and watercourses are assessed in separate unit categories. The metric’s trading rules also govern which replacements are acceptable.

For example, if a scheme has a baseline of 20 area habitat units, a 10% gain requires an outcome of at least 22 units in that category. Meeting that total alone is insufficient if trading rules or separate hedgerow and watercourse requirements remain unmet.

Collington Winter’s biodiversity net gain assessments help developers understand their baseline and test delivery options.

Can developers deliver gains on site or off site?

Developers can use on-site habitat creation and enhancement, off-site biodiversity units, or a combination. Statutory biodiversity credits provide a last-resort route where the other options cannot deliver the required outcome.

On-site biodiversity gains

On-site biodiversity gains are delivered within the development’s red-line boundary. Examples may include creating grassland, improving hedgerows or restoring suitable wetland habitats.

The available on-site biodiversity units depend on the layout, the habitats proposed and whether their target condition is achievable. Spaces subject to heavy recreational use may need different management from areas intended primarily for wildlife.

Off-site biodiversity units

Off-site delivery can involve a developer’s other land or purchasing units from land managers who create or enhance habitats elsewhere.

Off-site gains must meet registration and allocation requirements. Developers should check unit type, location, availability and legal security, rather than selecting units solely by price.

Habitat banking can provide a route for delivering these gains through land managed for biodiversity.

Statutory biodiversity credits

Government credits are different from privately purchased off-site units. They are a last resort and should not be treated as the default budget solution.

Whether delivery is on site or off site, it must follow the applicable rules. Since August 2026, qualifying small developments can consider on-site and off-site options together, without first exhausting on-site delivery.

Why does BNG matter commercially for developers?

BNG can influence how much land is available for buildings, access and other infrastructure. Retaining a valuable habitat may reduce the biodiversity deficit, while removing it can increase replacement requirements.

That creates a design decision worth testing early. The option with the most developable floorspace is not necessarily the option with the best overall financial outcome once habitat delivery, unit purchases and management are considered.

Budgets should cover surveys, calculations, habitat works, legal agreements, monitoring and any off-site purchases. Responsibilities also need to be understood when land is sold or transferred.

Comparing layouts before submission helps developers assess these costs alongside the scheme’s other priorities.

How does BNG affect planning permission?

For non-exempt development under the ordinary planning regime, the local planning authority needs the required biodiversity information with the application.

A biodiversity gain plan must then be approved before development can begin, subject to the relevant statutory arrangements. Obtaining planning permission does not, by itself, mean the BNG condition has been discharged.

Developers should establish a credible delivery strategy during design and application preparation. An unresolved unit shortfall or incomplete legal arrangements can affect the programme after permission is granted.

A preliminary ecological appraisal can identify wider ecological constraints, but it does not automatically replace a BNG assessment or protected species surveys.

Why does long-term habitat management matter?

BNG depends on habitats reaching their promised condition and remaining valuable. Off-site gains and significant on-site enhancements must generally be secured and maintained for at least 30 years.

This long term commitment requires clear responsibilities, realistic funding and appropriate monitoring. A habitat management and monitoring plan explains how habitats will be established, maintained and checked, including how problems will be addressed.

For development projects, these commitments should be considered alongside estate management and future ownership arrangements. Planting a habitat is only the beginning of delivery.

What about small sites and infrastructure projects?

Small sites are not automatically exempt. Developers need to check the applicable exemption, application date and habitat impacts. Where eligible, the Small Sites Metric offers a simplified calculation, but it has restrictions.

For nationally significant infrastructure projects (NSIPs) in England, mandatory BNG applies to development consent order applications made on or after 2 November 2026. These nationally significant infrastructure projects follow a separate framework and relevant biodiversity gain statements.

BNG support from Collington Winter

Collington Winter provides biodiversity net gain support alongside ecology and landscape services. We help developers assess habitats, understand delivery options and incorporate biodiversity into their proposals.

Contact our team to discuss your site, planning programme and BNG requirements.