Practical developer guide
Biodiversity Net Gain affects how most developments in England assess, replace and improve habitats. This guide explains how the requirement works, how biodiversity value is calculated, when off-site units may be needed and what developers should prepare before planning submission.
BNG requirements
Biodiversity Net Gain is mandatory for most developments that require planning permission in England. Unless an exemption applies, a development must deliver at least a 10% increase in biodiversity value compared with the site before development.
The requirement applies to habitats within the development’s red-line boundary, including habitats that will not be directly affected by the proposed work. Biodiversity value is measured using the statutory biodiversity metric.
Some developments are exempt from mandatory BNG. The exemptions depend on the type, scale and purpose of the development, so they should be checked against current government guidance before a planning application is prepared.
The exemption rules are changing for new planning applications made from 6 August 2026. Among the changes, the existing self-build and custom-build exemption will be removed, while new exemptions will apply in some circumstances. Applications submitted before that date generally remain subject to the earlier rules.
BNG requirements can affect site design, developable space, planning information and project costs. Developers should involve a suitably qualified ecologist and their planning team early in the process.
The biodiversity metric
Biodiversity Net Gain is measured using the statutory biodiversity metric. This converts the habitats affected by a development into standardised biodiversity units, allowing the site’s value before and after development to be compared.
The calculation is normally completed by a competent ecologist using habitat survey information and the official metric tool. Separate calculations are made for area habitats, hedgerows and watercourses where these are present.
The site is divided into habitat parcels and each habitat is identified, measured and assessed before development begins.
The metric assigns biodiversity units to the existing habitats using factors such as their size, type, distinctiveness, condition and strategic significance.
Habitats that will be retained, lost, created or enhanced are entered into the metric to estimate the site’s post-development biodiversity value.
New and enhanced habitats may be adjusted to reflect the time they will take to reach their target condition, the difficulty of creating them and, for off-site gains, their location.
The post-development total is compared with the baseline. The proposal must deliver the required uplift, usually at least 10%, while also meeting the metric’s trading rules.
The area of a habitat, or the length of a hedgerow or watercourse.
How ecologically valuable, unusual or difficult to replace the habitat type is.
The quality of the habitat when assessed against the relevant condition criteria.
Whether the habitat is located somewhere identified as important for local nature recovery.
Not by itself. The metric is an accounting tool rather than a complete ecological assessment. A proposal must also follow the applicable habitat trading rules, address protected species and wider ecological impacts, and provide suitable plans for habitat creation, management and monitoring.
Developers should avoid treating the calculation as a final check at the end of the design process. Running the metric early can reveal where habitat losses or difficult-to-replace habitats may affect the layout, cost or need for off-site units.
Planning priorities
The biodiversity gain hierarchy guides developers towards the most appropriate way to address habitat impacts and deliver the required Biodiversity Net Gain.
It is intended to encourage good site design rather than treating biodiversity units as a simple replacement for habitats lost through development. The first priority is therefore to avoid or reduce harm before deciding how any remaining shortfall will be addressed.
First priority
Where possible, development should be designed around existing habitats, particularly those that are valuable, distinctive or difficult to replace.
Where loss cannot be avoided
The layout, construction approach and landscaping strategy should be adjusted to retain as much existing habitat value as reasonably possible.
Preferred delivery
New or improved habitats can be included within the development boundary where the site has enough suitable space and they can be managed for the required long-term period.
Addressing a shortfall
Where the required gain cannot be delivered fully on site, developers can secure suitable off-site units from a registered habitat bank or another approved off-site location.
Last resort
Government statutory credits are intended for situations where suitable on-site or off-site gains cannot be secured. They are not designed to be the first or easiest option.
The hierarchy should be considered before the development layout is fixed. Retaining an existing habitat may be more practical and cost-effective than replacing its biodiversity value later, particularly where the habitat is highly distinctive or subject to strict trading rules.
Early metric calculations can also show whether the proposed on-site landscaping is likely to provide enough units, or whether off-site options should be explored before the planning application reaches an advanced stage.
Following the hierarchy does not remove the need to meet the statutory metric rules. Habitat losses must still be compensated for using suitable habitat types and the required trading rules must be followed.
Delivery options
Biodiversity Net Gain can be delivered within the development site, through registered habitat creation elsewhere, or through a combination of both.
The best approach depends on the site layout, existing habitats, development density, long-term management requirements and the number and type of biodiversity units needed.
Option 1
On-site BNG is delivered within the red-line boundary of the development. This may include retaining existing habitats or creating and enhancing grassland, woodland, hedgerows, ponds and other habitats as part of the scheme.
Option 2
Off-site BNG is delivered outside the development boundary. A developer can create gains on other land they control or secure biodiversity units from a registered habitat bank.
Off-site delivery may be useful where a site is constrained, where suitable habitat cannot be established reliably within the development, or where the amount of on-site habitat needed would significantly affect the proposed layout.
Off-site gains generally need to be secured through an appropriate legal agreement for the required management period. The gain site must be entered on the biodiversity gain sites register, and the relevant units must be formally allocated to the development.
The public register records information such as the location of the gain site, the habitats being created or enhanced and the gains allocated to individual developments.
Off-site units in Kent
Biodiversity & Habitat Solutions provides registered off-site BNG units from Chalksole Farm in Kent, with multiple habitat types, published starting prices and long-term habitat management included.
Developers making off-site gains on their own land can also register and allocate those gains. Buying units from a habitat provider is not the only off-site route.
From survey to approval
Biodiversity Net Gain should be considered from the earliest stages of a development. Habitat surveys, design decisions, metric calculations and delivery arrangements all need to work together before the development begins.
The exact documents and timescales will depend on the project and the Local Planning Authority, but most developments follow a similar sequence.
Early project stage
Confirm whether the development is subject to mandatory BNG and whether any exemption applies. Local planning policy should also be checked, as some authorities may seek more than the statutory minimum.
Before design is fixed
A competent ecologist identifies, maps and assesses the habitats within the development boundary. This information is used to calculate the site’s pre-development biodiversity value.
Design development
The layout should retain valuable habitats where possible and reduce unavoidable losses. Early metric calculations can help the design team understand how different layouts affect the BNG requirement.
Delivery strategy
Decide which habitats can be retained, created or enhanced within the development and identify any remaining shortfall that may need to be addressed through registered off-site units.
Planning application
The planning application should include the relevant habitat baseline and metric information, confirmation of whether the biodiversity gain condition applies and enough detail to explain the proposed approach to delivering BNG.
After permission
Once planning permission has been granted, the developer submits a Biodiversity Gain Plan showing how the development will meet its biodiversity gain objective. This usually includes the final metric calculation and details of on-site gains, off-site allocations and any statutory credits.
Before development begins
The Local Planning Authority must approve the Biodiversity Gain Plan before development can start. Long-term habitat delivery may also need to be secured through planning conditions, a Section 106 agreement or a conservation covenant.
Long-term delivery
Significant on-site gains and registered off-site gains must be managed and maintained for at least 30 years. Monitoring should show whether habitats are progressing towards the condition and biodiversity value set out in the approved plans.
The completed statutory metric showing the baseline, proposed gains and overall percentage achieved.
Plans identifying retained, enhanced and newly created habitats within the development.
Evidence of any registered off-site units allocated to the development.
Information explaining how significant habitats will be created, managed and monitored over the required period.
Planning an off-site allocation?
We can review your unit requirement, development location and planning timetable against the habitat types currently available at Chalksole Farm.
Local validation requirements and planning policies can vary. The project ecologist and planning consultant should confirm what the relevant Local Planning Authority expects at application and pre-commencement stages.
Common questions
These are some of the questions developers, landowners and planning teams commonly ask when preparing for Biodiversity Net Gain.
Most developments subject to mandatory BNG must deliver at least a 10% increase in biodiversity value compared with the site before development. Some Local Planning Authorities may seek a higher percentage through local planning policy.
Significant on-site habitat gains and registered off-site gains normally need to be managed and maintained for at least 30 years. The management period and responsibilities should be clearly set out in the approved plans and legal arrangements.
In some cases, yes. The correct approach depends on the development, the habitats affected and the biodiversity gain hierarchy. Developers should first consider avoiding habitat loss and reducing impacts, before addressing any remaining requirement through suitable on-site or registered off-site gains.
The calculation is normally completed by a competent ecologist using the statutory biodiversity metric. The result depends on the habitats present before development, what will be retained or lost, and the habitats proposed after development.
Developers can discuss availability and suitability before planning permission is granted. The final number and type of units should be confirmed against the completed metric and planning requirements before the allocation is finalised.
Not necessarily, but location can affect the value of off-site gains within the biodiversity metric. Units closer to the development and within relevant strategic areas may provide better value than gains located further away.
BNG units are created through habitat gains delivered on registered off-site land. Statutory credits are purchased from the government and are intended as a last resort where suitable on-site or off-site gains cannot be secured.
It is helpful to provide the development location, Local Planning Authority, draft or completed biodiversity metric, required habitat types, number of units and expected planning timescale.
The project ecologist should review the metric trading rules and available options. A different habitat type cannot always be used as a direct substitute, particularly where a distinctive or difficult to replace habitat has been lost.
Yes. Many developments retain or create some habitat on site and use off-site units for the remaining shortfall. The final metric should show how the combined package delivers the required gain.
Need off-site units in Kent?
Our brochure includes current starting prices, habitat specifications and availability at Chalksole Farm.