Biodiversity Net Gain Assessment: When are they required?

What is a biodiversity net gain assessment?

Following the introduction of The Environment Act 2021, developers on all new development projects should aim to protect ecosystems and habitats on site. A biodiversity net gain assessment should be carried out as early as possible within the development planning stages to identify and mitigate any potential habitat loss on the development site.

Mandatory biodiversity net gain will be introduced following the 2 year transition period in 2023 which will place a responsibility on developers to consider the environmental impact of a development in terms of biodiversity. The introduction of net gains for biodiversity also aims to ensure that the environment is left in a better state than before the development was completed.

Why is a biodiversity net gain assessment required for developments?

The Environment Bill gained royal assent in 2021, and under The Environment act 2021, all developments will need to ensure there is at least a 10% net gain to biodiversity, post development. This net gain is measured through the statutory biodiversity metric.

Following a two year transition period, biodiversity net gain will become mandatory in 2023. The Environment Act act will introduce the requirement to deliver biodiversity net gain for all developments in England.

In addition to this, the current national policy in England, The National Planning Policy Framework (NPPF) Paragraph 179 states:

“To protect and enhance biodiversity and geodiversity, plans should:

1. b) … identify and pursue opportunities for securing measurable net gains for biodiversity.”

As a result of these regulations, most planning applications for developments will be unable to proceed until local planning authorities are satisfied that the required biodiversity standards have been met.

Providing Biodiversity Net Gain assessments alongside methods for achieving Biodiversity Net Gain within a development’s proposal will soon become essential in order to receive the appropriate planning permission.

Calculating Biodiversity Net Gain

A biodiversity net gain assessment will use some sort of metric to assess each habitat type. The statutory biodiversity metric is the most widely used metric which is used to calculate biodiversity unit scores and translate them into the standards of the local authority.

For each habitat, the metric will assess:

• The condition and quality of the habitat
• The connectivity of the habitat, i.e., how well connected the habitat is to other similar habitats
• The strategic significance of the habitat

The government website provides a statutory biodiversity metric calculation tool that can help developments to determine their biodiversity unit score under the standards of their local planning authority.

Biodiversity Net Gain Plans

In order to obtain planning permission for a development, developers must be able to prove that they are taking the correct measures to increase biodiversity net gain. One of the initial steps in this process is to contact an experienced ecologist to create and develop a biodiversity net gain plan.

Depending on the ecologist’s findings within a biodiversity net gain assessment, the biodiversity net gain plan will help to determine the natural elements that could potentially be at risk as a result of the proposed development project, as well as any mitigation methods which should be used to prevent damage.

How can our team assist?

Our team have assisted numerous clients over the years, including policy guidance for biodiversity gain in England. It is important to note that BNG varies across each Local Planning Authority.

There are three stages of using a biodiversity net gain assessment, and we assist our clients during the very early stages of development, including promotions and land purchases. We are happy to complete an informal initial assessment for sites of interest. This helps our clients understand the probable implications and costings of Biodiversity Net Gain from the offset.

Project Feasibility

• Identifying implications for potential development projects.
• Audits of land for biodiversity gain capacity at the land acquisition stage.
• Providing advice on options for delivery of biodiversity gain on and off-site or potential unit costs to the Local Planning Authorities.

Assessment and design

• Baseline survey and habitat condition assessment – to provide data for the biodiversity metric (completed as part of a Preliminary Ecological Appraisal).
• Detailed design-phase input: aim to retain the highest valued ecological features and scope for creating or enhancing habitats.
• Combining green infrastructure, SANG and habitat provision.
• GIS expertise: managing metric data and supporting calculations.

Planning permission and delivery

• Planning conditions: working with a project team (especially landscape teams) with the aim to provide feasible, proportionate and practical final designs and management.
• Preparing long-term site management plans (or consult with Landscape Architects) and advising on future monitoring commitments.
• If offsetting is required, we will liaise with local authorities, conservation organisations, and other third parties for agreeing the delivery of biodiversity gain.

Please get in touch if you would like further information about Biodiversity Net Gain or Landscape Management Plans. We are happy to offer free CPD sessions on the Biodiversity Net Gain Principles and how we can help your schemes achieve this.

Establish whether mandatory BNG applies to your application

A biodiversity net gain assessment usually combines habitat evidence, metric calculations and an explanation of how a development will meet its biodiversity obligations. Whether it is required depends on the planning route, application date, exemptions and site circumstances. The size of the red-line boundary alone does not settle the question.

England’s mandatory BNG framework, introduced through the Environment Act 2021, generally applies to major development applications made from 12 February 2024 and small-site applications made from 2 April 2024. Transitional arrangements and exemptions mean that two apparently similar projects can have different requirements. Check applicability before commissioning a full assessment or claiming that none is needed.

Distinguish an exemption from a low-impact proposal

Some development is exempt from mandatory BNG, including qualifying householder development, qualifying self-build and custom-build development, and proposals meeting the de minimis criteria. Each exemption has specific conditions. A small footprint, limited landscaping or a previously developed site does not automatically establish an exemption.

The de minimis exemption concerns effects on habitats, not simply the total application area. Among its requirements, development must not affect priority habitat and must affect less than 25 square metres of other habitat with biodiversity value and less than five metres of linear habitat. Review the full BNG exemptions guidance and retain evidence supporting the exemption claimed.

Decide what evidence is needed to support the decision

A local planning authority may need plans, photographs, habitat information or an ecological explanation to understand an exemption claim. Where habitat type, impact extent or priority habitat status is uncertain, professional advice can prevent an unsupported declaration from delaying validation or determination.

A habitat baseline survey is often the clearest starting point for proposals within scope. Surveyors commonly use UKHab to classify habitats, with a condition assessment where required by the metric. The resulting evidence should identify relevant habitats accurately, rather than treating all undeveloped ground as equivalent or assuming that an urban site has no biodiversity value.

Select the correct metric for the assessment

The statutory biodiversity metric is the standard calculation tool for mandatory BNG. The small sites metric is a simplified statutory option for eligible developments, but eligibility depends on more than the planning definition of a small site. Certain ecological features and impacts prevent its use, so the current eligibility rules must be checked.

The assessment should record habitat areas and lengths, condition, distinctiveness and strategic significance using the applicable guidance. It must also apply relevant trading rules. A reliable biodiversity metric calculation explains its evidence and assumptions, allowing the planning authority to understand both the baseline and the proposed gain rather than relying on an unexplained percentage.

Check the relevant baseline date and site history

The baseline is normally assessed by reference to the planning application date, although an earlier date can be agreed with the planning authority. A recent survey is useful evidence, but survey timing and the legally relevant baseline date are not necessarily identical.

Earlier clearance or degradation may require the assessment to use the habitat value before that degradation, under the statutory anti-degradation provisions. Review aerial photographs, previous surveys, land-management records and site changes where relevant. Clearing vegetation before applying is therefore not a dependable way to reduce BNG requirements, and missing historical evidence can create additional uncertainty.

Separate application evidence from the biodiversity gain plan

For an in-scope application, required biodiversity information must accompany the planning submission. This includes the pre-development biodiversity value, the completed metric and supporting information required by the statutory framework. The authority may also need proportionate detail on how the proposal is expected to deliver the required gain.

The biodiversity gain plan is a separate approval stage, normally following permission and before development begins. Where the assessment identifies a deficit, the applicant should investigate feasible solutions early. A BNG consultant can help distinguish matters that must be resolved for the application from the detailed evidence needed to discharge the biodiversity gain condition.

Assess delivery requirements as well as the headline gain

An assessment should consider whether proposed habitat creation and enhancement are realistic. Significant on-site enhancements and off-site gains require long-term arrangements, with relevant habitats maintained for at least 30 years. A habitat management and monitoring plan should describe establishment, ongoing management, monitoring and corrective action.

If off-site biodiversity units are required, a habitat bank may provide suitable gains, but availability must be checked against the metric’s requirements. Off-site land must be legally secured through a section 106 agreement or conservation covenant, registered on Natural England’s biodiversity gain sites register and appropriately allocated. Statutory biodiversity credits should be considered only as a last resort.

Review strategic significance and other ecological duties

An LNRS, or Local Nature Recovery Strategy, can inform strategic significance where relevant, alongside the applicable metric guidance and local evidence. The assessment should explain why a score is justified rather than assuming that all habitat proposals qualify for the highest category. Local priorities can also help shape a more coherent delivery approach.

Finally, exemption from mandatory BNG does not remove separate obligations concerning protected species, designated sites or other planning policies. Conversely, a protected species survey does not replace a habitat-based BNG assessment. Where the scope is unclear, seek project-specific advice before submitting incomplete evidence or commissioning surveys that do not answer the relevant planning questions.