Biodiversity Net Gain Mitigation for Development

Biodiversity Net Gain (BNG) is an approach to construction and development that aims to leave the environment in a better state than it was before or at the same level that it was before any development took place.

Biodiversity Net Gain was initially introduced due to the fact that biodiversity has steadily declined in recent years in the UK. Construction and developments have had a considerable impact on the environment as they produce a high amount of pollution.

BNG is being requested more frequently by local planning authorities to inform planning applications for planning permission. Therefore, it can have a significant impact during the decision making process of planning authorities when they debate whether to grant or refuse a planning application.

As such, if local planning authorities determine that a proposed development may not meet the required BNG standards, developers may be required to specify key mitigation methods that must be put in place in order to deliver the net gain required to gain planning permission.

The Importance of Biodiversity Net Gain

Net gains for biodiversity are significant for a variety of reasons and can help to positively affect the environment almost anywhere. Some of the key benefits that biodiversity net gain provides include:

Enables the production of raw materials

• Supplies water and oxygen to surrounding ecosystems
• Enhances the environment’s visual appearance
• Provides jobs for local farmers and other agricultural occupations
• Facilitates a scientific understanding of the natural environment
• Offers recreational activities such as fishing, camping and hiking

The idea of mandatory biodiversity net gain is a structured and regulated method of prioritising and ensuring that all of the factors listed above are supported and encouraged in the years ahead. As biodiversity net gain applies to all development projects, it causes a universal approach from governing bodies. Therefore, as BNG is a government policy, it prevents any potential issues with key stakeholders.

Biodiversity Net Gain Mitigation

In order to achieve Biodiversity Net Gain, development proposals must follow the ‘mitigation hierarchy’.  This hierarchy encourages developers to avoid harm to biodiversity from the offset, then mitigate or compensate for any losses on site or off site.

Following the mitigation hierarchy is crucial for any development projects that are aiming to achieve no overall negative impact on local biodiversity or biodiversity net gain. The hierarchy is based on a series of steps that should be taken throughout a development project’s life cycle.

The biodiversity mitigation hierarchy contains the following steps:

• Avoidance – The first step in the mitigation hierarchy is comprised of measures taken to avoid a negative impact on biodiversity from the beginning. This can include timing construction to avoid disturbance to wildlife and carefully spacing infrastructure so that habitats are not affected. Avoidance is typically the easiest and most effective way of reducing potential negative impacts. However, it must be considered in the early planning stages of a project.

• Minimisation/Mitigation – This includes measures taken to reduce the duration, intensity, likelihood and extent of any impacts that cannot be avoided. Effective mitigation can help to eliminate some negative impacts. Examples of mitigation include building wildlife crossings on roads, designing powerlines to reduce harm to birds and measures to reduce pollution and improve air quality.

• Restoration/Rehabilitation – This step includes measures that will be taken when creating or enhancing habitats after development has taken place where mitigation or avoidance was not possible. Restoration aims to return an area to the original ecosystem that was present before any impacts occurred. Rehabilitation aims to restore basic ecological functions and ecosystem services. These steps are frequently required towards the end of a project.

• Offset – This includes measures that will be taken to compensate for any adverse impacts after the other steps of the mitigation hierarchy have been implemented. Offsetting can often be complex, so the steps beforehand are typically more preferable.

The Environment Bill

The Environment Bill was passed in 2020 as it received royal assent. Therefore, BNG will soon become mandatory through the forthcoming Environment Act in 2023. However, the National Planning Policy Framework also requires a net gain approach which should be achieved in a measurable way.

Within this Environment Bill, it aims to include the development of effective conservation covenants, increase the use of recycling, improve air and water quality, recall products that violate environmental standards, protect local wildlife species, regulate chemicals that may harm the environment, reduce plastic waste and use resources in an efficient manner.

As a result of this Bill, biodiversity net gain has been served as a core policy with numerous long term effects on ensuring the preservation of habitat types for a minimum of three decades.

Is Biodiversity Net Gain mandatory?

The Environment Act received Royal Assent in England in November 2021. It introduces a requirement to deliver biodiversity gain for developments in England. Following a transition period, mandatory biodiversity net gain came into force on 12 February 2024 for major development and 2 April 2024 for small sites.

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.”

Paragraph 180 states:

“When determining planning applications, local planning authorities should apply the following principles: … opportunities to improve biodiversity in and around developments should be integrated as part of their design, especially where this can secure measurable net gains for biodiversity…”

Numerous Local Planning Authorities have been requesting the assessment for numerous years. Many have introduced or are currently amending local developers’ plans to ensure it is mandatory as a part of Local Policy.

The Act will require the key points:

• Developers must deliver a minimum of 10% net gain through their schemes; this will be measured through the statutory biodiversity metric.
• A developer will need to demonstrate how biodiversity gain will be delivered. This will be demonstrated through the production of detailed Landscape Planting Schemes, Landscape Management Plans and Monitoring assessments on and/or off-site.
• A mitigation hierarchy is to be followed and demonstrated to avoid, minimise or compensate. If it is not possible to compensate on the development site, then offsetting will be required elsewhere. This will be done through discussions of third party land owners, the council, landbanks or wildlife charities.
• Developers will have to guarantee the biodiversity gain is maintained for at least 30 years (as outlined in Landscape Management Plans).
• New “local nature recovery strategies” will be prepared to geographically cover England by “responsible authorities”; this will encourage habitat creation and enhancement in the right places.
• Conservation covenants will be a mechanism used to deliver this .
• Natural England operates the biodiversity gain site register for off-site gains.
• Metrics are only concerned with habitats and do not take protected species into consideration.
• Other ecological legislation and policies still apply.

The aim is to get clients to think about biodiversity during the initial land acquisition and design stages and avoid retro-fitting the calculation once designs have been produced. Retrofitting will often lead to delays, unpredicted financial costs and difficulties with planning application determinations.

Biodiversity Net Gain Principles

Biodiversity Net Gain principles

According to the Chartered Institute of Ecology and Environmental Management (CIEEM), there are ten crucial good practice principles of Biodiversity Net Gain. These should help to achieve BNG if they are followed accordingly. The mandatory biodiversity net gain principles include the following:

• Utilise the mitigation hierarchy to minimise the impact on biodiversity
• Eliminate any negative impacts on biodiversity
• Communicate each BNG outcome with complete transparency
• Cover all areas of sustainability, including societal and economic factors
• Involve any pre-development and post-development stakeholders in creating mandatory net gain solutions
• Focus on producing long-term environmental benefits from BNG
• Understand the variable factors and potential risks in order to achieve and deliver biodiversity net gain
• Offer nature conservation that exceeds the stated BNG requirements
• Determine a suitable method when measuring biodiversity net gains
• Ensure the best possible results from biodiversity net gain

For case studies and a practical guide on biodiversity net gain principles, visit the CIEEM website. Following these principles alongside the mitigation hierarchy can help to ensure that your development proposal meets all of the biodiversity standards required by local planning authorities. This will mean that the planning application process will go smoothly and construction work can begin.

Calculating Biodiversity Net Gain

Metrics assign every habitat on a site a ‘biodiversity unit value’ according to its relative importance for biodiversity. This helps to compare between the existing value of a development site and what will be delivered through development or management. This may include an increase in natural habitats through retention and enhancement and/or creation, which goes over and above the environmental habitat originally on site.

Biodiversity net gain can be calculated through the the DEFRA statutory biodiversity metric, which requires a limited number of factors. These factors include:

The type of habitat (both on and off site)

• Any locations (if they are local environment priorities)
• The size of habitat parcels in kilometres or hectares
• The condition of any habitat parcels

The government website also provides a statutory biodiversity metric calculation tool, which can help to determine your biodiversity unit score that translates into the standards of your local authority.

Implementing BNG

If you are currently unsure of how mandatory Biodiversity Net Gain could affect your planning application for planning consent, you should contact your local planning authority immediately. Local planning authorities can discuss your development plans in detail and determine whether they align with the BNG standards that they need for planning approval.

If it is indicated that your development could be significantly impacted, we recommend that you contact an experienced ecologist as soon as possible. An ecologist can discuss whether your development plans meet the required standards as well as any mitigation measures you may need to take to meet biodiversity standards.

At Collington Winter, our team has strong experience completing BNG and will provide guidance throughout the process. We can identify and provide information on any mitigation methods that can be taken to assist with a planning application.

How can Collington Winter assist with Biodiversity Net Gain mitigation?

Our team of ecologists and landscape architects have helped numerous clients to achieve Biodiversity Net gain through mitigation over the years. Delivering BNG is something that we are qualified and experienced in, and we can offer advice and guidance on any mitigation methods your project may need to make in order to meet biodiversity net gain requirements for planning permission.

Please get in touch if you would like further information about Biodiversity Net Gain or Land 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 through mitigation methods.

Our Ecology Director, Olivia Collington, holds a Natural England license. Use the contact details below if you would like to find out more about the services we provide.

Start with avoidance, not a unit purchase

Biodiversity net gain should support better development decisions rather than provide a shortcut around ecological harm. Begin by understanding the site, avoiding damaging impacts where possible and reducing those that remain. Restore affected habitats where appropriate, then consider compensation for residual losses alongside the mandatory biodiversity gain hierarchy.

For development within England’s mandatory regime, the statutory objective is at least 10% gain, measured through the appropriate metric. This requirement generally applies to major applications made from 12 February 2024 and small-site applications made from 2 April 2024, subject to exemptions and transitional rules. A positive calculation does not remove separate duties relating to protected species, designated sites or irreplaceable habitats.

Use the baseline to identify what should be retained

A habitat baseline survey provides the evidence needed to make informed avoidance decisions. UKHab classification identifies habitat types, while a condition assessment establishes relevant habitat quality against the metric criteria. The assessment should distinguish genuine opportunities for enhancement from habitats whose existing value makes their loss difficult to compensate.

Retaining a mature hedgerow or established grassland may reduce ecological risk more effectively than replacing it after construction. Retention must nevertheless be realistic: construction access, drainage changes, shading and future maintenance can affect habitats beyond the building footprint. A BNG consultant should coordinate with the wider design team so that retained features are protected in practice, not merely shown on drawings.

Test on-site options against ecological and operational realities

The statutory biodiversity metric translates habitat losses and proposed gains into units, accounting for factors such as distinctiveness, condition, time to establishment and delivery difficulty. Its trading rules also influence which replacement habitats are acceptable. Where eligible, the small sites metric provides a simplified alternative for smaller proposals.

On-site delivery can include habitat creation or enhancement, but outcomes must be compatible with the development’s use. Recreation, lighting, utilities and drainage maintenance can all constrain target condition. Test these interactions before promising ambitious gains. A habitat that cannot realistically achieve its stated condition may leave the scheme dependent on an unreliable forecast.

Understand when off-site biodiversity units are appropriate

Where gains cannot be delivered appropriately on site, off-site biodiversity units can address the remaining requirement. A habitat bank is one potential source: land is managed to create or enhance habitats, with eligible gains made available for allocation to development. Other individually arranged off-site gain sites may also be suitable.

The off-site unit selection process should begin with the development’s actual metric outputs. Different habitat categories are not freely interchangeable, and area habitat, hedgerow and watercourse units are calculated separately. Check the required habitat types and trading rules before discussing commercial quantities. A generic offer of units may not satisfy the specific losses recorded in your assessment.

Compare location and strategic significance carefully

The location of off-site delivery affects the statutory metric through its spatial risk rules. A geographically distant gain site can require more units than a suitable local option. Compare proposals using the applicable metric outputs rather than simply comparing the quoted quantity of units in isolation.

Strategic significance is a separate consideration. An LNRS, or Local Nature Recovery Strategy, can identify priorities and areas where habitat action contributes to wider nature recovery. Use the relevant published strategy and metric guidance to support scoring. Neither proximity to the development nor a general claim of landscape connectivity automatically justifies a particular strategic significance category.

Check the legal security behind a habitat bank

For off-site gains to count, the habitat commitments must be legally secured for at least 30 years through a section 106 agreement or conservation covenant. A section 106 agreement involves the relevant local planning authority, while a conservation covenant is made with an eligible responsible body. The correct arrangement depends on the land and project circumstances.

The gain site must also be registered on Natural England’s biodiversity gain sites register, with the relevant gains allocated to the development. Registration, allocation and the commercial purchase agreement serve different purposes. Review all three rather than treating an invoice or unit reservation as complete evidence of compliance. Our habitat banking guidance explains the wider delivery model.

Make long-term habitat performance part of due diligence

A habitat management and monitoring plan should translate the proposed ecological outcomes into practical actions. It should cover establishment, routine management, monitoring methods, reporting, responsible parties and measures for addressing underperformance. Management prescriptions need to fit the land’s soils, hydrology, access and wider use.

Developers should understand who funds and implements these commitments and how responsibilities survive changes in ownership. Ask for evidence supporting habitat targets rather than relying solely on projected unit totals. No ecological outcome is risk-free, but clear responsibilities, realistic habitat design and a credible monitoring framework provide a stronger basis for delivery than unsupported assurances about future condition.

Reserve statutory credits for the last resort

Statutory biodiversity credits are the government’s last-resort route where the required gain cannot be achieved through suitable on-site and off-site delivery. They are distinct from privately supplied biodiversity units and are subject to their own purchasing and calculation requirements. Their use should follow the biodiversity gain hierarchy, not replace early investigation of realistic habitat solutions.

The final biodiversity gain plan must accurately describe the chosen combination of measures and supporting evidence. Align the approved metric, registered off-site allocations, legal documents and management commitments before development begins. This creates a clearer route from avoiding impacts to securing measurable, lasting biodiversity outcomes.