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Biodiversity Net Gain Mitigation for Development

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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. There is a two-year transition period before the net gain requirement becomes law (in autumn 2023).

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 a metric, currently Metric 4.0.
  • 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 (this approach is in preparation by Defra and Natural England).
  • A national register of land used for biodiversity gain will be established; this will involve setting up a new biodiversity credits market.
  • 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 DEFRA biodiversity metric 4.0, 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 biodiversity metric 4.0 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.

Registered Office

23 Bark Street East, 1st Floor, Bolton



01204 939 608


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