Biodiversity Net Gain (BNG)
Advice Note

Updated March 2024


  • The Environment Act 2021 requires that all qualifying developments requiring full planning permission within the National Park must deliver a 10% net gain for biodiversity from 2 April 2024.
  • For major developments, 10% biodiversity net gain became mandatory on 12 February 2024.
  • The biodiversity value of the site, as existing and as proposed, must be calculated using one of the Biodiversity Net Gain metrics, published by the Government.
  • The main Biodiversity Net Gain metric must be completed by a qualified ecologist. The ‘Small Sites Metric’ must be completed by someone capable of identifying habitats in line with UKHabs classifications and able to identify the management requirements for the proposed habitats.
  • The completed biodiversity metric, and other supporting information, will be required prior to the validation of any application.
  • The enhancements proposed will be secured, through planning conditions and legal agreements for a 30-year period.

Why have we produced this note?

This note provides an overview of Biodiversity Net Gain and the requirements for submission with planning applications. Although the principles of Biodiversity Net Gain were approved in 2021 through the Environment Act, and further guidance was released in November 2023, the principle of Biodiversity Net Gain is still new. This is a working document, which will be continually revised as new information is made available. It is solely intended to provide clarification on the current requirements for Biodiversity Net Gain within the North York Moors National Park.

Biodiversity Net Gain is the practice of, when carrying out most new development, leaving the natural environment in a measurably better state than it was before. This means ensuring that the biodiversity value of a site has increased post development. A metric must be used to convert the value of the habitat, pre and post development. This calculates how much habitat is lost through development and provides a numerical value as to how much needs to be provided to achieve a net gain.

Under the Environment Act 2021, new development in England must deliver at least 10% Biodiversity Net Gain, otherwise planning permission cannot be granted. This will be mandatory from 12 February 2024 for major development sites, and from 2 April 2024 for small scale sites.

Biodiversity Net Gain will be measured using a biodiversity metric and habitats will need to be secured for at least 30 years. A planning application should indicate generally how the Biodiversity Net Gain will be met with a pre-commencement condition attached to planning permissions requiring full details. This means that before development can take place the details of how a 10% net gain will be achieved must be submitted to and agreed by the National Park Authority.

Local Plan policies

Within the National Park, we already have local plan policies to require that development does not cause a detrimental impact on our habitats and wildlife. These policies include:

  • Strategic Policy A - Sustainable development means development which “maintains and enhances geodiversity and biodiversity through the conservation and enhancement of habitats and species”
  • Strategic Policy E - “The quality and diversity of the natural environment will be conserved and enhanced” and
  • Strategic Policy H - All development will be expected to; “Maximise opportunities to strengthen the integrity and resilience of habitats and species within the National Park and provide a net gain in biodiversity”

Major vs minor developments

For major development Biodiversity Net Gain became mandatory on the 12th of February 2024. Major developments are defined in article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2015. Small scale developments are developments which do not meet that definition and could include:

  • residential development where the number of dwellings is between 1 and 9 on a site of an area 1 hectare or less, or if the number of dwellings is unknown, the site area is less than 0.5 hectares
  • commercial development where floor space created is less than 1,000 square metres or total site area is less than 1 hectare
  • development that is not the winning and working of minerals or the use of land for mineral-working deposits
  • development that is not waste development

Biodiversity Net Gain is mandatory for small scale development from the 2nd of April 2024.


There are very few exemptions to the national requirement for Biodiversity Net Gain. Within the North York Moors these are:

  • Urgent Crown developments
  • Developments that fall below a ‘de minimis’ threshold of 25m2 for area habitats, or 5m of linear habitats (i.e., hedgerows)
  • Householder applications
  • Small scale custom or self-build housing
  • Developments undertaken for the purpose of fulfilling the Biodiversity Net Gain planning condition for another development
  • High speed rail transport networks
  • Development granted planning permission by a development order under Section 59. This includes permitted development rights
  • Some Section 73/96a applications. Please contact a planning officer if you need to check

Small scale custom or self-build housing are only exempt where there will be no more than nine dwellings, on a site no larger than 0.5 hectares, and consisting exclusively of dwellings that are self-build or custom housebuilding as defined in Section 1(A1) of the Self-build and Custom Housebuilding Act 2015. In practice, the initial owner of the home will have primary input into its final design and layout.

The ‘de minimis’ threshold is only applicable if the habitat impacted is not a priority habitat.

Applications under the de minimis threshold will still be scrutinised; It is not acceptable to damage the habitats onsite prior to the planning application submission. Where onsite biodiversity has been degraded recently, whether it is intentional or otherwise, the condition of the site prior to the degradation will be used.

The mitigation hierarchy

The implementation of Biodiversity Net Gain does not negate the need to apply the mitigation hierarchy.

The mitigation hierarchy is the order of which impacts are dealt with as part of a development. Every effort should be made in the first instance to avoid impacts from the development. If impacts cannot be avoided, they should be minimised, such as through measures to reduce the duration, intensity, or extent of impacts. The third step is restore - reinstating or improving degraded habitats where the development has had an impact. Finally, if the impacts cannot be avoided, minimised, or the natural system reinstated following the development, then offsetting is required. This aims to compensate for any residual, adverse impacts.

Diagram - the mitigation hierarchy

Measuring biodiversity

For applications that will require biodiversity net gain, there are two calculation tools, known as metrics, which have been developed by the Department for Environment Food and Rural Affairs. These are the Biodiversity Metric, and the Small Sites Metric. Both are available as excel spreadsheets, and can be downloaded from the Government website.

Biodiversity Metric

The Biodiversity Metric needs to be used for larger developments or more complex cases. This includes residential developments of more than nine houses on a site of more than 1 hectare, or any other development on a site of more than 0.5 hectare. It is important to use the most up-to-date version when calculating Biodiversity Net Gain.

Small Sites Metric

The Small Sites Metric is a slightly simplified version of the Biodiversity Metric, designed to be used in smaller scale development – most applications for development in the National Park are likely to fall below the thresholds where small sites metric can be used. The main difference is that this metric, unlike the main metric, does not factor in the condition of biodiversity habitats that are on-site before development. It should not be necessary to have specialist ecological experience to complete this metric. This metric can only be used when all the below criteria are met:

  • The development is a residential development where the number of dwellings to be provided is between one and nine inclusive on a site having an area of less than one hectare OR where the number of dwellings to be provided is not known but the site area is less than 0.5 hectares OR for all other development types where the site area is less than 0.5 hectares
  • There is no priority habitat within the development area (excluding hedgerows and arable margins)
  • There is no off-site biodiversity assessment included. If any off-site habitat creation or enhancement will be required, this must be assessed using Biodiversity Metric

A Phase 1 habitat map of the National Park was completed in 2021 and is available for review on our website. This covers the entire National Park and can be used to help assess the habitats present on the development site and assess the likelihood of priority habitats being present.

Using the metric

Baseline calculation

The metrics work by assessing the value of the habitats onsite prior to development and convert the biodiversity value onsite into numerical units. This calculation requires:

  • The habitat onsite and the area of habitat in hectares: The habitats are classified using the UKHabs habitat classification and are selected from a drop-down menu. This gives the “distinctiveness” score
  • Habitat condition: This is calculated using the Habitat Condition Assessments provided by Natural England, included as a spreadsheet to download, or in the Technical Supplement. This involves good ecological knowledge to complete and is therefore not included in the Small Sites Metric
  • Strategic significance: This relates to the identification of the site/habitat in local policy, plans or strategy. Recognising strategic significance gives extra value to habitats that are in optimal locations or are of a type that meet local objectives for biodiversity. This has been designed to tie into the Local Nature Recovery Strategies (LNRS), and an area identified in the LNRS will be of strategic significance

The Planning Advisory Service (PAS) states that, for the Biodiversity Metric, the person completing it “should be an ecologist” and “able to confidently identify the positive and negative indicator species for the range of habitats likely to occur in a given geographic location at the time of year the survey is undertaken”.

Whilst it is not essential that the Small Sites Metric is completed by a suitably qualified ecologist, the base requirement for is that it must be completed by a ‘competent person’. This is defined as someone who; “is confident in identifying habitats present on the site before the development AND identifying the management requirements for habitats which will be created or enhanced within the landscape design.”

Post development

To calculate the biodiversity value of a site after development the metrics require the proposed habitats, and their size and condition be included. As these habitats will not yet have been created/enhanced, this is an estimation of the proposed condition, based on appropriate management, and use of the Habitat Condition Assessment Sheets.

This will be modified by the following delivery risks:

  • The difficulty of creating/restoring/enhancing the habitat to the proposed condition
  • The time it will take to reach the proposed condition
  • A spatial factor, based on the distance between the site of habitat loss (the development site) and where the creation/enhancement will happen – the further away the biodiversity enhancement the less valuable it is

The baseline units are then compared with the post development units, to assess if the development can deliver 10% net gain in biodiversity.

Onsite vs off-site habitat enhancement/creation

There is a strong bias for on-site habitat enhancement and creation within the metrics, and onsite enhancement should be the first choice for development.

If it is not possible to deliver enough units post development, and 10% net gain cannot be achieved onsite, then off-site units will be needed. There are two main options for off-site units, using other land owned/controlled by the applicant to deliver additional units, or purchasing units from a third party.

If offsite land within the applicant’s control is being used, and the land is adjacent to the development site, the easiest way to show that the additional land will provide the required net gain is to include the extra land in the red line and measure the value of it as part of the baseline. This means that when it comes to calculating the biodiversity value after development the enhancement of the extra land will be measured along with the land that is to be developed. If the land is not able to be included in the red line boundary, then it needs to be treated as off-site units in the metric, which means that the spatial factor will apply and will reduce the value of the units.

Biodiversity Net Gain which is included on the development site will be secured as a condition of any consent. Off-site Biodiversity Net Gain will need to be secured via a legal agreement.

The market for biodiversity units is an emerging one, and there are already providers such as the Environment Bank, which are offering units for purchase. At the moment, there is no plan for the Authority to hold biodiversity units, and therefore external providers will be required.

What is required for validation?

For applications involving Biodiversity Net Gain the following information will be required before an application can be validated.

  • the pre-development biodiversity value(s), either on the date of application or an earlier proposed date (as appropriate);
    • where the applicant wishes to propose an earlier date (than the date of application) for the purposes of calculating pre-development onsite habitat value, the proposed earlier date and the reasons for proposing that date must be provided;
  • the completed biodiversity metric calculation tool with Macros enabled showing the calculations of the pre-development biodiversity value of the onsite habitat on the date of application (or an earlier date) including the publication date of the biodiversity metric used to calculate that value. It is essential that the whole metric is submitted with the planning application, rather than just the summary or extracts. This is so that the metric can be interrogated to ensure that all sections have been completed correctly and that there has been no attempt to come to a more positive result.
  • a statement whether the biodiversity value of the onsite habitat is lower on the date of the application (or an earlier date) as a result of activities carried out (known as ‘degradation’), and if degradation has taken place the applicant must provide:
    • a statement to the effect that these activities have been carried out;
    • the date immediately before these activities were carried out;
    • the pre-development biodiversity value on the date before degradation was carried out; o the completed metric calculation tool showing the calculations; and
    • any available supporting evidence of this.
  • a description of any irreplaceable habitat on the land to which the application relates, that exists on the date of application, (or proposed earlier date); and
  • plan(s), drawn to an identified scale and showing the direction of North, showing onsite habitat existing on the date of application (or an earlier date), including any irreplaceable habitat (if applicable). This should be completed either using the Phase 1 habitat definitions or the UK Habitats definitions.

What will be secured?

The National Park Authority, as the local planning authority is required to secure ‘significant’ Biodiversity Net Gain for 30 years. There is no legal definition of ‘significant biodiversity net gain’ and so this will be subject to local interpretation and definition.

Within the North York Moors, ‘significant biodiversity net gain’ will be defined as ‘where habitat creation/enhancement is proposed which will require regular management to ensure that it meets the criteria for the habitat, in line with UK Habs, and the correct condition, where relevant.’

For all developments requiring biodiversity net gain a standard deemed condition will be included on all planning permissions, requiring the submission of a Biodiversity Gain Plan. A template for this is available on the Government website. In more complex cases a Section 106 agreement will be required for which a fee is payable.

Where significant Biodiversity Net Gain is proposed, a Habitat Management and Monitoring Plan will be required. This should include detailed information on how the habitat creation/enhancement will be undertaken, and how the habitats will be managed and monitored for the next 30 years. It should set out a schedule of monitoring visits and the surveys that will be required, as well as a caveat for regular reviews of the management measures and may include a procedure for remedial measures if the habitat is not developing as expected.

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