What is Biodiversity Net Gain?

The idea behind Biodiversity Net Gain (BNG) is that development should leave biodiversity in a measurably better state than it was before any development took place.

BNG is defined as leaving more, bigger or better habitats for biodiversity and is demonstrated by using the DEFRA biodiversity metric.  

Paragraph 174 of the National Planning Policy Framework (NPPF, 2021) requires biodiversity net gains to be achieved.

The Government has made it a mandatory requirement for certain development to achieve a minimum of 10% BNG through the Environment Act 2021.

It's currently anticipated that mandatory BNG will be required from spring 2024. This is to bring the requirement in line with national implementation. Secondary legislation and guidance will be published to support implementation.  

Purpose of this interim Technical Advice Note

The purpose of this Technical Advice Note (TAN) is to provide interim guidance for planning applicants and decision-makers on how BNG will be achieved in Brighton & Hove in accordance with the policy requirements of City Plan Parts 1 and 2, prior to BNG becoming a mandatory requirement.

This TAN explains: 

  • the type of development required to provide BNG
  • the amount of BNG required
  • how BNG should be measured
  • the information required to be submitted with your planning application
  • information on the types, locations and extent of habitats that can provide BNG
  • how BNG will be secured
  • how BNG will be monitored

This TAN only provides guidance on BNG.

Biodiversity and Nature Conservation SPD11

The council has produced an updated Biodiversity and Nature Conservation SPD11 to assist planning applicants to better understand the importance of biodiversity more generally and ensure they meet legislative and policy requirements.

The SPD provides a step-by-step guide for ensuring biodiversity is properly considered throughout the planning process.

The SPD should therefore be referred to for all other matters concerning biodiversity, including the overarching principles that apply to development, such as the mitigation hierarchy which must be addressed prior to the provision of any BNG.   

Local policy requirements 

The key local planning policies which require BNG are:

  • City Plan Part 1 Policy CP10 Biodiversity
  • City Plan Part 2 Policy DM37 Green Infrastructure and Nature Conservation

Both policies seek BNG from development, with DM37 requiring BNG to be measurable.

Section 4.2 of SPD11 provides more details on the local policy requirements relating to BNG and biodiversity.  

Biodiversity Net Gain requirements

When will BNG be required in Brighton & Hove? 

The council is phasing the implementation of requirements for measurable BNG. Initially, only major planning applications submitted after the adoption of City Plan Part 2 (20 October 2022) will be required to provide measurable BNG.  

It's currently anticipated that minor applications will be required to provide measurable BNG from spring 2024. This is to bring the requirement in line with the national implementation of BNG.

This TAN will be updated and reviewed as necessary.   

How much BNG is required? 

A minimum of 10% BNG is sought reflecting the forthcoming requirements of the Environment Act.  

Net gains are additional to any mitigation or compensation that is required to address development impacts.

Development must therefore accord with the mitigation hierarchy and additionally achieve net gains for biodiversity. This is explained further in Section 5 of SPD11.  

What type and scale of development does BNG apply to? 

The following list sets out the interim BNG requirements expected for various types and scales of development and shows how this should be measured. The second list sets out the interim exemptions from BNG. 

This interim TAN sets out BNG requirements for development ahead of mandatory BNG. Forthcoming regulations will specifically identify the types of development that are exempt from providing BNG when it becomes a mandatory requirement.

This TAN will be updated once the regulations are published.

BNG does not apply to any development where the on-site habitat includes irreplaceable habitats, as defined in the NPPF Annex 2, nor does it replace existing protections, for example for designated sites or protected species.

Interim BNG requirements 

Residential Development 

  • Scale
    • development is 10 or more dwellings; or 
    • The number of dwellings is not known and the site area is greater than 0.5 hectares. 
  • Requirement 
    • Minimum 10% BNG; on-site opportunities maximised  
  • Biodiversity Metric 

Non-residential development

  • Scale
    • Floorspace is 1,000m2 or more; or 
    • The amount of floorspace is not known and the site area is greater than 1 hectare. 
  • Requirement 
    • Minimum 10% BNG; on-site opportunities maximised  
  • Biodiversity Metric 

Interim exemptions from measurable BNG 

Type of development  

  • minor development 
  • householder development 
  • permitted development 
  • prior approvals 
  • development that impacts less than 25sqm of habitat or 5m of linear habitat such as hedgerows
  • self-build and custom build providing no more than 9 dwellings

Interim requirements 

Measurable BNG is not currently required. 

Proposals should still include the application of the mitigation hierarchy and appropriate measures that support the enhancement of biodiversity. For example:

  • bee bricks
  • swifts bricks
  • biodiverse landscaping

See SPD11 Section 6 for guidance.

How is BNG measured? 

BNG should be measured using the most recent version of the relevant Biodiversity Metric.

The Biodiversity Metric calculates a proxy biodiversity value based on the type and condition of habitats present on site prior to development and proposed through development. This allows the pre and post-development biodiversity value to be calculated by an ecologist.

View the Statutory Biodiversity Metric and user guide.

Further details on carrying out the BNG assessment can be found in Section 7 of SPD11.  

At what point in the planning process should BNG be considered? 

BNG needs to be considered at the very start of the design process with baseline habitat surveys informing the design in accordance with the mitigation hierarchy.

Further details on the mitigation hierarchy, BNG and when to consider BNG can be found in Sections 5 and 7 of SPD11.

Where should BNG be provided?  

Government consultation confirms that BNG should be achieved in a way that is consistent with the mitigation hierarchy. BNG should therefore be provided on-site in the first instance and be designed appropriately for the development and local situation.

Government guidance is expected to clarify options for what BNG will look like for urban sites or those sites with a very low baseline.

Ahead of mandatory requirements, the priority in Brighton & Hove will be for onsite BNG.  

What if BNG can’t be achieved on-site? 

Where it's clearly demonstrated that meaningful on-site BNG cannot be achieved, off-site opportunities should be pursued.

SPD11 indicates that off-site BNG could be provided for at ‘local strategic sites’, having regard to any emerging Local Nature Recovery Strategy (LNRS).

An LNRS for East Sussex and Brighton & Hove will be prepared from 2023 to 2024. In the interim, the council is considering how it can provide a role in facilitating the provision of BNG in those instances where it cannot be delivered on-site.  

Further details about off-site provision are expected through forthcoming regulations.

What is required to support a planning application

All of the following information should be submitted with the planning application and will be required for validation purposes:

  • the completed relevant full Biodiversity Metric in excel and pdf format showing the pre-development biodiversity value - a summary is not acceptable
  • a biodiversity baseline assessment/ecological study which meets the requirements of the relevant Biodiversity Metric used
  • a plan showing onsite habitat
  • a description of any irreplaceable habitat

Prior to the commencement of development, a Biodiversity Net Gain Plan must be submitted.

This should include steps taken to minimise adverse effects, pre and post development value of the on-site habitat, any registered off-site BNG units, and any biodiversity credits purchased.

It should also include a description of how the habitat enhancement and/or creation will be implemented, managed and monitored for a minimum of 30 years, and pre-development and post-development plans showing location of onsite habitat.

Requirements for outline and reserved matters applications 

For outline applications, it will be necessary to demonstrate that the development can achieve BNG in principle.

Initial versions of the documents outlined in (a) to (d) above will be required based on information available at the time. This includes an initial run of the Biodiversity Metric, a proposed habitats plan based on an indicative layout plan and a Biodiversity Net Gain Feasibility Assessment report (CIEEM, 2021).

An up-to-date habitat survey will still be needed to establish the baseline for the site. An indication of the post-development value using the Biodiversity Metric should be provided based on the proposed habitats plan.   

Reserved Matters applications should continue to demonstrate that BNG will be achieved and should add more detail to the Biodiversity Metric and other required evidence as appropriate, based on finalised layout plans.  

Where development is to be phased, a biodiversity net gain strategy must be submitted at the outline stage, which shows how individual phases deliver a predetermined proportion of the biodiversity value. Reserved matter applications will then be required to demonstrate exactly how each phase will meet its biodiversity requirements with the majority of BNG provided in the first phases of development.

How to complete a Biodiversity Metric

Defra have produced a comprehensive user guide for completing the Statutory Biodiversity Metric which should be referred to.

Key points include: 

  • the Biodiversity Metric must be completed by a suitably qualified and experienced ecologist
  • baseline habitat surveys must be undertaken during the appropriate survey season to inform the completion of the metric
  • habitat surveys must include an assessment of their condition and distinctiveness
  • the UKHab is the default format for habitat surveying for the metric

Strategic significance 

The Biodiversity Metric includes a section on strategic significance.

Strategic significance is whether the location of each habitat parcel is identified in a local plan or other strategic document as an important area for biodiversity.

Strategic documents for biodiversity often include planning documents such as supplementary planning documents and local plans, or within local nature recovery strategies, biodiversity opportunity areas and biodiversity action plans.  

Prior to the production and adoption of a Local Nature Recovery Strategy, the following are considered to be of high or medium strategic significance in Brighton & Hove:  

High significance:

  • Special Areas of Conservation
  • Sites of Special Scientific Interest
  • Local Nature Reserves
  • Local Wildlife Sites

Medium significance:

  • Biodiversity Opportunity Areas

Which types of biodiversity enhancement count as measurable BNG

Measurable BNG is based on enhancements to existing habitats or the creation of new habitats.

The Biodiversity Metric lists all the different types of habitats that can either be present within the application site, or that could be provided as measurable BNG. This includes locally important habitats such as:

  • lowland calcareous grassland
  • broadleaved woodland
  • mixed scrub

The Biodiversity Metric also lists a variety of habitats that are specifically found or could be provided as BNG within an urban context. This includes:

  • allotments
  • biodiverse green roofs
  • green walls
  • shrubs
  • urban trees
  • sustainable urban drainage features

Some of these habitats and measures may be more achievable for proposals situated within the built-up area of Brighton and Hove.  

All proposed measures should be appropriate to the development, site location and surroundings.  

Although species-based measures such as swift bricks do not count as measurable BNG, these types of measures are still important for biodiversity and should be provided where possible.  

Various examples of species and habitat-based measures that can help incorporate biodiversity into development can be found in Section 6 of SPD11

Management and monitoring

BNG will be required to be managed and monitored for a period of 30 years. It will be the landowner or developer’s responsibility to ensure monitoring and reporting obligations are fulfilled.

Monitoring should be proportionate. The number of monitoring assessments will depend on the size of the proposal, and the habitat types and extent, but it's envisaged that typically reports will be required for years: 2, 5, 10, 20 and 30.

As a minimum, reports should include a summary of habitat type, extent and condition, with a comparison against the expected condition proposed within the submitted BNG Report.

Future provisions for the management and monitoring of mandatory BNG will be set out in forthcoming government regulations.

Securing BNG


The implementation and ongoing monitoring of on-site BNG will be secured through planning conditions.

Off site

It's envisaged that the implementation and monitoring of off-site BNG will be secured through a Section 106 agreement or conservation covenant.

A conservation covenant is a legally binding agreement between a landowner and a designated “responsible body” to provide BNG on their land.

It will be the developer’s responsibility to take out a conservation covenant. Forthcoming government regulations will set out provisions for the use of conservation covenants.