Biodiversity net gain

Table of Contents

Introduction to BNG

Biodiversity Net Gain (BNG) is the process by which any major development that goes through the planning application process must deliver at least 10% net gain on the biodiversity baseline pre-development.

This can be delivered with a hierarchy of 1) onsite, 2) mix of on-site and off-site with a private landowner or on their own off-site land, 3) off-site statutory credits from the Government as a last resort.

This became statutory for major developments from Monday 12 February 2024 and minor developments from Tuesday 2 April 2024, therefore, relevant applications that are submitted on or after these dates will be required to follow the BNG process.

The amount of biodiversity needed to reach 10% net gain is calculated using a metric developed by Department for Environment, Food and Rural Affairs (DEFRA). 

A baseline ecological survey will be undertaken at the development site to assess the ‘before’ condition and, when input into the Statutory Metric, the baseline units can be calculated. The number of these baseline units that will be lost during development must be replaced +10% of the baseline amount. This applies for off-site and on-site.

For example: if a site has a baseline of 10 units and six are lost during development, you will need to put a total of seven units back on-site (the six that you lost + one as 10% of the baseline 10). Alternatively, you could put those seven units off-site where the uplift of that land due to the Habitat Management and Monitoring Plan (HMMP) results in a seven-unit enhancement, or a mixture of both approaches, as long as you achieve an overall unit total of 11.

Where BNG must be delivered off-site, an ecological assessment will be conducted on the proposed off-site land to calculate its baseline condition and to determine the uplift required to achieve those units.

The biodiversity enhancements must be monitored, maintained, and legally secured for 30 years under a S106 or a Conservation Covenant (CC). A CC is a private, voluntary agreements (between a landowner/leaseholder and a responsible body (e.g. LPA or conservation body/organisation) to conserve the natural or heritage features of the land, in this case to ensure specific land management processes and outcomes.

Our Role as the Local Planning Authority (LPA)

Whether a developer achieves their BNG on-site, or off-site with a private habitat bank or landowner, the LPA will need to secure that land and any significant habitat enhancements with either a S106 or CC. We may also condition any non-significant on-site enhancements.

We will be required to monitor this agreement for 30 years by requesting ecological progress assessments at given intervals (e.g. year 2, 5, 10, 15, 20, 25, 30) depending on the habitat being created/enhanced.

The council will levy a fee to monitor the net gain as required by law, in addition to any financial penalties raised in the event of a breach or unsuccessful habitat enhancement. 

Your Role as the Developer 

A major development includes:


  1. Where the number of dwellings being proposed is ten (10) or more
  2. If the specific number of dwellings to be provided is not known, a site area exceeding 0.5 hectares.


  1. The provision of a building or buildings where the allocated floor space being created because of the development is 1,000 square metres or more
  2. Where the development has an area of 1 hectare or more
  3. Change of use applications over 1,000 square metres or more
  4. All full applications concerning mineral extraction and waste development

A minor development, or small site, includes:

  1. 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.
  2. Commercial development where floor space created is less than 1,000 square metres or total site area is less than 1 hectare. 
  3. Development that is not the winning and working of minerals or the use of land for mineral-working deposits.
  4. Development that is not waste development.


  1. Development below a de minimis threshold
  2. Householder developments
  3. Small Scale Self-Build and Custom Housebuilding
  4. HS2
  5. Biodiversity gain sites

More information on exemptions.

If your units are satisfied on-site, you must manage the land enhancements to the standard set out in your Habitat Management and Monitoring Plan (HMMP) or 30 years, submitting habitat progress assessments in the years set out in your S106. 

If your units are off-site, you (if using off-site land that you own) or the land manager (if using a private habitat bank) must manage the land enhancements to the standard set out in your HMMP for 30 years, submitting habitat progress assessments in the years set out in your S106. You must also register your biodiversity net gain site and any allocations of off-site gains on the National Register. You do not need to register on-site gains or statutory biodiversity credits.

Mandatory BNG and Local Planning Policy 

If your development is exempt from mandatory BNG (major or minor submitted after mandatory dates) but falls under local policy that requires a net gain (e.g. NR2), it will remain exempt as per the national exemption guidelines above.

Non-Mandatory BNG and Local Planning Policy

If your development does not fall under mandatory BNG (majors submitted before Monday 12 February 2024 and minors before Tuesday 2 April 2024) but is captured by another net gain policy (e.g. NR2), it will be required to satisfy any biodiversity net gain requirements regardless of its pre-mandatory status.

What is required from you

When making a planning application, applicants must submit specific information to show that they can deliver habitat improvements to meet a required minimum 10% BNG. Avoiding impacts on biodiversity which already exists on the development site should be the first consideration so design of the development is very important.

Biodiversity net gain still relies on the application of the mitigation hierarchy to avoid, mitigate, or compensate for biodiversity losses.

Developers will need to submit as part of their planning application: 

  1. Biodiversity Net Gain statement confirming whether:
    • Mandatory BNG applies to the development or not
    • Habitat degradation has taken place before or after the Statutory Metric calculations were completed
  2. The Statutory Metric calculations in Excel with:
    • Pre-development baselines and post-development calculations fully completed (either on the date of application or earlier proposed date (as appropriate); where the applicant proposes to use an earlier date, this proposed earlier date and the reasons for proposing that date)
    • Date of metric completion
    • Name of competent/accredited person
    • This can be replaced with the Small Sites Metric for small sites that do not contain priority habitats and does not need to be written by an accredited person.
  3. A scaled map showing:

This is in addition to or as part of the Preliminary Ecological Appraisal/ ECIA/protected species surveys that are required for planning applications. Please ensure that the trading rules have been satisfied within the metric.

We recommend that you submit as much information as possible, in particular any off-site gains or plans for excess units.

A Draft Biodiversity Gain Plan can also be provided as part of the application, or the plan will be required as part of pre commencement conditions. 

Small Site Metric (SSM) changes

You can use the either the Statutory Metric or the simpler Small Sites Metric (SSM).
However, the SSM cannot be used on sites where the following are present: 

  • Priority habitats (excluding some hedgerows and arable field margins) 
  • Protected sites
  • European protected species

If you use the SSM, you do not have to do a condition assessment as part of your survey of existing habitat, as the values are fixed for all habitats assessed.  
If you qualify to use the SSM, you do not need an ecologist for BNG, though you may still choose to seek ecological advice. You can either use the simpler small sites biodiversity metric tool yourself if you are familiar with the site and the habitats on it, or someone else who can do a survey and make a calculation. This could be the project manager, a gardener, or a landscape architect.

Full guidance.

What you should expect

  1. Developer submits a major planning application with the provisional biodiversity metric and a draft BGP if they wish.
  2. BNG and ecological information assessed and validated by in-house Ecology team.
  3. Planning decision issued and the proposed BNG is secured through the S106 process. A monitoring fee will be incurred for the legally required monitoring on the habitat enhancement for 30 years.
  4. If approved, the developer must provide a completed BGP with the finalised metric and HMMP. You must register any allocations of off-site biodiversity gains before your BGP can be approved.
  5. The LPA will request habitat monitoring assessments from the developer/land manager in the specified years, which will subsequently be checked by the Ecology team.
  6. If deemed accurate, business as usual continues.
  7. If deemed inaccurate or insufficient, enforcement process begins.

Biodiversity Net Gain is a dynamic area of planning policy, therefore this guidance may be updated over time.

Biodiversity Net Gain : Contact details

For further information, please contact us by:

Sustainability and Climate Change Lead  - Royal Borough of Windsor and Maidenhead
Town Hall, St Ives Road
Maidenhead SL6 1RF