Biodiversity Net Gain : A Bitesize Guide (January 2024)

What is Biodiversity Net Gain?

To quote the official Defra definition, “Biodiversity Net Gain is an approach to development which aims to leave the natural environment in a measurably better state than beforehand” (Defra 2019).

How does BNG work?

Biodiversity net gain delivers measurable improvements for biodiversity by creating or enhancing habitats in association with development. Habitats are used as a proxy for biodiversity and are assigned a value using Defra’s Statutory Biodiversity Metric. The percentage of Biodiversity Net Gain (or loss) is calculated by comparing the difference between the pre-development and post-development habitat data. Biodiversity Net Gain can be achieved on-site, off-site or through a combination of on-site and off-site measures. Area, linear and river habitats must be measured separately and each achieve 10% net gain or higher. Protected species are not directly addressed by BNG and a separate ecological assessment is still required to accompany your planning application. Any habitat creation must be managed and monitored for a period of 30 years.

When is BNG coming into effect?

Biodiversity Net Gain (BNG) will be soon be mandatory across the whole of England for almost all new planning applications. It is part of the Environment Act, which was made law in November 2021. This was followed by a two year transition period, which has now been extended by two months to January 2024, at which point mandatory BNG will come into effect. The exact date is still tbc, but it’s widely believed to be towards the end of the month. For “small sites”, mandatory net gain will apply from April 2024. BNG is already a requirement of many Local Planning Authorities that have their own locally-specific policy in place.

What is the definition of a “small site” in BNG terms?

A small site for BNG is a minor development of nine or fewer residential units on a site smaller than 1 ha, or any other type of development on a site smaller than 0.5 ha. There is simplified version of the Defra Statutory Biodiversity Metric called the “Small Sites Metric” that can be used for small sites, although equally the standard Defra metric can also be used for small sites.

Which types of applications are exempt from BNG?

There are some exemptions to Biodiversity Net Gain:

  • Householder applications
  • Developments impacting an area smaller than 25 m2 or linear habitats less than 5m.
  • Biodiversity gain sites
  • Small scale self-build and custom housebuilding.
  • Any sites that already benefit from outline consent.

Does BNG only apply to new planning applications?

Although initially it was suggested that BNG would apply retrospectively to any live planning applications in the system, it has now been confirmed that it will only apply to new applications submitted after the start dates in January/April 2024.

Will the requirement be the same across all Local Planning Authorities?

Mandatory net gain, as set out in the Environment Act and its secondary legislation, will be the minimum requirement across the country. But LPAs have the option to create their own locally-specific policies in addition, which could potentially demand a greater level of net gain for some types of development.

What is involved in a Biodiversity Net Gain assessment?

The BNG assessment consists of a site visit by a qualified ecologist, in which all habitat types present on the development site and their condition are classified using the UKHab habitats index. The Defra Statutory Biodiversity Metric assigns a value to habitat type and condition, which is then multiplied by the extent of that habitat to give a number of biodiversity units. The total of these units will give a baseline biodiversity score and represents the biodiversity value before development.

UEEC has been carrying out BNG assessments for several years and they are now a standard component of our service for the projects we work on. After calculating the baseline score, we can advise on how you can design your scheme to maximise biodiversity value, which habitats to retain, where best to build and how you can improve and enhance your site to achieve maximum net gain. We will then use your layout, together with landscape and planting plans to calculate the post-development biodiversity value. This is compared to the baseline to give the net gain or loss, usually expressed as a percentage.

We can provide various iterations of the BNG calculation as your designs develop, making assumptions if necessary in the early stages.  Once the layout and landscape design is  finalised, we will produce a Biodiversity Gain Plan to submit with your planning application.

UEEC is able to complete all elements of the BNG process. This includes consultants with appropriate training and accreditation to conduct River Condition Assessment (RCA), which inform the watercourse elements of metric calculations.

When is the best time of year to carry out a BNG assessment?

A BNG assessment can be done at any time of year, but for some habitats, particularly grassland, it can be difficult to establish the exact habitat type and condition at certain times of the year, and a repeat site visit may be needed during May or June. If your project timescales don’t allow for this, the higher-value habitat must be assumed.

Habitat Distinctiveness Categories

Some habitat types are treated differently than others. Arable, improved grassland and urban environments are examples of “low distinctiveness” habitats and any losses can be compensated by any kind of habitat creation. “Medium distinctiveness” habitats include semi-natural grassland, recent woodland, scrub and individual trees and these must be replaced by the same broad type of habitat. Examples of “high distinctiveness” habitat include Lowland Mixed Deciduous Woodland, Lowland Heathland and ponds (priority habitat). Any losses of high distinctiveness habitat are difficult or very expensive to mitigate, so these habitats should be retained where possible. The highest category of “very high distinctiveness” applies to habitats such as ancient woodland, veteran trees and chalk streams. BNG is considered undeliverable where losses to these habitats are incurred and bespoke compensation is necessary.

Biodiversity Gain Plan

The purpose of the biodiversity gain plan is to provide a clear and consistent document in which the developer sets out how the proposals meet the biodiversity net gain objective and the planning authority can determine whether or not it has been met. The Biodiversity Gain Plan is submitted after planning permission has been granted to discharge the mandatory net gain condition. A template is now available for the Biodiversity Gain Plan and more detailed instructions will be forthcoming in the coming weeks.

Habitat Management and Monitoring Plans

A Habitat Management and Monitoring Plan (HMMP) is a detailed plan that outlines how the land will be managed for at least 30 years to create and enhance habitats for BNG and how that net gain will be managed and monitored. This will be conditioned, but the definition for significant on-site gains (, makes it likely this will be needed for most.

Biodiversity Net Gain Off-Setting

The ideal is that 10% net gain is achieved on site, but how achievable this is does depend on what kind of habitat you start off with.

Sometimes it’s just not viable to deliver 10% BNG on your development site, in which case you can provide the net gain elsewhere. If this is done on land within the same local planning authority or same National Character Area, you are not penalised and the number of units required are the same as if it was on your development site. If your off-setting site is in a neighbouring LPA, an “area risk multiplier” applies and you must provide 50% more biodiversity units and for a location beyond that, you need to provide twice as many biodiversity units. The off-setting BNG market is in its infancy, but there are landowners who are already in a position to sell biodiversity units to developers and many more in the pipeline. Any BNG off-setting off-site must be subject to a 30-year management plan and secured via either a S106 agreement or a conservation covenant. The site must also go on the BNG Register.

At UEEC, we can help you find a suitable solution if you wish to go down the off-setting route.

Biodiversity Credits

If appropriate off-setting land cannot be found, as a very last resort, you can buy statutory biodiversity credits from Natural England. The prices of these credits depend on the habitat type required but have purposely been set at an uncompetitive level. It important to bear in mind that you also must buy two credits for every unit needed. This is therefore likely to be the most expensive option and should be avoided if possible.

If you need advice on any aspects of BNG, please get in touch and we’d be very happy to help.

Ecological Surveys

Impact Assessments

Environmental Planning