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BNG – An Introduction

Policy introduction

Abby Robinson provides an introduction to BNG.  The Government in England is launching a new policy that from November 2023, it is mandatory for building developments to ensure a Biodiversity Net Gain (BNG) of at least 10%; this agreement being for a minimum of 30 years. Developers must try to deliver as much biodiversity as they can onsite, if they cannot then they can offset this somewhere nearby. Therefore, if a developer decides that they cannot produce the necessary 10% BNG uplift on the site being developed, then they must purchase ‘Biodiversity units’ from an offsite provider. Purchase options include statutory credits from Natural England, or from third party land managers such as farmers implementing a change in their agricultural practices.

DEFRA Metric 4.0

This is a calculation tool, as specified by UK Government’s DEFRA department, to identify how the biodiversity units on a piece of land will change with a change in land use or management. The metric uses vegetation habitat as a proxy to measure and monitor biodiversity; it considers habitat type (e.g., wetland, grassland etc.), size by area, distinctiveness (ecological importance e.g., priority habitats), condition and strategic significance (location of the habitat e.g., connected to a nature reserve).

Baseline assessment

Before a development occurs, a BNG baseline assessment must take place. Existing habitats must be entered into the metric, this will calculate any units lost from the development. The developer then must produce these units, plus the relevant minimum 10% uplift, elsewhere, either onsite or offsite.

If a landowner wants to become an offsite provider, then they must also complete a baseline assessment in the metric. Again, the landowner will be inputting existing habitats for the baseline.

BNG works on the principle ‘like for like, or like for better’, this means that if a habitat is lost, the habitat must be replicated, or one that has higher distinctiveness must be created. This means that habitats that are very ecological important are protected.

Post development

This part of the metric should be filled out once the proposed habitats are known, as the creation or enhancement of habitats depends on many factors e.g., the type of management, sediment, soil, local plans etc. These factors should be taken in consideration before committed to.

A developer should already know what existing habitats they are going to impact (identified in the baseline assessment), and they therefore need to tailor their post development BNG plan around creating habitats that are either more ecologically important (higher distinctiveness), or if the habitats being developed on are already very ecologically important, they should then be replicated  elsewhere.

If a developer needs a particular habitat to be created, then they should seek out a landowner that is able to create the needed habitat. Therefore, early engagement from the developer to a nearby landowner is advantageous.


For the units to viable, then the trading rules on the metric must be satisfied. If not, then the BNG project needs to be altered. Trading rules will be satisfied once the 10% net gain has been achieved, and any losses have been correctly compensated, e.g., the like for like, or like for better principles have been accomplished. The trading rules box within the results tab in the metric will turn green once satisfied and the units can then be traded.

Any units left over, can either be sold or banked.

How CSX measures BNG

Through the use of remote sensing and earth observation data, CSX accurately map habitats across the whole of a BNG site, and then gather data that audits and demonstrates the BNG progress over time.

RGB drone imagery is captured to a resolution of less than a centimetre per pixel, and processed into orthomosiacs of a site in great detail. The resulting imagery, which can be used in conjunction with a site walkover survey, supports the BNG assessment. Having the photographic data means that habitats can be mapped more accurately, important ecological features can be highlighted, and vegetation data can be collected. Attaching scientific data that is repeatable and robust to BNG ensures enforcement and transparency, which will be vital to delivery.

CSX offers further opportunity to go beyond BNG as defined by Government, by accounting for various aspects of biodiversity other than just vegetation habitat. Through the use of new technology and innovative data sources, we can provide a more holistic view of the BNG project that goes above and beyond the metric.

What to consider before entering BNG

What are my current management practises?

Your current management practises will have an impact on what habitats can be created e.g., if fertiliser has previously been used then the land parcel may need to be nutrient stripped first for certain habitats to be created.

What management practices can I realistically undergo for 30 years?

If you’d prefer a more relaxed management regime, then this will influence the proposed habitats. The habitats that require more complex managements generate more units, however, you will have to commit to the management regime for 30 years.

What is my financial model?

Deciding how you plan to finance this BNG project will also influence your post development. For example, if you require funding before the project carries out then it may be better to ask us to find a potential buyer needing units/habitats the farmer or land owner can supply.

Who will carry out the works?

If the habitat you want to create has a complex management regime, then you may need multiple people carrying out the works. Understanding the resources, you have available, and how long you have the resources for will help identify which habitats are feasible.


BNG is a new governmental initiative impacting property developers, that provides farmers and landowners with an opportunity to change their land management practices to ones that are more environmentally favourable, and then use the results of these changes to trade as an offsite provider and be financially rewarded by developers for having done so.   

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