05 September 2019

The government is to introduce a mandatory requirement for development in England to deliver ‘biodiversity net gain’ of 10 per cent. Provisions of the new Environment Bill will be aimed at ensuring, wherever possible, that developers leave wildlife habitats in a measurably better condition than they were before development started. Following consultation earlier this year, the government is still exploring the practical mechanisms to deliver biodiversity net gain and deciding which organisations will be involved.

What is biodiversity net gain?

Biodiversity net gain ('BNG') is both an approach and an outcome. It involves not only avoiding or mitigating harm to natural areas, but also seeking to improve them through the creation or enhancement of habitats over and above what is there already. This will require baseline assessment of existing habitats on a given site and evidence to demonstrate that a net gain has been achieved.

To facilitate this, the government proposes to improve environmental mapping across England by supporting the creation of local nature recovery strategies (LNRSs). Each LNRS will include a statement of biodiversity priorities for the area covered by the strategy, as well as a local habitat map. It is intended that LNRSs will inform planning decisions and underpin local action to protect or enhance biodiversity.

How will net gain be measured?

In the first instance, measurements will be made using the Biodiversity Metric 2.0 (beta version), an online tool published by Natural England. This will be subject to further updates during the two-year transition period to address the issue of connectivity between habitats and to allow for technical developments. The beta version does not yet take account of specific species. Instead it uses broad habitat categories as a proxy for the species communities they support. Different habitats are scored according to their relative biodiversity value with scarce or declining habitats typically scoring more highly than those that are more common. Depending on the condition and location of the habitat, this value is then used to calculate the number of existing ‘biodiversity units’ within a specific development site.

The metric uses habitat area as its core measurement and linear habitats such as hedgerows, rivers and lines of trees are assessed according to their length. Developers will need to understand the different ‘habitat parcels’ on a site, each of which will have its own biodiversity unit value. They will then use their design plans to calculate the biodiversity unit value of the habitats to be retained after works finish, together with the value of any enhanced or newly created habitats. The change in biodiversity is calculated by subtracting the total number of projected post-intervention units for each habitat type. The figure arrived at should show a net gain of at least 10 per cent to be maintained for at least 30 years.

What happens if a 10 per cent gain is not achieved?

If a development scores less than 10 per cent, off-site compensation, or a revised design may be needed. According to the Biodiversity Metric 2.0 rules, losses of habitat must be compensated on a ‘like for like’ or ‘like for better’ basis (rule 3). Ecological equivalence or better is not determined by the area of habitat created, but by the net change in biodiversity units (rule 5).

Developers who are unable to mitigate biodiversity net loss or purchase biodiversity units locally, will be required to pay a cash tariff on their shortfall against net gain obligations. The tariff system will strongly incentivise protecting existing habitats and creating local habitat creation by imposing a fixed charge of between £9,000 and £15,000 per biodiversity unit (subject to review), which will be used to fund compensatory habitats.

Conservation covenants

New legislation will also make provision for conservation covenants – voluntary agreements (similar to section 106 agreements) between a landowner and a ‘responsible body’ – designed to give long-term assurance that compensatory habitats will be maintained. The covenants, which will run with the land, will be legally binding.

Are there any exemptions?

Irreplaceable habitats such as ancient woodland, ancient and veteran trees, blanket bog, limestone pavement, sand dunes, salt marsh and lowland fen will all remain out of scope of the mandatory net gain requirement. These sites will continue to be protected by requirements of existing law and policy which are typically more stringent than the BNG approach.

Other types of development which, for the time being, will not need to provide BNG are: 

  • Nationally significant infrastructure projects (NSIPs)
  • Marine development
  • Urban brownfield sites which do not contain protected or priority habitats and face genuine viability issues
  • Permitted development and householder extensions

The government also proposes that some developments, for example brownfield and small sites, could be subject to a simplified assessment process. 


Biodiversity net gain requirements will come into effect during a two-year transition period which begins when the Environment Bill receives royal assent. Before then, there is much detail to be settled. Roles and responsibilities (e.g. creation of LNRSs, tariff collecting and spending) have yet to be allocated. Even once the legislation is in place, due to the complexity of the issues BNG seeks to address, the legislation will allow some flexibility so that the system can evolve and adapt. It seems likely therefore that the BNG will be a work in progress for some time.

For advice or further information on biodiversity net gain, please contact either Liz Dunn or Sarah Sutherland.

Key contact

Elizabeth Dunn

Elizabeth Dunn Partner

  • Energy and Utilities
  • Infrastructure
  • Real Estate

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