FAQs: Biodiversity Net Gain (1)

As part of the Environment Act 2021 it is now mandatory for all development schemes in England to deliver a 10% biodiversity net gain and this is to be maintained for at least 30 years. In response we have produced a series of simple frequently asked questions.

What is Biodiversity Net Gain (BNG)?

Biodiversity Net Gain (BNG) is all about “leaving biodiversity in a better state than before”.  To deliver a BNG, a project must incorporate a measurable increase in natural habitat over and above what is lost. It can be achieved on-site, off-site, or through a combination of measures.

By considering BNG as early as possible in the design process you can reduce the risk of completing a scheme with a net loss. Creating a development with a net loss is not only a risk on the local biodiversity, but it could result in substantial offset costs.

Click here to read more about BNG on CIEEM’s website.

What is a BNG assessment?

A BNG Assessment compares baseline conditions to post-development plans. BNG is achieved if the post-development plans provide a net improvement to the biodiversity of a site.

As biodiversity uses habitats as a proxy for measuring biodiversity and ecological value a number of Good Practice Principles  must be applied within the assessment to ensure a holistic and well-rounded development which truly improves the ecological value of the site. The mitigation hierarchy ensures that all onsite opportunities for improving biodiversity have been maximised before offsite mitigation is considered.

Avoidance – Avert from negative impacts on the habitat if possible. Examples include finding an alternative site, retaining habitat features, or changing work timings. This is the most preferred option.

Minimise/Mitigate – Measures that aim to reduce impacts to the point where they have no adverse effects.

Compensate – Measures that compensate for residual losses of biodiversity.

Offset – Where losses of biodiversity can’t be compensated for on site. This should be a last resort.

Although we recommend that BNG is considered as early as possible in the development process, the assessment can be undertaken at several stages which dictates the specific assessment required;


  • Identifying implications for potential development projects at any scale from single unit to whole estate and portfolio
  • Audits of land for biodiversity gain capacity at land acquisition stage, akin to risk assessment or due diligence.
  • Providing advice on options for delivery of biodiversity gain on and off-site.
  • Modular River Survey Assessment.

Assessment and Design Stage

  • Baseline survey and habitat condition assessment
  • Design-phase Biodiversity Enhancement Strategy: advising on the retention of ecological features, scope for habitat creation and enhancement.
  • Combining green infrastructure, Local Nature Recovery Strategies, SANG and habitat provision.
  • GIS expertise: managing metric data and supporting calculations.

Planning Consent and Delivery

  • Planning conditions/obligations: working with a project team to ensure that the proposed requirements are reasonable, proportionate and practical.
  • Preparing long-term (30 years) Habitat Management Plans and advising on future management commitments.
  • Site-based advice and expertise in working with contractors to deliver habitat creation, restoration and enhancement.
  • Liaison and negotiation with local authorities, conservation organisations, and other third parties for agreeing the delivery of biodiversity gain.
  • Identification and negotiation of land and schemes for offsite compensation.

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