What biodiversity net gain units actually involve

Biodiversity Net Gain (BNG) is a legal requirement in England under the Environment Act 2021, mandating that new developments deliver a 10 percent improvement in biodiversity value compared to the pre-development baseline. This improvement is measured using the DEFRA biodiversity metric, which converts habitat area, condition and distinctiveness into a unit figure. Developers must either create habitat gain on-site, off-site, or purchase statutory biodiversity credits from government, and the gain must be secured for at least 30 years via a legal agreement.

Why it's harder in practice than it looks

The DEFRA metric is technical and easy to miscalculate

The biodiversity metric requires accurate habitat mapping, condition assessments conducted by a qualified ecologist, and correct application of distinctiveness, connectivity and strategic significance multipliers. Errors in baseline assessment directly affect the number of units required and the cost of compliance.

On-site habitat creation is often constrained by development design

Most developments cannot deliver the full 10 percent net gain on-site without compromising developable area or increasing infrastructure costs. Developers who leave the BNG calculation to late in the planning process often discover that on-site delivery is insufficient, requiring expensive off-site or statutory credit purchases.

Off-site BNG market is early-stage and variable in quality

The market for off-site BNG units is growing rapidly but lacks mature price transparency and quality standards. Buyers face real risks of purchasing units from habitat banks that will not deliver the ecological outcomes committed, or that will not meet legal requirements for the 30-year maintenance obligation.

BNG must be secured before planning permission is granted

Under mandatory BNG, developers must demonstrate compliance with the 10 percent net gain requirement as a condition of planning permission. This means the BNG strategy, including securing off-site units if needed, must be resolved during the planning process, not after it.

What good looks like

A well-managed BNG process starts at the pre-application stage with an accurate baseline habitat survey, a realistic on-site habitat creation assessment, and a BNG strategy that is integrated into the masterplan rather than added as an afterthought. Where off-site units are required, legal agreements with habitat bank providers are in place before planning is submitted. Post-consent, a habitat management and monitoring plan covering the full 30-year period is prepared and agreed with the local planning authority.

When to bring in external support

The combination of ecological expertise, legal knowledge and commercial understanding required for BNG makes it difficult to manage with generalist internal teams. Ecology consultants who understand both the technical metric and the emerging off-site market can significantly reduce compliance costs and de-risk the planning programme. Leafr's network includes BNG specialists who have delivered net gain strategies across residential, commercial and infrastructure projects, helping developers meet planning requirements without unnecessary overpayment on off-site units.

Frequently asked questions

What is biodiversity net gain and when did it become mandatory?

Biodiversity Net Gain is the requirement that development projects leave biodiversity in a measurably better state than before. It became mandatory for most major developments in England in February 2024 under Schedule 14 of the Environment Act 2021, and was extended to small sites in April 2024. It requires a minimum 10 percent uplift measured using the DEFRA biodiversity metric.

How are biodiversity units calculated?

Units are calculated using the DEFRA biodiversity metric, which assigns a value to each habitat type based on its area, condition and distinctiveness. The pre-development baseline is compared against the post-development outcome, and the difference determines whether net gain has been achieved. The calculation must be prepared by a suitably qualified ecologist.

What are statutory biodiversity credits and when would a developer use them?

Statutory biodiversity credits are units sold by government as a last resort where on-site or off-site gain cannot be achieved. They are priced by DEFRA at a premium above market rates to incentivise habitat creation in preference to credit purchase. Developers should explore genuine on-site and off-site options before using statutory credits.

How long must biodiversity net gain be maintained?

BNG habitat must be secured and maintained for a minimum of 30 years. This must be legally secured through a planning condition, Section 106 agreement or conservation covenant before planning permission is granted. Landowners providing off-site habitat are responsible for ensuring the habitat meets its condition targets throughout this period.

Does biodiversity net gain apply to all development types?

Mandatory BNG applies to developments requiring planning permission in England under the Town and Country Planning Act, with exemptions for householder applications, permitted development, and developments affecting a habitat area of less than 25 square metres or five metres of linear habitats. Nationally significant infrastructure projects are subject to a separate BNG requirement under the Planning Act 2008.