Biodiversity Net Gain (BNG) Surveys
What is Biodiversity Net Gain?
Biodiversity Net Gain (BNG) is a planning requirement in England. It means most new development must leave the natural environment in a measurably better state than it was before. Developers have to show at least a 10% increase in biodiversity value, measured in biodiversity units using the government's statutory biodiversity metric. The metric scores each habitat on a site by its size, type, condition and location which gives a baseline figure the post-development design has to exceed.
The most effective approach is to bring an ecologist into the design process early, while there is still room to keep the most valuable habitats and plan new ones around them. If the full gain can't be delivered within the red line boundary, developers can buy biodiversity units from a registered off-site gain site. Statutory biodiversity credits from government are available as a last resort.
How the Requirement Works
Mandatory BNG comes from the Environment Act 2021, which inserted Schedule 7A into the Town and Country Planning Act 1990. It has applied to major development since 12 February 2024 and to small sites since 2 April 2024. It extends to NSIPs from 2 November 2026.
Before work can begin, the developer must submit a biodiversity gain plan to the local planning authority showing how the 10% gain will be delivered. Significant on-site enhancements and all off-site gains have to be legally secured through a section 106 agreement or a conservation covenant. They then have to be managed and monitored for at least 30 years.
Recent Changes to BNG
The regulations were amended on 6 August 2026 after a government review of how BNG was working for smaller schemes. Sites of 0.2 hectares or less are now exempt, as long as the development doesn't affect priority habitat. A new exemption, with the same safeguard, covers temporary permissions where the land is reinstated within 5 years. The exemption for self-build and custom housebuilding has been removed, so those schemes now need to meet the 10% requirement if they are above the 0.2 hectare threshold. Minor developments can also now secure off-site gains from the outset, where previously on-site options had to be ruled out first.
Further reforms are still in progress, including a proposed exemption for some residential brownfield development and updates to the metric itself. If you're unsure whether your project falls within scope we can review it against the current regulations.
What Happens During a Biodiversity Net Gain Assessment?
For Developers
Baseline Survey
A BNG assessment usually begins alongside a Preliminary Ecological Appraisal. One of our ecologists will survey the site and map every habitat using UKHab classification, assessing the condition of each habitat against the criteria set out in the statutory biodiversity metric. Those findings set the site's baseline value in biodiversity units, which is the figure the development will need to exceed by at least 10%.
Design and Metric Calculations
We then work with your architect and landscape team to test the proposed layout against the baseline. We look at which habitats should be kept and where new or enhanced habitat will deliver the most units. Running the metric before the layout is fixed tends to cost far less than finding a shortfall once the application is ready to submit.
Biodiversity Gain Plan and HMMP
Once the design is settled, we prepare the biodiversity gain plan for the local planning authority. Alongside it we write a Habitat Management and Monitoring Plan (HMMP) setting out how new and enhanced habitats will be managed and monitored over 30 years. Typical measures range from sowing species-rich grassland to planting native hedgerows and trees. Each one has condition targets that match the assumptions made in the metric.
Off-Site Units
If the full gain can't be achieved on-site, the shortfall can be met by buying units from a gain site listed on Natural England's Biodiversity Gain Sites Register. We can calculate how many units you need and of which habitat type, so that any units you buy meet the metric's trading rules.
For Landowners
Land that can deliver a measurable uplift in biodiversity can be registered as a biodiversity gain site. The resulting units can then be sold to developers who need to meet their off-site requirement. Units can be sold directly or through brokers and local schemes, such as the South Downs National Park Authority's ReNature Credits.
The process starts with a baseline survey of your land. That is followed by an HMMP setting out the habitat creation and enhancement to be carried out, and how it will be managed over 30 years. From these, we use the statutory biodiversity metric to calculate how many units the site could generate. Before any units can be allocated to a development, the land must be secured through a conservation covenant or section 106 agreement and added to the Biodiversity Gain Sites Register.
Our team can also deliver the HMMP on the ground - from sowing wildflower meadows and laying hedges to tree planting and ongoing monitoring.
If you're planning a development, or considering whether your land could become a gain site, get in touch to discuss how BNG applies to your project.
Get in touch
info@ecologyco-op.co.uk
+44 (0) 1798 861800
Unit 4, Langham Stables
Langham Lane
Lodsworth
Petworth
GU28 9BU
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Explore our other services, including protected species surveys, biodiversity net gain assessments and more.