Biodiversity net gain is now a legal requirement in England, not an optional aspiration. Since mandatory biodiversity net gain came into force through the Environment Act 2021, local planning authorities have become the delivery engine of a statutory system designed to secure measurable biodiversity gains from development. For most planning permissions, biodiversity net gain is not something that can be “worked out later”. It is embedded in law through a deemed biodiversity gain condition, and it must be satisfied before development can begin.
For clients working with Civity, understanding what local planning authorities must do to deliver biodiversity net gain helps reduce planning risk, speed up approvals, and ensure that biodiversity value is genuinely enhanced. This blog explains the duties and practical steps planning authorities must take, aligned with the latest government guidance.
The statutory role of local planning authorities
The Government’s Biodiversity Net Gain Planning Practice Guidance makes clear that biodiversity net gain (BNG) is mandatory for most Town and Country Planning Act development.
The legal framework introduces a deemed biodiversity gain condition on planning permissions granted for development projects within scope. As a result, the relevant local planning authority must approve a Biodiversity Gain Plan before works can commence.
This changes local planning profoundly. Authorities are not simply reviewing ecological reports. They must validate baseline evidence, interpret statutory biodiversity metric results, enforce the mitigation hierarchy, and monitor post-development outcomes for at least thirty years.
Setting expectations early in the planning process
Delivery of biodiversity net gain begins at the planning application stage, where local planning authorities must communicate biodiversity gain requirements clearly. Government guidance expects applicants to show how they will enhance biodiversity and achieve net gain, and authorities should embed these expectations in pre-application advice, validation requirements, and local policy.
A key part of this early stage is ensuring that development proposals align with local nature recovery strategies. Authorities should reference any adopted strategy and explain how it informs habitat creation priorities, strategic significance scoring in the biodiversity metric, and wider nature recovery.
Local planning authorities should also emphasise the mitigation hierarchy. Applicants must show that they have first avoided impacts on existing habitat, then minimised losses, then sought to create biodiversity on-site through on-site habitat retention, habitat enhancement, and habitat creation. Only after these steps should off-site biodiversity gains be considered, and buying statutory biodiversity credits should be treated as a last resort.
Requiring robust baseline evidence within the red line boundary
Biodiversity net gain depends on a trustworthy starting point. Local planning authorities must verify that the pre-development biodiversity value has been assessed correctly within the red line boundary of the development site. That means checking that existing habitat has been accurately mapped and classified by habitat type, condition, and distinctiveness, and that surveys follow appropriate seasonal and methodological standards.
Authorities may require additional information through their local validation lists, provided these changes are consulted on and published. The purpose is to make sure baseline biodiversity value and biodiversity units are not underestimated, which would distort the entire net gain calculation.
Applying the statutory biodiversity metric consistently
Local planning authorities must assess applications using the statutory biodiversity metric produced by Defra and Natural England.
This metric converts habitats into biodiversity units by considering area, distinctiveness, condition, and strategic significance. Authorities need to interpret metric outputs confidently and challenge assumptions that inflate predicted post-development biodiversity value.
This applies equally to major developments and small sites, even where the Small Sites Metric is used. The calculation must still demonstrate that biodiversity net gain requirements will be met and that the post-development biodiversity value exceeds pre-development biodiversity value by at least ten percent after the metric’s risk and time-to-target rules are applied.
Determining scope and exemptions
Government guidance includes exemptions and transitional arrangements. Local planning authorities must decide, at the validation stage, whether a planning application is subject to mandatory BNG and record this clearly.
This is essential for legal certainty. If a development is within scope, the biodiversity gain condition applies. If it is exempt, the authority must make the exemption basis transparent.
Approving the Biodiversity Gain Plan before commencement
The Biodiversity Gain Plan is the statutory template a developer uses to discharge the biodiversity gain condition. Local planning authorities must review and approve it before any development begins.
In doing so, authorities must confirm that the gain plan includes accurate baseline biodiversity value, the final statutory biodiversity metric calculations, a clear explanation of the mitigation hierarchy, and a deliverable package of on-site and, where necessary, off-site biodiversity units.
For phased developments, the plan must show how each phase contributes to the overall net gain objective and how biodiversity units will be tracked and secured over time.
Scrutinising off-site biodiversity gains and the register
Where net gain cannot be achieved entirely on-site, developers may rely on off-site units. Local planning authorities must verify that any off-site biodiversity gains are genuine, additional, and secured for at least thirty years. This includes checking that off-site gain sites are, or will be, recorded on the Biodiversity Gain Site Register operated by Natural England.
Authorities should also confirm that off-site biodiversity units meet trading and spatial risk rules and that an appropriate legal agreement is in place with the land manager, whether through a planning obligation or a conservation covenant.
Ensuring statutory biodiversity credits remain a last resort.
The government explicitly positions statutory biodiversity credits as a last resort. Local planning authorities must ensure that applicants only propose to buy statutory biodiversity credits when on-site habitat measures and off-site gains have been exhausted.
Where credits are proposed, authorities should require a clear justification and evidence that the credit purchase will be made through Natural England. This protects the integrity of the biodiversity net gain hierarchy and keeps local biodiversity value uplift central to development.
Treating irreplaceable habitat with heightened care
Irreplaceable habitat cannot be traded or offset through standard biodiversity units in the same way as other habitats. Local planning authorities must identify whether an irreplaceable habitat is affected, ensure that the statutory biodiversity metric is applied correctly, and require avoidance or bespoke compensation consistent with government guidance.
This responsibility is particularly important in sensitive settings where biodiversity net gain interacts with broader aims for green infrastructure, transport network delivery, or strategic local planning priorities.
Securing delivery through enforceable legal agreements
Authorities must ensure that biodiversity net gain commitments are enforceable. This is typically done through planning obligations under section 106 or through conservation covenants in off-site situations.
The legal agreement should lock in the biodiversity gains promised in the gain plan, set out responsibilities for habitat management, define reporting cycles, and establish remedies if net gain delivery fails.
Without that legal backbone, net gain becomes a projection rather than a secured outcome.
Monitoring post-development outcomes for thirty years
Biodiversity net gain is achieved through long-term ecological performance, not simply by passing a metric test. Local planning authorities must therefore require a credible monitoring plan and habitat management approach that runs for at least thirty years. They also need a practical system for reviewing monitoring reports and ensuring compliance. Many authorities are drawing on best practice and support from bodies such as the Planning Advisory Service and the Local Government Association as they build internal frameworks and resourcing for long-term delivery.
Preparing for extension to nationally significant infrastructure projects
Mandatory BNG currently applies to most TCPA development, but the government has proposed extending a parallel requirement to nationally significant infrastructure projects from May 2026.
Local planning authorities and other consenting bodies should build capacity now, because NSIPs will introduce larger schemes where biodiversity net gain must be balanced against major strategic needs. Early preparation will help ensure consistency across the planning system as these changes come into effect.
Conclusion
Local planning authorities play a defining role in making biodiversity net gain work. They must set expectations early, verify baseline biodiversity value, apply the statutory biodiversity metric consistently, approve the biodiversity gain plan before commencement, scrutinise off-site biodiversity units and the gain site register, and keep statutory biodiversity credits as a genuine last resort. Once planning permissions are granted, they must also secure delivery through enforceable legal agreement structures and ensure compliance through monitoring for at least thirty years.
When this system is applied well, it supports the natural environment, strengthens local nature recovery, and enables development that leaves biodiversity better than it found it. Civity supports local authorities, land managers and developers across every stage of this process, from biodiversity metric interpretation to gain plan strategy and long-term habitat management.
Disclaimer
This blog provides a general overview of what local planning authorities must do to deliver biodiversity net gain in England, based on current government guidance. It is not legal advice. Mandatory BNG includes exemptions, transitional arrangements, and site-specific factors such as irreplaceable habitat or phased development that may alter obligations. Always confirm requirements with the relevant local planning authority and qualified ecological or legal advisers before finalising a planning application or biodiversity gain plan.
