The Biodiversity Gain Plan is the document that makes BNG enforceable. It's where the numbers, the delivery mechanism, and the 30-year commitments get pinned down in a form the local planning authority can assess and approve. Until it's approved, development can't lawfully start—regardless of what the planning permission says.
When planning permission is granted for a development subject to BNG, it comes with a biodiversity gain condition. That condition requires the applicant to submit a Biodiversity Gain Plan and have it approved before commencing development.
The Gain Plan isn't a vague statement of intent. It's a technical document that must demonstrate, with evidence, how the development will achieve at least 10% net gain in biodiversity value. The LPA reviews it against the statutory framework and either approves it, requests amendments, or refuses it.
This makes the Gain Plan a pre-commencement condition. Developers sometimes underestimate what this means: you can hold a valid, unconditional planning permission and still be legally unable to break ground. If the Gain Plan isn't approved, the biodiversity gain condition isn't discharged, and starting work is a breach of planning control.
The Gain Plan needs to establish both the baseline (what's there now) and the outcome (what will be there after development), calculated using the Defra Biodiversity Metric. The metric is a standardised calculation tool that converts habitat area, type, condition, and strategic significance into biodiversity units.
Pre-development biodiversity value is the starting point. This requires survey data—habitat mapping, condition assessments—fed through the metric to produce a baseline unit figure. The surveys need to be recent enough to reflect current conditions, and the metric must be applied correctly. Errors here cascade through the whole calculation.
Post-development biodiversity value is what the site (and any off-site land) will score after the development is complete and habitats are established. This accounts for habitat lost to buildings and infrastructure, habitat retained and enhanced, and new habitat created. The target is at least 110% of the baseline—a 10% net gain.
The delivery mechanism must be specified. Will the gain be achieved on-site, off-site, or through a combination? If off-site units are involved, from where? If statutory credits are being used (the government's last-resort option), how many and for which habitat types?
For off-site delivery, the Gain Plan must evidence that units are properly secured. This typically means showing that the habitat bank or receptor site is registered on Natural England's Biodiversity Gain Sites Register, with legal agreements in place. LPAs won't approve a Gain Plan that relies on off-site units that haven't been locked in.
Habitat management and monitoring arrangements for the 30-year period must be set out. What management interventions will occur? Who will do them? How will habitat condition be monitored and reported? What happens if targets aren't being met?
Legal security must be evidenced. The Gain Plan should demonstrate that the commitments aren't just aspirational—they're backed by section 106 agreements, conservation covenants, or equivalent mechanisms that will bind the land for 30 years.
Once submitted, the LPA has eight weeks to determine the Biodiversity Gain Plan. In practice, this can involve queries, requests for clarification, and negotiation over details—particularly where the metric calculations are complex or the off-site arrangements need verification.
The LPA is checking several things:
Are the calculations correct? The metric is detailed, and mistakes are common. Habitat classifications, condition assessments, and multipliers all need to be applied accurately. An LPA ecologist (or consultant acting for the authority) will review the numbers.
Does it follow the mitigation hierarchy? BNG isn't supposed to be a licence to destroy habitat and pay it off elsewhere. The expectation is that developments avoid and minimise harm first, then compensate for residual impacts. A Gain Plan that proposes clearing high-value habitat and offsetting with low-value creation elsewhere may face pushback.
Is the delivery credible? Will the proposed habitats actually establish? Are the management prescriptions realistic? Is the off-site provider legitimate and the land properly registered? LPAs are wary of approving plans that look good on paper but won't deliver in practice.
Is it properly secured? Legal agreements must be in place or demonstrably imminent. A Gain Plan that says "we'll sort out the section 106 later" won't be approved.
If the LPA refuses the Gain Plan or requests changes, the developer can't proceed until the issues are resolved. This can create programme delays if BNG wasn't properly planned from the outset.
The Gain Plan sits in an awkward position in the development timeline. It's required after planning permission is granted but before development can start. This creates a gap that can catch developers off guard.
The sensible approach is to prepare the Gain Plan in parallel with the planning application, so it's ready to submit immediately upon permission being granted. This requires having baseline surveys done, metric calculations completed, and off-site arrangements secured (or at least well advanced) before the planning decision.
Developers who wait until permission is granted to start thinking about BNG often find themselves stuck. Securing off-site units takes time. Habitat banks may not have immediate availability in the right location or habitat type. Getting legal agreements finalised isn't instant. An eight-week determination period becomes much longer if the LPA is waiting for evidence that doesn't yet exist.
Pre-planning unit reservation—locking in off-site supply before permission is granted—removes this bottleneck. The units are secured, the legal agreements are in place, and the Gain Plan can be submitted with full evidence immediately after permission.
Several issues recur in Gain Plan submissions:
Baseline errors. Habitat surveys misclassify vegetation, overstate or understate condition, or miss features entirely. The metric is only as good as the input data.
Optimistic post-development assumptions. Proposed habitats are assigned high target conditions that may not be achievable, inflating the predicted unit gain. LPAs increasingly scrutinise whether targets are realistic.
Incomplete off-site evidence. The Gain Plan references units from a habitat bank but can't demonstrate they're registered, allocated, and legally secured. This is a common reason for delays.
Inadequate management plans. Thirty years of habitat management is glossed over with vague commitments. LPAs want to see credible prescriptions, funding arrangements, and monitoring schedules.
Late preparation. The Gain Plan is treated as an afterthought, prepared hurriedly after permission is granted, with gaps that take months to fill.
EcoCapital supplies off-site BNG units that are already registered with Natural England and backed by conservation covenants—so the legal security evidence required for Gain Plan approval is ready from day one. Units can be reserved pre-planning, meaning developers aren't scrambling to secure supply after permission is granted.
For developers preparing Gain Plans, EcoCapital can advise on unit requirements, provide allocation documentation, and ensure the off-site elements of the submission will satisfy LPA scrutiny. The goal is a Gain Plan that gets approved within the eight-week window, without delays that hold up the build programme.
