Biodiversity net gain and nutrient mitigation together

Developers in a nutrient catchment now carry two environmental obligations that look similar and work nothing alike. Biodiversity net gain is a planning condition requiring a measured 10 per cent uplift in biodiversity units, secured for at least 30 years. Nutrient mitigation is a prior legal requirement flowing from habitats law, measured in kilogrammes of nitrogen or phosphorus a year, and it has to be resolved before permission can lawfully be granted at all. Asking whether BNG and nutrient mitigation can be delivered on the same land is reasonable. The answer depends on additionality, on how each obligation is secured, and on durations that do not match.

Two regimes with different legal roots

Biodiversity net gain comes from the Environment Act 2021, which inserted Schedule 7A into the Town and Country Planning Act 1990 and created a biodiversity gain condition deemed to be attached to most planning permissions in England. It applied to major development from 12 February 2024 and to small sites from 2 April 2024. The gain is calculated with the statutory biodiversity metric, set out in a biodiversity gain plan, and the plan has to be approved by the authority before development begins.

Nutrient neutrality comes from the Conservation of Habitats and Species Regulations 2017. Where development would increase nutrient loading on a protected site that is already in unfavourable condition, the authority must carry out an appropriate assessment and may only grant permission if it can conclude there will be no adverse effect on the integrity of that site. The catchments affected are identified by Natural England advice and include the Solent, Stodmarsh, the Somerset Levels and Moors, the River Wye, the Tees and the Eden among others.

The difference in legal character matters. A biodiversity gain condition can be discharged after permission. A nutrient conclusion cannot. If the appropriate assessment cannot reach a favourable conclusion, there is no permission to condition.

One caveat on the policy direction. Government has legislated for a more strategic approach to environmental obligations through environmental delivery plans and a central fund that developers can pay into in place of site-specific mitigation. The commencement, the catchments covered and the charging arrangements should be checked at the time you are advising, because this area has moved repeatedly and will move again.

What each one actually measures

The units are not comparable, and that is the root of most confusion.

A hectare of arable land reverted to species-rich grassland generates biodiversity units because the habitat distinctiveness and condition improve. The same reversion reduces nitrogen leaching because fertiliser inputs stop. Those are two different outcomes from one land-use change, which is exactly why the stacking question arises.

Whether the same land can do both

In principle yes, and in practice with care. Government has published guidance on selling more than one type of environmental unit from the same land, and the position has been refined since the markets opened, so check the current guidance rather than relying on a note from 2024.

The distinction that does the work is between stacking different interventions and stacking different outcomes from one intervention. Where a parcel carries two separate interventions, for example a wetland built in one corner to intercept drainage and deliver phosphorus removal, and species-rich grassland created across the remainder to deliver biodiversity units, the case is straightforward. Two interventions, two outcomes, two sets of units, both additional.

Where a single intervention is claimed twice, the case is contested. Arable reversion sold once as nutrient mitigation and again as biodiversity units is the example everyone argues about. Some authorities accept it on the basis that the regimes protect different things and neither would have happened without the payment. Others treat the second sale as non-additional, because the land-use change was already paid for.

My own view is that the second position is the safer one to plan around. Where a scheme depends on stacked units from one intervention, build in the possibility that the authority will not accept it, and have a fallback.

Additionality and the double counting objection

Additionality is a requirement in both regimes, and it is where claims fail.

For biodiversity net gain, habitat enhancement that was going to happen anyway cannot count towards the gain. That includes work required by another legal obligation, work already funded through an agri-environment scheme in a way that overlaps, and habitat created to satisfy a protected species licence. The metric is unforgiving about baselines, and a baseline recorded after the land has been degraded is a common cause of a plan being rejected.

For nutrient mitigation, the reduction has to be real and attributable. Land already out of production, or land where the nutrient reduction was secured by a different mechanism, does not deliver new headroom.

So the question to put to a habitat bank is specific: which physical intervention on which parcel delivers which unit, and what is the counterfactual for each. A bank that cannot answer that parcel by parcel is selling you risk. The same discipline applies to the baseline evidence from the site itself, and the habitat creation detail matters as much as the paperwork, which is covered in the note on soil translocation and habitat creation on stripped ground.

Hedgerows deserve a separate check, because they are counted in units of length rather than area and carry their own regulatory overlay, as set out in the note on hedgerow units and the hedgerows regulations overlap.

Securing each obligation separately

Two obligations need two legal mechanisms, even where the land is the same.

Off-site biodiversity gain is secured either by a conservation covenant with a responsible body or by a planning obligation, and the gain site must be entered on the biodiversity gain site register with the allocation to your development recorded. The register entry is what prevents the same units being allocated twice, and it is worth checking before contracts are exchanged.

Nutrient mitigation is secured by a planning obligation, by a credit certificate from a mitigation scheme, or by a combination. The obligation has to bind the land, survive changes of ownership and run for the required period, and the authority's legal team will want its own drafting rather than a template.

Do not try to collapse the two into one agreement clause. If the biodiversity obligation is later varied or released, a single clause can take the nutrient mitigation with it, and that is an unwinding nobody wants to be explaining to a habitats regulations assessment.

The duration mismatch nobody plans for

This is the practical trap. Biodiversity net gain requires habitat to be maintained for at least 30 years. Nutrient mitigation has to last for the lifetime of the development, which authorities commonly take as 80 years or longer and sometimes treat as perpetuity.

A parcel secured for 30 years therefore cannot support a nutrient claim on its own, however good the ecology is. Either the land has to be secured for the longer period for both purposes, which changes the price and the landowner's willingness, or the nutrient mitigation has to come from land secured separately for the longer term.

Resolve this at heads of terms. Discovering at the section 106 drafting stage that the habitat bank will only commit for 30 years, on a scheme whose appropriate assessment depends on 80, costs months.

Evidence the authority and the habitats assessment need

The bundle is larger than most developers expect, and the two strands have to be internally consistent.

1. A baseline habitat survey of the development site and of each off-site parcel, with the metric calculation and the trading rules satisfied, presented in a biodiversity net gain assessment.
2. A biodiversity gain plan in the prescribed form, with the significant on-site enhancements identified and the 30 year management and monitoring arrangements set out.
3. A nutrient budget calculation using the catchment methodology the authority applies, covering the development and the mitigation land.
4. A shadow appropriate assessment the authority can adopt, addressing the protected site, its condition, the pathway and the conclusion.
5. A management plan for the mitigation land showing which intervention delivers which outcome, parcel by parcel.
6. Draft legal mechanisms for both obligations, with the register entry reference for any off-site gain site.

Where the site also has protected species or a scoping question, the earlier ecological work feeds all of it, and a sound preliminary ecological appraisal at the outset avoids repeating survey work later.

Pricing and contractual risk

Unit prices move with catchment scarcity and are negotiated privately, so take current quotations rather than last year's assumptions. Three contractual points are worth insisting on.

Warranties that the specific units have not been and will not be sold, allocated or claimed for any other purpose, with the register entry identified. An unambiguous allocation of the risk that the authority refuses to accept stacked units, so that the developer is not left paying for units it cannot use. And step-in rights or a replacement obligation if the land manager fails, because a 30 year management commitment from a thinly capitalised company is a covenant worth very little in year twelve.

Buy the mitigation late enough to know what the authority will accept and early enough that the catchment has not run out. That window is narrower than it sounds.

Enter your postcode on PlanScope to see which ecology and environmental documents your scheme is likely to need.

This is general guidance, not legal advice. The statutory biodiversity metric must be completed by a competent person under the Environment Act 2021 regime and the biodiversity gain plan approved by the local planning authority, and an AI-assisted draft is a starting point for that professional rather than a substitute. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.