Nutrient neutrality: what a scheme has to demonstrate

In an affected catchment, a scheme has to demonstrate that it adds no net nitrogen or phosphorus load to the protected site. That is the whole of the nutrient neutrality requirement, and everything else follows from it: a nutrient budget showing the load the development would generate, mitigation that removes at least as much, and a legal mechanism that keeps the mitigation in place for as long as the development exists.

The obligation is not a planning policy. It arises because the local planning authority, as competent authority under the Conservation of Habitats and Species Regulations 2017, cannot grant permission unless it is satisfied that the proposal will not adversely affect the integrity of the European site. Where Natural England has advised that additional nutrient load cannot be ruled out as a cause of harm, neutrality is the route to that conclusion.

Which catchments are affected, and why

Natural England has issued catchment-specific advice where designated sites are in unfavourable condition because of nutrient enrichment and where new development would add load. The catchments have been added to over time and have included the Solent, Stodmarsh in Kent, the River Wye, the Somerset Levels and Moors, and areas in the Tees and Eden catchments, among others.

Do not work from a list in an article, including this one. Check the current catchment map and the current advice with Natural England and the local planning authority before scoping any work, because both the geography and the methodology have changed more than once.

The reason the advice bites on housing is that the load comes largely from wastewater. Anything that adds population served by a treatment works discharging into the catchment adds load, which is why the requirement catches small schemes as readily as large ones.

The nutrient budget and its inputs

A nutrient budget compares the nutrient export from the site now with the export after development, and expresses the difference in kilograms of nitrogen or phosphorus per year. Three inputs drive it.

Occupancy. Use the published occupancy rates by dwelling size that the methodology relies on, not an assumption of one person per bedroom. A scheme weighted towards larger units carries more load than the unit count suggests.

Water use per person per day. The figure used has a direct effect on the load, so a scheme securing a tighter water efficiency standard by condition produces a smaller budget. Where nothing is secured, the assessment has to assume the less efficient case.

Nutrient concentration in the final effluent. This comes from the treatment works that will serve the site, and it is the input most often taken on trust.

The budget then applies a precautionary buffer, because every element of it carries uncertainty. Show the buffer separately rather than folding it into the figures, so the officer and Natural England can see what has been assumed.

Wastewater treatment assumptions

Two questions decide this part. Which works will serve the development, and what concentration should be assumed for its discharge.

The first is a matter for the sewerage undertaker, and the answer is not always the nearest works. Get it in writing, because a budget built on the wrong catchment is worthless.

The second has moved. Sections in the Levelling-up and Regeneration Act 2023 placed a duty on sewerage undertakers to upgrade designated wastewater treatment works to a stated standard by a date set in that legislation, and provided for competent authorities to take the upgrade into account when assessing effects. That changes the concentration an assessment may assume for works within the scope of the duty. Check whether those provisions are in force, and check the current list of designated works, before relying on it, and state in the report which assumption you have used and why.

Where the development will not connect to mains drainage, the calculation is different again. A package treatment plant or a septic tank has its own discharge characteristics, and replacing an existing failing system can itself be mitigation.

Land use change within the budget

The existing use of the land counts. Arable land in intensive production exports nutrients through run-off and leaching, so developing it removes that export and reduces the budget. Permanent pasture exports less, and previously developed land with no agricultural export gives no credit at all.

That produces an outcome people find counter-intuitive. A greenfield scheme on intensively farmed arable land can have a smaller budget than a brownfield scheme of the same size, because the brownfield site was contributing nothing to start with.

Land use change after development matters too. Public open space, sustainable drainage features and landscaped areas have their own export rates, and a scheme that converts arable land to amenity grassland is doing something measurable. Set this out in the Drainage Strategy as well, because the drainage design and the nutrient case rely on the same layout and they must agree.

On-site against off-site mitigation

On-site mitigation is limited in what it can achieve. Constructed wetlands and treatment trains can remove nutrients, and they need land, a maintenance regime and monitoring, which is why they suit larger schemes and rarely resolve a small one on their own. Water efficiency reduces the load at source and is the cheapest measure available, so secure it by condition whatever else you do.

Off-site mitigation is the usual answer. The recognised routes are taking agricultural land out of production and managing it under a long-term agreement, creating wetlands or woodland, and upgrading or replacing failing private treatment systems in the catchment.

An illustrative example. A scheme of 30 dwellings on an arable field in an affected catchment calculates a surplus after allowing for the existing arable export and a secured water efficiency standard. The developer buys credits from a council-operated scheme rather than assembling its own land, and the credits are allocated against the application before determination. The whole nutrient case runs to a short technical note plus a credit allocation letter, because the hard work was done by the scheme operator.

Buying credits is almost always faster than creating mitigation. What it costs and how the transaction is documented is a separate question, covered in nutrient neutrality credits and the section 106.

Securing it, and for how long

Mitigation has to last as long as the effect. Since the development is permanent, the mitigation must be secured for the lifetime of the development, which authorities commonly express as a period of many decades rather than as an open-ended covenant.

The mechanism is normally a section 106 agreement binding the mitigation land, with a management plan, a monitoring regime, a funding mechanism and a default provision. Where a credit scheme is used, the agreement instead secures the purchase and allocation and relies on the scheme's own agreements for the land.

Three practical points. The mitigation must be additional, so land already required to be managed that way under another agreement does not count. It must be in place before the effect arises, which in practice means before occupation. And it must be located where it affects the same designated site, so a wetland in the wrong sub-catchment does nothing.

Recording the conclusion

The developer does not carry out the appropriate assessment. The authority does. What the applicant submits is the evidence for it, usually as a shadow assessment setting out the budget, the mitigation, the securing mechanism and the conclusion the authority is invited to reach. That relationship, and what it means for how the document is written, is set out in nutrient neutrality and the Habitats Regulations assessment.

Natural England will normally be consulted, so write the document for that reader. Show the calculation in full, state every assumption, and attach the correspondence confirming the treatment works, the credit allocation and the occupancy source.

Biodiversity net gain runs alongside and does not overlap. Nutrient mitigation land can sometimes deliver habitat units as well, but the two regimes are secured separately and the Biodiversity Net Gain Assessment needs its metric completed by a competent person regardless of what the nutrient case says.

This area has moved repeatedly since the first catchment advice was issued, and further reform has been brought forward. Check the current statutory position, the current Natural England methodology and the catchment list at the time you write rather than relying on any fixed description.

This is general guidance, not legal advice. Check the position with your local planning authority, Natural England and a suitably qualified ecologist before relying on it.

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