Nutrient neutrality and the appropriate assessment duty

Nutrient neutrality is not a planning policy. It is the practical consequence of a statutory duty: regulation 63 of the Conservation of Habitats and Species Regulations 2017 requires a competent authority to carry out an appropriate assessment where a project is likely to have a significant effect on a European protected site, and to refuse permission unless it has established that the project will not adversely affect the integrity of that site. Where a catchment's protected habitat is already in unfavourable condition because of nitrogen or phosphorus, additional wastewater from new homes cannot pass that test unless the loading is offset. That is the whole of the nutrient neutrality appropriate assessment problem.

Where the duty comes from

The duty falls on the local planning authority as competent authority, not on the applicant. In practice the applicant supplies the evidence and the authority adopts it.

Regulation 63 has two stages. First, a screening judgement on whether the project is likely to have an effect of that kind, applied on a precautionary basis. Second, where that threshold is crossed, an appropriate assessment of the implications for the site in view of its conservation objectives. Permission can only be granted if the authority has established that there will be no adverse effect on integrity, with the derogation in regulation 64 as the only alternative.

That wording does a lot of work. Court decisions on the habitats rules have made clear that the assessment must remove reasonable scientific doubt rather than find that harm is unlikely, which is why mitigation has to be certain and secured rather than probable and intended.

As at October 2026, reform of this area has been proposed more than once and the duty in regulation 63 remains the operative test. Check the current position on gov.uk and follow changes through the planning law updates timeline rather than relying on summaries written during a consultation.

Why unfavourable condition triggers it

A habitats site in favourable condition can usually absorb a marginal increase in loading without an adverse effect on integrity. One already failing its conservation objectives because of excess nutrients cannot.

Natural England's position, which authorities have generally accepted, is that in those catchments any net increase in nutrient loading from new overnight accommodation contributes to that failure. The logic applies to housing, student accommodation, hotels, care homes and tourism uses, because the driver is overnight population rather than floorspace.

This is why the issue appeared suddenly in catchments that had seen housing consented for years. Nothing changed in the law. The evidence on condition changed, and the precautionary application of an existing duty followed.

Calculating a nutrient budget

A nutrient budget compares the nutrient load leaving the site after development with the load leaving it before, usually expressed in kilogrammes of nitrogen or phosphorus a year.

The calculation has four parts:

1. Wastewater from the new development, derived from the number of bedrooms or units, an occupancy rate, a per capita water use figure and the consented discharge quality of the receiving treatment works.
2. The existing land use load, based on the current agricultural or other use of the site and published export coefficients.
3. The future land use load for the developed site, including any areas of open space or habitat.
4. A precautionary buffer applied to the net figure.

Use the catchment calculator Natural England publishes for the relevant area rather than a generic method, because the occupancy rates, coefficients and buffer differ between catchments. Get the treatment works discharge permit position right, since an upgrade already committed by the water company changes the answer materially, and a works operating close to its permit limit changes it the other way.

Where a site drains partly by septic tank or private treatment, that has to be modelled separately. The Foul Sewage Assessment is the natural place to set out the drainage arrangements the budget relies on.

Mitigation options and how each is secured

Mitigation has to be additional, enforceable and in place for as long as the development has an effect.

Three traps recur. Mitigation land must not already be delivering the same benefit for another purpose, so double counting against biodiversity net gain units is checked. The securing period has to match the lifetime of the effect, which for housing means a very long obligation. And the mitigation has to be deliverable at the point of permission rather than at the point of occupation, because the authority has to be satisfied before it grants.

Where the same land is doing more than one job, read our note on irreplaceable habitats and the biodiversity gain exception, which deals with a related overlap between regimes.

The derogation route and its tests

Regulation 64 allows permission where an adverse effect cannot be ruled out, on three cumulative tests: there must be no alternative solutions, there must be imperative reasons of overriding public interest, and compensatory measures must be secured to maintain the overall coherence of the network of protected sites.

For a housing scheme those tests are hard. The absence of alternatives is difficult to show where other sites exist, and general housing need has not readily been accepted as an imperative reason of overriding public interest. Where a priority habitat or species is involved, the public interest test is narrower still and may require the Secretary of State's view.

Treat the derogation as a route for infrastructure rather than as a fallback for a residential scheme with an awkward budget.

Affected catchments and how they are identified

Natural England publishes the catchments where it advises that nutrient neutrality applies, and that list has been extended more than once since the advice first appeared. Check the current list rather than a map from a previous scheme.

For a specific site, three checks settle the position: which habitats site the catchment drains to, which treatment works serves the site and where it discharges, and whether the authority has published its own position or interim guidance. The second check surprises people, because a site can sit outside a catchment boundary and still drain to a works that discharges inside it.

Drainage design interacts with all of this, so the Drainage Strategy should be developed alongside the nutrient budget rather than after it.

What the authority needs to adopt the assessment

The authority has to be able to sign the assessment as its own. Give it a document it can adopt rather than a consultant's advocacy.

That means a shadow habitats regulations assessment written in the authority's voice, the nutrient budget with every input stated and sourced, the calculator output, the mitigation evidence including credit certificates or land agreements, the draft section 106 heads of terms, and a conclusion expressed in the language of regulation 63. Natural England will be consulted and its response is usually decisive, so engage with it before submission rather than after.

One honest caveat. Where an authority has no adopted interim position, the determination timetable becomes unpredictable, and the biodiversity gain condition that applies to most permissions adds a further step before work can start. Build both into the programme.

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