Imperative reasons of overriding public interest

Imperative reasons of overriding public interest, shortened to IROPI, is the second limb of the derogation in regulation 64 of the Conservation of Habitats and Species Regulations 2017. It allows a competent authority to agree to a plan or project that will adversely affect the integrity of a protected habitats site, but only once it has ruled out alternative solutions and only where compensatory measures are secured. It is a route of last resort. For an ordinary housing scheme it almost never opens.

The phrase is often used loosely, as though a strong public benefit case could tip a habitats judgement the way it tips a planning balance. That is the first thing to correct when you brief a junior drafting a statement of case. The derogation is a statutory sequence with gates, and the public interest argument is the third gate, not the first.

Where the test sits in the habitats regime

A Habitats Regulations Assessment runs in stages. Screening asks whether the proposal is likely to have a likely significant effect on a habitats site, alone or in combination with other plans and projects. If that cannot be excluded, the competent authority carries out an appropriate assessment under regulation 63 and may only agree to the proposal once it has established, beyond reasonable scientific doubt, that site integrity will not be harmed.

Mitigation cannot be used to dodge the screening stage. The Court of Justice decision in People Over Wind settled that measures intended to avoid or reduce harm belong in the appropriate assessment, not in the screening judgement. So a scheme with careful avoidance measures still goes through a full assessment on paper.

Only when the appropriate assessment cannot reach a conclusion of no adverse effect does regulation 64 come into play. Most schemes never get there, because the applicant redesigns, relocates the discharge point, buys mitigation in a nutrient neutrality catchment or agrees a recreational disturbance contribution. That is the normal outcome and it is the one to aim for.

The three limbs of the derogation

The order matters. An authority that reaches the public interest question before it has properly tested alternatives has made the classic error, and it is the error most often picked apart on judicial review. Compensation under regulation 68 is a condition of relying on the derogation, not a sweetener that strengthens the public interest case.

No alternative solutions and how it is tested

The alternatives test starts with the objective of the plan or project, framed at the right level of generality. Frame it too narrowly, as "deliver 400 homes on this field", and every alternative is excluded by definition, which is why authorities and inspectors push back on self-serving objectives. Frame it as "meet an identified housing need in this district within the plan period" and real alternatives appear.

What counts as an alternative is wider than a shortlist of sites. It includes a smaller scheme, a different layout, a different access arrangement, a different drainage solution and refusing to proceed at all. Cost is relevant to whether an alternative is really available, but a higher price on its own does not make an option unavailable.

The comparison is specific. You are asking whether an alternative would avoid the adverse effect on integrity while still meeting the objective. An alternative that reduces harm but still harms integrity does not take the scheme out of the derogation, though it may change what compensation is needed.

Compensatory measures and their timing

Regulation 68 requires the authority to secure compensatory measures that maintain the overall coherence of the national site network. Four practical points decide whether a package is deliverable.

1. The measures must be additional to anything already needed as mitigation or as part of site management.
2. They must be ecologically equivalent, targeting the same qualifying feature that will be harmed, at a scale that replaces the lost function.
3. They must be working before or as the harm happens, which usually means land assembly, consents and establishment years ahead of the development programme.
4. They must be secured for the long term with funding, management and monitoring, normally through a planning obligation with step-in rights.

Biodiversity net gain does not do this job. The 10 per cent uplift under the Environment Act 2021 is a separate statutory requirement measured in habitat units, and a Biodiversity Net Gain Assessment cannot stand in for habitats compensation. Some applicants submit one and assume the other is covered. Natural England will say otherwise.

Priority habitats and the narrower grounds

Where the site hosts a priority natural habitat type or a priority species, regulation 64 narrows the acceptable reasons to human health, public safety and beneficial consequences of primary importance for the environment. Anything else needs an opinion from the appropriate authority before it can be relied on.

An illustrative example shows how that bites. A coastal flood defence realignment that will permanently lose an area of intertidal mudflat on a designated site can argue public safety directly, and the compensation is familiar territory, creating replacement intertidal habitat by managed realignment elsewhere in the same estuary. A distribution shed on the same mudflat has no equivalent argument, whatever its economic case.

Who may rely on the derogation

The competent authority decides. For a planning application that is the local planning authority, for a major infrastructure project consented under the Planning Act 2008 it is the Secretary of State, and for a local plan it is the plan-making authority at examination. The applicant does not grant itself the derogation. It supplies the evidence, usually as a shadow assessment and a separate derogation case.

Natural England must be consulted on the appropriate assessment and its advice carries real weight. Where it does not accept the no-alternatives case, an authority taking a different view needs reasons on the record that an inspector or a judge can follow.

Evidence an authority needs

Expect to produce an alternatives appraisal with the objective stated and each option tested against it, a public interest case supported by named documents rather than assertion, a compensation strategy with location, quantum, timetable, funding and monitoring, and a draft obligation that secures all of it. This is a statutory threshold with its own vocabulary, and borrowing planning balance language into it weakens the case.

Survey evidence sits underneath all of that. A Preliminary Ecological Appraisal will identify the pathway to the designated site, but the functional linkage work, the water quality modelling or the disturbance survey that the derogation needs goes well beyond it, and the survey seasons involved often set the programme. A Protected Species Survey and any licence application sit with a licensed ecologist, not with the planning agent.

Why the route is rarely used in housing

Housing delivery is a public benefit, and it is rarely an imperative one in the sense the regulation uses. There is almost always an alternative, because the plan process itself exists to find less harmful sites. And compensation for a European site feature at the scale a housing scheme would need is slow, expensive and hard to prove.

That is why practice in nutrient neutrality and recreational disturbance catchments has moved towards mitigation markets rather than derogation. Buying credits, funding a greenspace or delivering wetland treatment lets the appropriate assessment reach a conclusion of no adverse effect, which keeps the decision in ordinary planning territory. If you are advising on a site within a catchment, the useful question is what mitigation is available and when, not whether the public interest case is strong enough. Exception site arguments on affordability, such as those used for First Homes exception sites, do not change the habitats analysis at all.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a licensed ecologist is legally required for protected species survey and licensing work under the Wildlife and Countryside Act 1981 and the 2017 Regulations.

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