Short answer
Courts first identify the regime. The EIA Directive assesses projects; the SEA Directive assesses qualifying plans and programmes. Both operate before adoption or consent and address alternatives, cumulative effects and participation. Courts examine the report, consultation timing and final reasons.
The Habitats Directive 92/43/EEC applies a separate, stricter rule to plans or projects affecting Natura 2000 sites. Screening must consider effects alone and in combination. If significant effects cannot be excluded objectively, an appropriate assessment must use the best scientific knowledge and resolve reasonable scientific doubt about harm to site integrity before ordinary authorisation. If it cannot, the proposal may proceed only through the exceptional Article 6(4) route: no alternative solutions, imperative reasons of overriding public interest and compensation.
Courts do not ordinarily design the project or choose scientific models. They test timing, scope, evidence, reasons and the statutory threshold. Missing cumulative analysis, unexplained expert concerns or inaccessible consultation material can make a decision unlawful.
The three assessments answer different questions
EIA asks what significant effects a project may cause. SEA assesses a plan or programme and reasonable alternatives. A project may need EIA even where its enabling plan had SEA.
Habitats Article 6(3) is not merely a biodiversity chapter within EIA or SEA. It asks whether a plan or project may adversely affect the integrity of a protected site in view of that site's conservation objectives. The procedures can run together, but their conclusions cannot be substituted for one another. A broad EIA conclusion that impacts are “acceptable” does not establish the certainty required by Article 6(3).
National courts apply procedural law subject to EU effectiveness, equivalence and access-to-justice rules.
Alternatives must be real, relevant and explained
Under the current EIA Directive, the developer's report must contain:
“a description of the reasonable alternatives studied by the developer, which are relevant to the project and its specific characteristics, and an indication of the main reasons for the option chosen, taking into account the effects of the project on the environment”. —
Annex IV mentions design, technology, location, size and scale. The rule does not require every imaginable option, but the file must disclose studied reasonable alternatives and environmental reasoning. In Holohan, C-461/17, the predecessor wording covered all main alternatives studied, including one rejected early.
SEA Article 5 requires evaluation of reasonable alternatives for the plan's objectives and geographical scope. Article 9 then requires reasons for the selected plan. Courts can test whether alternatives were merely labelled away or evaluated strategically.
Habitats Article 6 uses alternatives differently. If an appropriate assessment is negative or cannot remove doubt, ordinary authorisation under Article 6(3) is unavailable. Article 6(4) permits an exceptional route only “in the absence of alternative solutions”, followed by imperative reasons of overriding public interest and compensatory measures. In Commission v Portugal, Case C-239/04, a motorway through a protected area breached Article 6(4) because the authorities had not demonstrated the absence of alternative routes. Compensation does not excuse skipping that sequence.
Cumulative effects prevent fragmented assessment
EIA Annex IV covers direct, indirect, secondary, cumulative, transboundary and temporal effects. Existing or approved projects and shared pressures may become significant together. In Commission v Spain, C-404/09, mine assessments omitted direct, indirect and cumulative effects; a post-authorisation report could not cure that omission.
SEA Annex I likewise includes secondary, cumulative and synergistic effects. Its strategic scale can expose effects that a project-by-project view misses: several sites allocated by one plan, traffic generated across a network or successive pressures on the same water body or habitat.
Habitats Article 6(3) expressly requires screening and assessment of effects “either individually or in combination with other plans or projects”. The combination analysis is tied to the site's conservation objectives, not administrative boundaries or the identity of the promoter.
Participation must happen while it can change the decision
EIA Article 6 requires early and effective opportunities for the public concerned to comment while all options remain open. Relevant environmental information must be available, reasonable time frames must allow preparation and participation, and Article 8 requires consultation results to be duly taken into account. The duty is substantive as well as chronological: publishing a large file does not help if affected people cannot reasonably find or access it.
In Flausch, Case C-280/18, the Court held that participation arrangements at a regional headquarters were unlawful if they did not ensure actual enjoyment of public rights at the project location. It also rejected reliance on an internet announcement to start the challenge period where the public had not had an adequate opportunity to learn of the consent procedure.
In Land Nordrhein-Westfalen, Case C-535/18, the Court held that EIA consultation information must include data necessary to assess the project's water effects. A participation defect can therefore be material where it deprives a claimant of the right to participate, even though national remedy rules may ask whether a defect could affect the decision.
In Petón do Lobo, C-461/24, the Court held that authorities and the public may be consulted simultaneously. If the public receives the Article 6 information, especially the EIA report, EU law does not invariably require a later opportunity to comment on the consulted authorities' opinions; later relevant information must still be made available.
SEA Article 6 uses the same essential timing logic:
“The authorities referred to in paragraph 3 and the public referred to in paragraph 4 shall be given an early and effective opportunity within appropriate time frames to express their opinion on the draft plan or programme and the accompanying environmental report before the adoption of the plan or programme”. —
The report and opinions must influence preparation, not be attached after the choice is fixed. Habitats Article 6(3), by contrast, obtains public opinion “if appropriate”. Other applicable rules may still require consultation; that wording alone is not the full EIA procedure.
Scientific uncertainty has different legal consequences
EIA and SEA are predictive. EIA Annex IV requires the forecasting methods or evidence, difficulties, lack of knowledge and main uncertainties to be disclosed. SEA requires information that may reasonably be demanded in light of current knowledge, methods, level of detail and the assessment stage. Neither directive promises perfect prediction, and neither applies the Habitats “no reasonable scientific doubt” authorisation formula to every environmental effect.
Courts require enough current evidence to evaluate likely significant effects and a reasoned conclusion based on the report and consultations. An authority cannot hide a material gap, postpone core assessment or reject relevant expert concerns without reasons.
Habitats screening starts at a low threshold. In Waddenvereniging and Vogelbeschermingsvereniging, Case C-127/02, the Court held that appropriate assessment is required where significant effects cannot be excluded on objective information. At the authorisation stage the standard becomes stricter:
“That is the case where no reasonable scientific doubt remains as to the absence of such effects.” —
The assessment must identify, using the best scientific knowledge, every aspect capable alone or in combination of affecting conservation objectives. It must contain complete, precise and definitive findings capable of removing doubt. In Holohan, the Court required an explicit, detailed explanation where the authority rejected an expert recommendation for more information. Conditions leaving construction details open are permissible only if strict enough to guarantee no adverse effect on site integrity.
Site-specific conservation objectives are indispensable. In SU and Wild Ireland Defence, Joined Cases C-27/25 and C-356/25, the Court held that no valid appropriate assessment can be completed before those objectives are adopted. They are also needed at screening to decide whether effects on a protected site within the identified geographical area are likely.
What a court-ready assessment record contains
- reasoned EIA, SEA and Habitats screening decisions;
- a baseline and geographical scope matching impact pathways;
- alternatives, environmental comparison and selection reasons;
- existing, approved and foreseeable interacting pressures;
- methods, assumptions, data limits and expert disagreements;
- accessible consultation while options remained open, with responses to material comments;
- separate EIA/SEA and site-specific Habitats conclusions;
- enforceable mitigation assessed before consent; and
- a dated record proving that evidence and conclusions preceded authorisation.
Conclusion
Courts match the record to the governing directive. Under EIA and SEA, they ask whether alternatives and cumulative effects were evaluated at the right level, participation preceded the decision, uncertainties were disclosed and reasons engaged with the evidence.
For Natura 2000 sites, Article 6(3) demands assessment against site-specific objectives using the best scientific knowledge; ordinary authorisation is barred while reasonable doubt remains. Only the sequenced Article 6(4) derogation can then permit a damaging proposal.
Sources
This article provides general legal information, not advice for a specific matter. Laws, facts and procedural deadlines may change the result. Follow the linked primary sources and obtain local advice before acting.