What is an environmental impact assessment?
A formal process requiring the likely environmental effects of a proposed development to be identified, assessed and taken into account before consent is granted — a procedural requirement rather than a prohibition on harm.
What it is not. It does not prevent damaging projects from proceeding. It requires the effects to be properly assessed, published and considered by the decision-maker. A project with significant environmental effects can lawfully be approved, provided the process was followed. This is the most common misunderstanding, and it explains why people are surprised when a project with an unfavourable assessment goes ahead.
When it is required. For specified categories of development above thresholds, and for other projects where screening determines that significant effects are likely. Screening is itself a formal, challengeable decision.
What the process involves:
Screening — is an assessment required?
Scoping — what must it cover? Consultation with statutory bodies at this stage sets the terms.
The Environmental Statement, assessing effects on air, water, soil, biodiversity, landscape, cultural heritage, population and health, climate, and the interactions between them — plus cumulative effects with other projects, which is frequently the weakest part.
Alternatives considered, and why the chosen option was selected.
Mitigation proposed, following the hierarchy: avoid, reduce, then compensate.
Public consultation, with the statement made available and representations invited.
Consideration by the decision-maker, who must give reasons.
Monitoring of significant effects.
Why the process has legal force. Failure to carry out a required assessment, or carrying out a defective one, is a ground for judicial review — and projects have been quashed on exactly these grounds. The procedural obligation is enforceable even though the outcome is discretionary.
Related regimes: Strategic Environmental Assessment applies to plans and programmes rather than projects; Habitats Regulations Assessment applies where protected sites may be affected and imposes a genuinely stricter test; and biodiversity net gain requirements now apply to many developments in England.
The criticisms: statements are long and inaccessible; they are commissioned by the developer; cumulative effects are under-assessed; and mitigation commitments are inconsistently monitored.
General information, not legal advice.