The limits of “iterative” evidence in local plan decision-making
In local plan decision-making, the term “iterative” is frequently relied upon to justify proceeding in the absence of completed evidence.
The word is often used loosely, as if iteration itself relaxes evidential requirements.
This page explains what “iterative” evidence properly means, how the concept is commonly misused, and why reliance on iteration can create material governance risk.
What “iterative” evidence actually means
Local plan preparation is, by design, a staged process.
Evidence may be commissioned, refined, or expanded as a plan progresses from early options through to publication and examination.
That is the legitimate sense in which plan-making is iterative.
Iteration allows evidence to be improved between stages. It does not permit decisions to be taken without a sufficient evidential foundation at the stage in question.
The decision-stage evidential boundary
Each formal step in the plan-making process involves a decision.
At that point, the authority must be able to demonstrate that it had sufficient evidence to justify the judgment it reached at that stage.
- Whether to consult on a draft plan.
- Whether to progress a preferred option.
- Whether to submit a plan for examination.
Iteration does not suspend this requirement.
If the authority proceeds while acknowledging that evidence is incomplete, it is not iterating — it is deferring assessment.
The common misuse of “iterative” reasoning
Authorities frequently rely on formulations such as:
- “The evidence base will continue to evolve.”
- “Further evidence will be produced at the next stage.”
- “Any gaps can be addressed as part of later plan-making.”
These statements are often used to justify progressing a plan despite acknowledged evidential gaps.
However, where the missing material goes to the justification of the decision being taken, iteration cannot cure its absence.
When decision-makers themselves identify evidential insufficiency
A particularly acute governance issue arises where the decision-making body expressly records that further evidence is required.
In such cases, it is not credible to later assert that the evidence was nevertheless sufficient “for that stage”.
If the decision-maker has concluded that evidence is incomplete, the authority cannot retrospectively reframe that position by reference to iteration.
Iteration does not convert an identified evidential deficiency into sufficiency.
Why later evidence does not retrospectively justify earlier decisions
Authorities sometimes point to the later production or publication of evidence as confirmation that earlier decisions were justified.
This is a form of hindsight reasoning.
Iteration explains how evidence may develop over time. It does not alter the evidential position at the moment a decision was made.
A decision taken without sufficient evidence does not become lawful because that evidence is produced later.
Why misuse of “iterative” evidence matters in judicial review
In judicial review, courts are concerned with whether a decision was lawful at the time it was taken.
Arguments that rely on the iterative nature of plan-making to excuse evidential gaps are unlikely to succeed where those gaps go to the justification of the decision itself.
The court will examine:
- what evidence existed at the relevant decision stage,
- what conclusions were drawn from it, and
- whether unresolved matters were improperly deferred.
Where iteration is used to mask evidential absence, the decision is exposed to challenge.
Analytical focus in plan-making governance reviews
Governance analysis of plan-making decisions focuses on whether the authority crossed a decision threshold without having resolved matters that were necessary to justify that step.
It does not treat later evidence, consultation responses, or examination-stage material as capable of curing earlier insufficiency.
The critical boundary remains the same at every stage: legality is fixed at the moment the decision is taken, not as later explained.
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