Simplex: material error in a planning decision
Simplex is an important Court of Appeal authority on a straightforward question: can a planning decision stand when a material part of its reasoning is wrong?
The full proceedings involved Simplex G.E. (Holdings) Limited and Rialto Builders Limited v Secretary of State for the Environment and City and District of St Albans District Council. The judgment was delivered on 6th May 1988 and was reported at (1989) 57 P&CR 306, [1988] 3 PLR 25 and, much later, [2017] PTSR 1041.
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- Neutral citation
- (1989) 57 P&CR 306; [1988] 3 PLR 25; [2017] PTSR 1041
- Court
- Court of Appeal (Civil Division)
- Judgment date
- 6th May 1988
- Judge
- Purchas LJ, Staughton LJ and Sir Roualeyn Cumming-Bruce
- Case number
- 1986 No. CO 944/86
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There were two challenges, with different outcomes
The litigation concerned land at Napsbury Lane, St Albans, and its treatment as Metropolitan Green Belt. One challenge concerned the development plan. The other concerned the Secretary of State’s dismissal of planning appeals.
The development-plan challenge failed. Although the council had not complied with the formal requirement to give reasons in the prescribed way, the appellants had been informed of the reasons and the court did not find the necessary substantial prejudice.
The planning-appeal challenge succeeded. It is that part of the judgment which supplies the familiar Simplex principle about material error and the outcome of a decision.
The study did not answer the question attributed to it
The Secretary of State proceeded on the basis that the council had undertaken a study relevant to whether the appeal site should remain in the Green Belt. The study had instead considered possible uses on the assumption that Green Belt treatment continued.
Those are different exercises. A study of acceptable uses within the Green Belt is not necessarily a study of whether the land should be within it in the first place.
The mistake mattered because it formed a significant part of the Secretary of State’s explanation for rejecting the appellants’ case.
A likely result is not necessarily an inevitable result
The Court of Appeal considered whether the decision could nevertheless stand because the same outcome would have followed without the error.
The court could not reach that conclusion. It was not enough to consider the same result probable, or to substitute the court’s own view of the planning merits for the decision-maker’s judgment. The Secretary of State’s planning-appeal decision was therefore quashed.
Simplex is often cited for the distinction between identifying an error and deciding whether that error could have affected the outcome. It does not mean that every inaccurate statement in a decision letter automatically requires the decision to be set aside.
How to use Simplex carefully
The practical task is to identify the precise mistake, show where it entered the reasoning, and explain why it mattered. Simply disagreeing with the weight given to a lawful material consideration is a different complaint.
The later Court of Appeal judgment in E v Secretary of State for the Home Department [2004] EWCA Civ 49 discusses Simplex in its treatment of mistake of fact and unfairness.
For a current court challenge, the applicable statutory provisions, relief tests and later authorities must also be checked. The historical Simplex formulation is not a substitute for identifying the correct legal route and its requirements.
The original transcript is now available
Planning Geek now holds the complete approved transcript, including both appeals and the final orders. This page is no longer based only on later descriptions of the case.
Gold Members can read the 46-page transcript below. The 1988 judgment date should not be confused with the years in which the different reports were published.
Simplex Case Law Page Updated: 12th September 2026