Arbitrariness or deficiencies of logic in the decision-making process may, if sufficiently serious, make the decision unreasonable.
A decision may be judged to be unreasonable if it is based on a conclusion that is not supported at all by evidence or good reasons, or where no reasonable person would consider the evidence to be sufficient to support the conclusion drawn. In order to be found to be unreasonable, the conclusion drawn must have been material or integral to the decision-maker’s reasoning for the decision (e.g. a planning development decision based on a conclusion that the development would only partially affect the view from the claimant’s window was unreasonable since, in fact, the development would obstruct the view completely) (Jagendorf v Secretary of State  JPL 771).
Depending on the context, a decision may also be unreasonable if the Court finds that the weight of evidence pointing to one course of action is overwhelming but that the decision-maker has opted to decide the matter in a different and opposing way (Emma Hotels Ltd v Secretary of State for the Environment (1980) 41 P&CR 255). Similarly, a decision may be unreasonable if “manifestly excessive” or if “manifestly inadequate” weight has been given to a particular relevant consideration (Secretary of State for Trade and Industry ex p BT3G Ltd  Eu LR 325).
A decision which is made without alternatives being considered may be unreasonable. Unreasonableness may also be inferred from a decision-maker’s failure to give reasons for the decision made where they are required (Padfield v Minister of Agriculture Fisheries and Food  AC 997).
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