In proprietary estoppel by acquiescence (Ramsden v Dyson type), the owner's knowledge of the claimant's mistaken belief is critical, but it is unnecessary for the claimant to show that the belief was reasonable — it suffices that the belief was held, known to the owner, and acted upon. An uncommunicated change of position by the owner cannot render the claimant's continued reliance unreasonable. The principle of proportionality applies only in unusual cases where proprietary relief would be out of all proportion to the detriment; where the detriment involves life-changing decisions and irreversible consequences of a profoundly personal nature, the appropriate remedy is to make good the assumption. Detriment in proprietary estoppel is to be evaluated as opportunity cost, not as quantum meruit.
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