Proprietary estoppel is claimed to rest, variously, on unconscionability, or on the unjust enrichment of the party who makes the promise. Both of these approaches are problematic. Instead, and borrowing from the unjust enrichment concept of failure of basis, a cause of action in proprietary estoppel arises where A assures B that B has or will have a property right in A’s land, and B incurs detriment on the objective basis that A will fulfil his promise. Should A resile, the basis on which B incurred her detriment will fail, giving her a cause of action in proprietary estoppel. The quantum and mode of relief should be that which is necessary to remedy B’s detriment incurred on the failed basis.
Failure to estop: rationalising proprietary estoppel using failure of basis. Conveyancer & Property Lawyer, Sweet & Maxwell