Promissory estoppel and proprietary estoppel are both legal doctrines that prevent a person from going back on their word under certain conditions, but they apply in different contexts and have different requirements.
Promissory Estoppel is generally used in the context of contract law. This doctrine comes into play when one party makes a promise to another party, who then relies on that promise to their detriment. The essential elements of promissory estoppel are: a clear and unambiguous promise, reasonable and detrimental reliance by the promisee on the promise, and an injustice that can only be avoided by enforcing the promise. For example, suppose a landlord promises a tenant that the tenant can live rent-free for a year if they improve the property. The tenant, relying on this promise, spends money on improvements. Even though there was no formal contract, the landlord may be estopped (prevented) from charging rent, as it would be unjust to allow the landlord to renege on the promise after the tenant has incurred expenses based on it.
Proprietary Estoppel is used in the context of property rights. This doctrine applies when someone is led to believe that they will acquire rights to a property, they rely on this belief to their detriment, and it would be unjust to deny them some form of right over the property. For example, consider a situation where an elderly parent tells their child that they will inherit the family home, and in reliance on this, the child spends years caring for the parent and investing in the upkeep of the home. If the parent later decides to will the home to someone else, proprietary estoppel could be invoked to give the child some rights to the property, because the child acted to their detriment based on the parent's assurances.
Both doctrines serve to enforce fairness and prevent parties from acting against their promises or assurances when others have relied on those to their detriment. However, the key distinction lies in the nature of the promise: promissory estoppel is about promises in general that lead to financial or other detriments, while proprietary estoppel specifically relates to promises or assurances about rights to use or own property.
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