In law, the doctrine of estoppel is a legal principle by which a claimant may be prevented from asserting a legal right or depending on a set of facts to support a claim if that claimant has said or done something that contradicts his current claim. This doctrine attempts to avoid injustice or harm to one party due to inconsistencies of another party. Although there are several forms of estoppel, a doctrine of estoppel generally involves a promise or representation by one party that influences the behavior of the second party, who relies on the veracity of the promise or representation. For example, if a dog breeder agrees to give a customer a free dog, he cannot make a claim for the price of the dog six months later. The doctrine of estoppel prevents him from asserting his otherwise legitimate right to payment for the dog due to the representation that he made to the customer that the dog would be free.
Promissory estoppel refers to a situation in which a promise made by one person leads to actions by another party. If the promising person reneges, the second party suffers damages. For example, a hotel agrees to purchase 100 muffins from a small bakery, and the bakery consequently purchases additional supplies and adds an employee to complete the order. At the last minute, the hotel backs out of the deal. Promissory estoppel, if claimed by the bakery, will force the hotel to purchase the muffins, because otherwise the bakery that relied on the hotel's commitment will suffer harm.
Proprietary estoppel occurs when Party A promises to give property to Party B, and Party A knows that Party B will either spend money or carry out some other action that he would not otherwise do. For example, a father-in-law promises that he will give his son-in-law a piece of lakefront property. Based on the promise, the son-in-law builds a house on the property, landscapes it, and builds a dock. Five years later the father-in-law changes his mind and decides to give the land to his son instead. In such a case, the son-in-law can use a proprietary doctrine of estoppel to compel the father-in-law to abide by his original promise.
Estoppel may also exist due to a person's failure to answer or respond to a legal notice. When a plaintiff makes a claim, the defendant must respond to the complaint within a given period. Failure to respond appropriately implies the defendant's acquiescence to the claim, thereby forfeiting his right to present a counterclaim. For example, the owner of a storage facility sends a certified letter to a delinquent renter to remove his property from the facility or the owner will take possession of it. If the renter fails to respond, the owner may lawfully take possession of the storage compartment contents, becauise the failure to reply constitutes a doctrine of estoppel by acquiescence.