An unenforceable contract is one that some rule of law bars a court from enforcing. For example, Tom owes Pete money, but Pete has waited too long to collect it and the statute of limitations has run out. The contract for repayment is unenforceable and Pete is out of luck, unless Tom makes a new promise to pay or actually pays part of the debt. (However, if Pete is holding collateral as security for the debt, he is entitled to keep it; not all rights are extinguished because a contract is unenforceable.) A debt becomes unenforceable, too, when the debtor declares bankruptcy.
A bit more on enforceability is in order. A promise or what seems to be a promise is usually enforceable only if it is otherwise embedded in the elements necessary to make that promise a contract. Those elements are mutual assent, real assent, consideration, capacity, and legality. Sometimes, though, people say things that seem like promises, and on which another person relies. In the early twentieth century, courts began, in some circumstances, to recognize that insisting on the existence of the traditional elements of contract to determine whether a promise is enforceable could work an injustice where there has been reliance. Thus developed the equitable doctrine of promissory estoppel, which has become an important adjunct to contract law. The Restatement (Section 90) puts it this way: “A promise which the promisor should reasonably expect to induce action or forbearance on the party of the promisee or a third person and which does induce such action or forbearance is binding if injustice can be avoided only by enforcement of the promise. The remedy granted for breach may be limited as justice requires.”
To be “estopped” means to be prohibited from denying now the validity of a promise you made before.
The doctrine has an interesting background. In 1937, High Trees House Ltd. (a British corporation) leased a block of London apartments from Central London Properties. As World War II approached, vacancy rates soared because people left the city. In 1940 the parties agreed to reduce the rent rates by half, but no term was set for how long the reduction would last. By mid-1945, as the war was ending, occupancy was again full, and Central London sued for the full rental rates from June on. The English court, under Judge Alfred Thompson Denning (1899–1999), had no difficulty finding that High Trees owed the full amount once full occupancy was again achieved, but Judge Denning went on. In an aside (called a dicta—a statement “by the way”—that is, not necessary as part of the decision), he mused about what would have happened if in 1945 Central London had sued for the full-occupancy rate back to 1940. Technically, the 1940 amendment to the 1937 contract was not binding on Central London—it lacked consideration—and Central London could have reached back to demand full-rate payment. But Judge Denning said that High Trees would certainly have relied on Central London’s promise that a reduced-rate rent would be acceptable, and that would have been enough to bind it, to prevent it from acting inconsistently with the promise. He wrote, “The courts have not gone so far as to give a cause of action in damages for the breach of such a promise, but they have refused to allow the party making it to act inconsistently with it.” 1
In the years since, though, courts have gone so far as to give a cause of action in damages for various noncontract promises. Contract protects agreements; promissory estoppel protects reliance, and that’s a significant difference. The law of contracts continues to evolve.