Preexisting duty rule is a common-law rule of contract. It says that the rule that performance of an act by which a party is already contractually bound to perform does not constitute valid consideration for a new promise. In other words, a party's offer of a performance already required under an existing contract is an insufficient consideration for modification of the contract.
For example, if a builder agrees to construct a building for a specified price but later threatens to walk off the job unless the owner promises to pay an additional sum. Then the owner’s new promise will not be enforceable because under the preexisting duty rule, there is no consideration for that promise.