A more thorough explanation:

The pre-existing duty doctrine is a legal principle that applies to contracts. It states that if a party to a contract is already obligated to perform a certain duty, then any modification to the contract that does not provide additional consideration is not valid.

For example, imagine that a construction company agrees to build a house for a homeowner for $100,000. Halfway through the project, the homeowner decides that they want some additional features added to the house, such as a swimming pool. The construction company agrees to make the changes, but only if the homeowner agrees to pay an additional $50,000. This modification to the contract is valid because the homeowner is providing additional consideration (the extra money) in exchange for the additional work.

However, if the construction company simply agreed to make the changes without any additional compensation, this modification would not be valid under the pre-existing duty doctrine. This is because the construction company was already obligated to complete the original contract, and the homeowner is not providing any additional consideration for the extra work.

Another example of the pre-existing duty doctrine can be seen in employment contracts. If an employee is already obligated to perform certain duties as part of their job, the employer cannot modify the contract to require additional duties without providing additional compensation.