A long stop date is the latest date that parties must fulfill their duties in a contract. In respect of your new build purchase, this will be the latest date that your purchase must complete, before either party can terminate the contract. The long stop date entered into your contract will be the latest date that the developers believe the property will be ready to move into. If this date passes and the property is not ready, you will usually be able to terminate your contract to purchase and the developer should return your deposit and reservation fee to you.