To protect buyers, there is a 7-day 'cooling-off' period after the property purchase contract is signed. The buyer can opt-out any time during this period.
After this 7-day period, if a buyer decides not to proceed with the purchase, they will lose their deposit. The only exception to this is if there is a legal justification for not proceeding (for example, if one of the 'conditions suspensives' is not met). Consequently, if after you sign the contract you have second thoughts, you must cancel within the 7-day period or you will lose your deposit.
If you decide to cancel during this cooling-off period, it is advisable that you have written proof (e.g. a signature from a registered letter) that your cancellation was done within the 7 days. This will avoid potential disputes as to whether the cancellation was received by the seller or not.
Some dishonest estate agents will ask you to pre-date your signature of the property contract. In other words, to sign with a date which is earlier than the current date. By doing so, you can forfit your right to the cooling-off period. It is inadviseable to agree to any such requests.
If you hoping to take advantage of the 7-day period to do any final checks on the property, such as having a building surveyor examine it, be aware that such arrangements are often delayed and you may not be able to complete them within seven days. Consequently, if there are items you need to verify, you should add a condition to the contract that the contract can be cancelled without penalty if the specified conditions are not met (these are known as 'conditions suspensives').