No, annual leave is calculated based off someone’s contractual hours per week so you cannot adjust it.

The only way to treat this differently would be to renegotiate the contractual hours. However, if the amount of unpaid annual leave that your employees are taking is a problem then you always have the option to refuse the leave.

Unless it is unpaid time off that is protected by statute then they still need your permission to be absent even though they are not getting paid.

Consider what they are taking the unpaid leave for and whether or not this is time that they should be taking out of their paid annual leave entitlement.

Unless it is a statutory right, unpaid leave is discretionary and should only be sought within reason.

If the time is needed for appointments then you only need to approve the time for them to travel to and from the appointment, as well as the appointment itself, rather than a full or half day, and only on production of an appointment card.

Once your employees have to use their paid annual leave entitlement they are likely to reconsider just how important it is that they have that particular time off.