Ultimately it depends on the form of annual leave. One form of annual leave – statutory EU annual leave – requires employees be given four weeks of annual paid leave. For this form of statutory EU annual leave, it is not possible to carry it over.
The second type of annual leave – under UK law that gives employees 1.6 paid weeks leave – can be carried over. However, only if the employer and employee have agreed to do so in writing, such as in an employment contract.
The final third type of annual leave is that in which the employer offers above and beyond the basic amount. For this form of annual leave an employer and employee must agree as to whether this form of annual leave can be carried over. This sort of agreement would commonly be found in an employment contract or written agreement drawn up by an employer and given to an employee.
There are exceptions to these rules, however. An employee could be allowed to carry over any unused leave if they have been in long-term sickness, or even for serious non-sickness related circumstances. However, for the latter, this is still a hotly debated issue, which is even currently being considered by the European Court of Justice.
Overall, for an employee, the usual scenario they will find in the workplace is that four weeks of annual leave is expected to be used during the year in which it is assigned, rather than any future year. However, it is possible to carry over annual leave in certain circumstances such as minimum annual leave amounts or exceptional circumstances such as ill health.