Your basic statutory holiday entitlement under the EU Working Time Directive (four weeks of holiday) must be taken during the holiday year. Otherwise it will be lost.
There is an exception for these four weeks of holiday if you are unable to take your holiday because you are sick, injured, pregnant or on maternity leave, or if your employer has made it impossible for you to take your paid holiday in the correct holiday year. In these circumstances, you can carry forward your holiday into the next holiday year.
The right to carry forward holiday when you are sick is not without its limits. In general, employers are allowed to have a policy allowing you to you should be allowed to carry it forward for a maximum of 18 months after the end of the year in which it accrued. Any holiday left unused beyond that point is likely to be lost.
As well as the first four weeks of statutory holiday based on the Working Time Directive, UK workers are entitled to an extra 1.6 weeks of leave based on national law. These extra days result from trade union campaigning and provide a minimum annual holiday allocation of 28 days in total (inclusive of bank holidays) for every full-time worker. You can carry forward this extra 1.6 weeks of leave into the next holiday year if a “relevant agreement” allows this. That relevant agreement is usually your employment contract.
If you have a contractual right to more than 5.6 weeks’ holiday a year, then your employer is allowed to agree any rules they want about whether or not you can carry forward this part of your holiday entitlement and for how long.