The pension annual allowance effectively sets the maximum pension contributions from all sources (including your employer) on which you may be able to claim income tax relief. In recent times it has been the subject of much controversy because of the way the allowance is tapered from the ‘standard’ £40,000 to as little as £10,000 for high earners.
Carry forward rules allow you to mop up unused annual allowance from up to three tax years ago, ie 2016/17 onwards during the 2019/20 tax year. In theory this could mean that before 6 April 2020 tax-relievable pension contributions of up to £160,000 could be made (£40,000 a year for 2016/17 – 2019/20 inclusive).
Calculating how much can be carried forward is sometimes a difficult exercise, requiring detailed contribution records, so if you want to beat the deadline for using up your remaining allowance from 2016/17, start seeking advice now.