For the tax year 2019-20 onwards individuals can carry forward any annual allowance they have not used in the previous three tax years.
To carry forward unused annual allowance to a current tax year from one or more of the previous three, or previous four, tax years then the individual must have been a member of a registered pension scheme at some point in the previous tax year concerned.
The unused annual allowance for the previous tax year is the difference between the total pension input amount for that year and the annual allowance for the same year.
Where an individual has unused annual allowance from a previous tax year that they can use to set off against an annual allowance charge for a current tax year, they do not have to include this on their Self Assessment tax return for the year in which the unused allowance arose or for the year that the unused annual allowance is being used to set off against an annual allowance charge.
Further guidance on pension allowance carry forward can be found at PTM055000.
PTM055000 - Annual allowance: carry forward