Do I need to make payments on account for Self Assessment?
Some people need to submit a Self Assessment tax return each year so HMRC can calculate how much tax they owe. If your Self Assessment tax bill is greater than £1000, then HMRC will require you to make payments on account.
They basically assume you will earn a similar amount the following year, and ask you to pay part of next year’s tax bill in advance. It’s worked out by dividing last year’s tax bill in half. The first payment on account is due by 31st January; the same deadline as the previous year’s tax bill. The second instalment must be paid by 31st July.