Payments on account: why January costs one and a half bills

Nobody warns you about the first big January. You expect to pay the year you just declared; HMRC also wants half of NEXT year, in advance, the same day. Here is the machine, with numbers.

The machine

Once a Self Assessment bill reaches 1,000 GBP (and less than 80% of your tax came through PAYE), HMRC assumes next year will match it and collects it in advance: 50% on 31 January, 50% on 31 July. Checked 2026-08-22 against gov.uk’s payments on account guidance. The January date therefore stacks two things: the balance of the year you declared plus the first advance. On a 6,000 bill with nothing prepaid, January is 9,000. Our payments on account planner does your version.

Why year two feels better

From the second year, the advances you paid last year offset the new bill: if the years match, January is just the new advance. The pain is concentrated in year one, which is exactly when nobody has warned you and the money is least likely to be sitting there. The fix is boring: set aside from every invoice from month one, as if the bill already existed.

Reducing them without burning yourself

Income dropping? You can apply to reduce payments on account through the return or your online account, and it is legitimate. The trap is optimistic reduction: if the real bill comes in above what you paid, HMRC charges interest on the shortfall, backdated. Reduce to what you honestly project, not to what January would like.

What they do not cover

Advances cover income tax and Class 4. Capital gains and student loan repayments sit outside and land with the January balance, which is how a share sale in a good year produces a January surprise even for veterans.