Payment on account, explained

Payment on account is HMRC collecting your Self Assessment tax in advance, in two instalments, rather than a year after you earned it. It's what surprises people the first January.

Why the first bill is bigger

That first year you pay the tax you owe AND a payment on account toward next year, usually half again, so the January bill can be about one and a half times the tax for the year. The second half is due 31 July. It's not extra tax; it's next year's, paid early, and set against your actual bill later.

Worth looking into

Because it front-loads that first January, setting money aside through the year softens the landing. HMRC sets the exact thresholds and figures. Gov.uk and an accountant can tell you what your own payments will be.

This is general information, not accounting or tax advice. An accountant or HMRC is the right source for your situation.