You finish your first profitable year of self-employment, put aside what you calculated you would owe, file the return, and HMRC asks for half as much again. Nothing has gone wrong. This is how the system works, and almost nobody is told about it in advance.
What is actually happening
Self Assessment collects tax after the year it relates to. That leaves HMRC a year behind, so it asks for part of next year’s tax in advance. Those advances are called payments on account, and GOV.UK sets each of them at half of the tax you owed last year.
There are two, due on 31 January and 31 July. The awkward part is the timing of the first one: it lands on the same 31 January as the balancing payment for the year that has just ended.
So on that single date you pay:
- the whole of the tax for the year just finished, and
- half of it again, as the first instalment towards the year you are currently in.
That is roughly 150% of your bill, in one go. The second instalment follows six months later, and then you are on the same footing as everyone else.
Why it feels like a penalty and is not
Nothing extra has been charged. You are paying the same tax you would have paid anyway, just earlier — and only once. From the second year onwards the two instalments cover most of the bill before it is due, and the January payment becomes a much smaller balancing figure plus the next advance.
The reason it hurts is entirely a cash-flow one. You budgeted for a number, and the demand is 50% larger, at the point in the year when trade is usually slowest.
When it does not apply
GOV.UK gives two exemptions. You do not make payments on account if:
- the tax you owed last year was less than £1,000, or
- you paid more than 80% of the tax you owed at source — through a tax code, for instance, or because a bank had already deducted tax on savings interest.
The first is the one that catches people out in the other direction. A first year that just scrapes over the threshold triggers the whole mechanism; a first year just under it does not, and the following year then arrives with both a bill and a first advance at once.
The other half: what is actually in the bill
Payments on account cover income tax and Class 4 National Insurance. They do not cover everything.
Student loan repayments and capital gains are part of the balancing payment rather than the instalments, so they arrive in one lump each January and are never spread. If you are self-employed and repaying a student loan, that is a meaningful amount arriving at the worst possible moment, and it is invisible in any estimate that only models the advances.
Class 2 National Insurance, by contrast, is not in the bill at all for most people. Since the 2024 reform, anyone with profits at or above the small profits threshold has Class 2 treated as having been paid — the record is protected and nothing changes hands. Plenty of calculators still add a weekly Class 2 charge; if yours does, it is inventing a liability.
What to do about it
Not advice, just arithmetic: the amount to set aside in a first profitable year is your calculated tax plus half of it again, and then half again six months later. The self-employed calculator works out both figures rather than leaving the January total as a footnote, because the tax bill on its own is not the number you need to have in the bank.
If your income falls in the following year you can ask HMRC to reduce the payments on account — but reducing them below what you actually end up owing means interest on the shortfall, so the estimate has to be honest rather than hopeful.
Why nobody warns you
Because the first year is the only year it looks like this, and by the time you understand it you have already been through it. Accountants know; people filing their own first return generally do not. It is the single most common self-employed cash-flow surprise, and it is entirely predictable a year in advance — which is the only reason this page exists.