Every January, thousands of sole traders open their first real tax bill and assume HMRC has made a mistake. It asks for half as much again as the tax they carefully calculated. There is no mistake. It is payments on account, the least-explained mechanism in Self Assessment, and it is the single most common reason a profitable first year ends in a cashflow crisis.

Nothing about it is unfair once you can see it coming. Everything about it is brutal if you cannot.

The mechanism, in one paragraph

Once your Self Assessment bill exceeds £1,000, HMRC starts collecting next year's tax in advance on the assumption that next year will look much like this one. It splits that advance into two instalments, each 50% of last year's bill, due on 31 January and 31 July. So in the first January the system applies, you pay last year's tax in full plus half of next year's — 150% of the number you were expecting, in one payment.

There is one exemption worth knowing: you are outside the system if more than 80% of your tax was already collected at source, typically through PAYE. Someone with a substantial employed salary and a modest sideline often falls under that test and never sees a payment on account at all.

The consolation is real, if cold: you are not paying more tax, only paying it earlier. Once you are inside the cycle it settles down, because each January's balancing payment is only the difference between what you prepaid and what you actually owed.

Worked example: three years of one sole trader

Illustrative figures, 2026/27 rates. A sole trader with taxable profit of £45,000 in 2026/27, their first full year.
  • Income tax: (£45,000 − £12,570) × 20% = £6,486
  • Class 4 National Insurance: (£45,000 − £12,570) × 6% = £1,946
  • Total 2026/27 bill: £8,432

31 January 2028: £8,432 balancing payment + £4,216 first payment on account = £12,648
31 July 2028: second payment on account = £4,216

Year two, profit rises to £52,000 and the 2027/28 bill comes to £10,529. They have already prepaid £8,432 of it.

31 January 2029: £2,097 balancing payment + £5,265 first payment on account = £7,362
31 July 2029: second payment on account = £5,265

Look at what happened between the two Januaries. The tax bill went up by £2,097, and the January payment went down by £5,286. That is the whole shape of it: the pain is front-loaded into one year, and it never repeats with the same force.

How much to actually set aside

The advice to "put 20% away" is roughly right at modest profits and badly wrong once you approach the higher-rate threshold, because both income tax and National Insurance change gear at £50,270.

  • £45,000 profit: £6,486 income tax + £1,946 Class 4 = £8,432, which is 18.7% of profit.
  • £70,000 profit: £15,432 income tax + £2,657 Class 4 = £18,089, which is 25.8% of profit.

So a working rule: put aside 25% of every payment you receive, and 30% once your profits are clearly through £50,270. The margin is not waste — it absorbs the payment-on-account year and, once you are past it, becomes the buffer that stops a slow quarter turning into a missed deadline. Class 2 National Insurance no longer needs paying separately: it is £3.65 a week for 2026/27 and voluntary, with profits of £7,105 or more treated as though it were paid, and our National Insurance guide covers what that means for your state pension record.

Reducing your payments on account — and when it backfires

If you genuinely expect lower profits — a lost contract, going part-time, a large equipment purchase coming — you can apply to reduce your payments on account, either through your online account or on form SA303. This is entirely legitimate and often sensible. HMRC's assumption that next year mirrors last year is a default, not a judgement.

The trap is reducing them optimistically. If the final bill turns out higher than the reduced payments justified, HMRC charges interest on the difference, backdated to the original due dates. The current late payment interest rate is 7.75%, set at the Bank of England base rate plus four percentage points and in force since 9 January 2026; the base rate was held at 3.75% on 30 July 2026. Repayment interest, if you overpay, runs at just 2.75% — base rate minus one. The asymmetry is deliberate and it is not in your favour.

Worked example — an optimistic reduction. Our trader expects a quiet 2028/29 and reduces both payments on account from £5,265 to £2,500. The year turns out fine and the bill lands at £10,529, so each instalment was £2,765 short. The catch-up is paid on 31 January 2030.
  • First instalment, due 31 January 2029, one year late: £2,765 × 7.75% = £214
  • Second instalment, due 31 July 2029, 184 days late: £2,765 × 7.75% × 184/365 = £108
  • Interest cost of the optimism: about £322, on top of the £5,530 that still has to be found.
An honest reduction costs nothing. A hopeful one is a loan from HMRC at 7.75%.

The 31 July trap

The July payment is the deadline sole traders forget most, and the reason is structural: nothing arrives with it. No return to complete, no reminder that feels urgent, no accountant's email. It simply falls due in the middle of the summer against a bank balance that has had six months to look comfortable.

There is a way to make July work for you. If your return for the year just ended is already filed and shows a lower bill, the July payment shrinks automatically to match. Filing in April, May or June — rather than the following January — means you know your real number nine months early, and it can reduce the payment you make in July. It is the quietest piece of leverage in Self Assessment and almost nobody uses it. The full sequence of dates is set out in our key tax dates guide.

What happens if you simply cannot pay

Interest starts the day after the deadline, but interest is not the expensive part — penalties are, and the penalty regime is changing underneath sole traders right now.

Under the long-standing Self Assessment rules, a late balancing payment attracts a penalty of 5% of the tax outstanding at 30 days, another 5% at six months and another 5% at twelve months. From April 2026, taxpayers mandated into Making Tax Digital for Income Tax — combined self-employment and property income over £50,000 — move to a different regime: 3% of the tax outstanding at day 15, a further 3% of the tax outstanding at day 30, and then 10% a year accruing daily from day 31. Those percentages rise to 4% from April 2027, and the new regime extends to all Self Assessment taxpayers from that point. There is a first-year concession: in your first year in the new system, nothing is charged if you pay within 30 days. Our guide to MTD for sole traders explains who is in scope and when.

Payments on account themselves do not attract late payment penalties, only interest — but the balancing payment does, and an unpaid payment on account rolls straight into a bigger balancing figure. If you know you cannot pay, contact HMRC before the deadline and ask about a Time to Pay arrangement. Interest still runs, but a plan agreed in advance is treated very differently from silence.

Your defence, in four steps

  1. Open a separate savings account for tax and move 25% of every payment you receive into it on the day it lands. Not monthly, not quarterly — on the day, before the money feels like yours.
  2. Keep a live tax estimate in your bookkeeping software so the pot tracks reality rather than a rule of thumb, and you can see the January number building from about month four.
  3. Write both dates in the calendar now — 31 January and 31 July — with a reminder six weeks before each, which is enough time to do something about a shortfall.
  4. File in April or May, not January. You find out the real number nine months early, the July payment can fall, and you stop paying the annual tax on procrastination.

One more thing to watch alongside all this: VAT registration is triggered by turnover, not profit, and by a rolling twelve months rather than the tax year — the detail is in our post on the VAT threshold for sole traders. Crossing it in the same year the payments on account arrive is a genuinely difficult few months if neither was forecast.

The bottom line

Payments on account are not a penalty and they are not a trap for the unwary — they are simply a cashflow mechanism that nobody explains before it happens to you. Understood in advance, they become a transfer between two of your own accounts. Understood in January, they become the reason a good first year ends badly.

Every one of our packages includes us telling you the exact number to expect and the exact date, months ahead, along with the software that keeps the estimate live. If you would rather never be surprised by a tax bill again, get started.