Most people who are late filing a Self Assessment return know there will be a fine. What they do not know is exactly how much, or how quickly it grows. The HMRC penalty system is not a single fine — it is an escalator with four stages, each more expensive than the last.
Here are the exact numbers.
Stage 1 — Day 1: £100 Fixed Penalty
Automatic. Instant. Applies to everyone regardless of their tax liability. A person with zero tax to pay gets exactly the same £100 fine as someone who owes £50,000. No warning letter precedes it — the fine is applied from the date of the deadline, not the date of any correspondence.
Total penalty at day 1: £100.
Stage 2 — 3 Months Late: £10 Per Day (Up to £900 More)
Three months after the filing deadline, if your return is still outstanding, HMRC begins charging £10 per day. This runs for up to 90 days — a maximum additional penalty of £900. The charges accumulate quietly. Many people discover months of daily fines at once when the notice arrives.
Total penalty at 6 months (3 months of daily charges): £1,000.
Stage 3 — 6 Months Late: 5% of Tax or £300
At six months, a surcharge applies equal to 5% of the unpaid tax shown on the return, or £300 — whichever is higher. This is the first penalty that scales with your income, meaning higher earners face proportionally larger charges at this stage.
- Tax bill of £0: surcharge is £300 (the minimum)
- Tax bill of £3,000: surcharge is £300 (5% is only £150, so the minimum applies)
- Tax bill of £6,000: surcharge is £300 (5% equals £300 — the minimum, exactly)
- Tax bill of £10,000: surcharge is £500
- Tax bill of £20,000: surcharge is £1,000
- Tax bill of £50,000: surcharge is £2,500
Stage 4 — 12 Months Late: The Same Again
At twelve months, the identical 5%-or-£300 surcharge applies for a second time. Additionally, HMRC gains the right to issue a determination — their own legally-enforceable estimate of your tax liability — which can be pursued through the courts without a further court order.
Worked example — contractor with a £15,000 tax bill, 13 months late:
- £100 initial fixed penalty
- £900 daily charges (90 days × £10)
- £750 six-month surcharge (5% of £15,000)
- £750 twelve-month surcharge
- £1,460 estimated interest on £15,000 at 7.5% for 13 months
- Total penalty and interest exposure: £3,960
That £100 letter turned into nearly £4,000 in thirteen months. And that is before accounting for the underlying £15,000 tax itself.
The most important date: 3 months
The three-month threshold is the most critical because it is where fixed charges end and daily charges begin. If you act before three months, your total penalty exposure is £100 — a significant fine but a manageable one. If you slip past three months, the meter starts running at £10 per day, and you cannot recover those accumulated charges even by filing immediately after.
Use our penalty calculator to see your exact exposure based on your specific situation.
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