The £100 that arrives the day after the deadline — and the daily charges that turn a forgotten return into a four-figure bill if you leave it.
The late filing penalty is the most expensive £100 in the tax system — not because of the £100 itself, but because of what follows it. Miss the Self Assessment deadline and HMRC charges you automatically, even if you owe nothing. Leave the return unfiled and the charges escalate in waves, so a return forgotten in January can cost well over £1,000 by the following winter.
Self Assessment penalties for filing late come in four stages, and they stack. The clock starts the day after the online deadline of 31 January.
The first stage is a £100 fixed penalty, applied immediately and regardless of your tax position. The second, once the return is three months late, is a daily penalty of £10 for up to 90 days — a maximum of £900. The third, at six months late, adds the higher of £300 or 5% of the tax due. The fourth, at twelve months, adds the higher of £300 or 5% again (and more in deliberate cases).
Take Sofia, a freelance illustrator. Her 2024/25 return was due online by 31 January 2026, but life got in the way and she finally files in September 2026 — about seven months late. Assume she owed £2,000 in tax. Here is how the penalties build.
| Stage | Trigger | Penalty |
|---|---|---|
| Initial | Missed 31 Jan 2026 | £100 |
| Daily | 3 months late, 90 days at £10 | £900 |
| Six months | Higher of £300 or 5% of £2,000 (£100) | £300 |
| Total late filing penalties | £1,300 |
That £1,300 is purely for filing late. On top of it, the unpaid £2,000 attracts separate late-payment interest and surcharges. Run your own dates and figures through the late filing penalty calculator to see the full picture before it grows.
It is vital to separate the two. The late filing penalty punishes a late return; late-payment interest and surcharges punish late tax. You can owe one without the other. Someone who files on time but pays late avoids the £100 fixed penalty entirely but still pays interest. Someone owed a refund who files late still gets the £100 and daily penalties, because those do not depend on tax being owed.
The cruelty of the late filing penalty is that it does not care whether you owe a penny. File a nil return a day late and HMRC still wants £100 — which is why filing early, even with figures you will later refine, is almost always the safer move.
If a genuine, unexpected event stopped you filing — serious illness, a death in the family, a flood, or an HMRC service outage — you can appeal, usually within 30 days of the penalty notice. You must still submit the outstanding return, and HMRC expects you to file once the obstacle has passed. Forgetting, finding the system confusing, or relying on someone else who let you down are generally not accepted as reasonable excuses.
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