The Self Assessment calendar has a handful of fixed dates. Missing any of them triggers penalties that are entirely avoidable with a bit of planning.
| Date | What's due |
|---|---|
| 5 October | Deadline to register for Self Assessment if this is your first year needing to file |
| 31 October | Paper tax return deadline |
| 31 January | Online tax return deadline, and balance of tax owed for the previous year |
| 31 July | Second payment on account for the current tax year |
What happens if you miss the 31 January deadline
- 1 day late: automatic £100 penalty, even if you owe no tax
- 3 months late: £10 per day, up to £900
- 6 months late: a further £300 or 5% of tax owed, whichever is higher
- 12 months late: another £300 or 5%, sometimes more in serious cases
Late payment (as opposed to late filing) also accrues interest separately — the two penalties stack.
What counts as a reasonable excuse
HMRC does accept appeals in genuine circumstances — serious illness, bereavement, or system failures on HMRC's own side. "I forgot" or "I was busy" won't qualify. If something genuinely prevented you from filing, document it and appeal promptly rather than staying silent.
The smartest move: file early
You can file any time after the tax year ends on 5 April — you don't need to wait until January. Filing early doesn't mean paying early (the payment deadline stays 31 January either way), but it does mean:
- You know your tax bill months in advance, so there are no surprises
- You have time to query anything that looks off
- You avoid the January rush, when accountants and HMRC's helpline are both stretched thinnest
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