The key dates
| Deadline | What's due |
|---|---|
| 5 October | 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 tax year |
| 31 July | Second Payment on Account due, if applicable |
The tax year runs 6 April to 5 April, and the 31 January deadline falls in the calendar year after the tax year ends — for example, the return for the tax year ending 5 April is due by the following 31 January.
Penalties for late filing
- £100 automatic penalty for missing the deadline, even by a single day, even if no tax is owed.
- Further daily penalties after three months.
- Additional percentage-based penalties after six and twelve months.
Penalties for late payment
Separately from filing penalties, interest and additional percentage-based charges apply to tax paid late, calculated from the original due date — so even a return filed on time but with an unpaid balance will start accruing interest and penalties.
Reasonable excuses
HMRC can waive penalties for a genuine "reasonable excuse" — a serious illness, bereavement, or a technical failure with HMRC's own systems, for example. Simply forgetting, finding the process confusing, or being busy generally isn't accepted, so appeals need a legitimate, evidenced reason.
What to do if you're going to miss a deadline
Filing as soon as possible after a missed deadline limits how much the penalties escalate, since several of the harsher penalties apply after longer periods of continued non-filing. If you can't pay in full, contacting HMRC to arrange a Time to Pay agreement before the deadline passes can reduce penalties compared with simply not paying and not communicating.
Key takeaways
- 31 January is the key deadline for online returns and paying the balance owed.
- A £100 penalty applies immediately for late filing, even with no tax due.
- Late payment penalties and interest are separate from late filing penalties.
- Contact HMRC proactively if you can't pay or file on time — silence makes penalties worse.