Self Assessment deadlines and penalties
The Self Assessment calendar barely changes from year to year, which makes it easy to plan around, and easy to forget. Here are the dates that matter and what happens if you miss them.
The key dates
| Deadline | What it's for |
|---|---|
| 5 October | Register for Self Assessment (if it's your first year) |
| 31 October (midnight) | File a paper return |
| 31 January (midnight) | File your online return and pay the tax due |
| 31 July | Second payment on account (if you make them) |
The tax year itself runs from 6 April to 5 April. So the 2025-26 tax year ended on 5 April 2026, and the online return and payment for it are due by 31 January 2027.
Quarterly updates under Making Tax Digital
Since 6 April 2026, sole traders and landlords with qualifying income over £50,000 also have to send quarterly updates to HMRC through compatible software under Making Tax Digital for Income Tax. The first was due by 7 August 2026 and the next by 7 November 2026. These updates do not replace the 31 January deadline: the end-of-year return and payment are still due as normal. The threshold drops to £30,000 from April 2027.
Registering
If you're newly self-employed or need to file for the first time, you must tell HMRC by 5 October following the end of the tax year in which the income arose. Registering can take a little time because HMRC posts out your Unique Taxpayer Reference, so don't leave it to the last week.
The two things due on 31 January
It's worth stressing that 31 January is both the filing deadline and the payment deadline. Filing on time but paying late still triggers interest. If you also make payments on account, the first one is due on this date too, on top of the balancing payment for the year just filed.
Penalties for filing late
- 1 day late: an automatic £100 penalty, even if you owe no tax.
- 3 months late: £10 a day, up to £900, on top of the £100.
- 6 months late: a further penalty of 5% of the tax due or £300, whichever is greater.
- 12 months late: another 5% or £300, and in serious cases more.
Penalties for paying late
Late payment attracts interest from 1 February, plus separate late-payment penalties at 30 days, 6 months and 12 months. If you genuinely can't pay in full, contacting HMRC to arrange a Time to Pay plan is far better than simply missing the payment. It can head off the penalties. Missing a date is near the top of our list of common Self Assessment mistakes.
Know the number early
You don't have to wait until you file to know what you'll owe. Once the tax year ends on 5 April, put your figures into the estimator and you'll have a solid estimate straight away, plenty of time to set the money aside before the January deadline.
Frequently asked questions
When is the Self Assessment deadline for the 2025-26 tax year?
31 October 2026 for a paper return and 31 January 2027 for an online return. The tax owed for 2025-26 must also be paid by 31 January 2027.
What happens if I miss the 31 January deadline?
You get an automatic £100 penalty even if you owe no tax. After 3 months it becomes £10 a day for up to 90 days, then further penalties at 6 and 12 months. Interest runs on any unpaid tax from 1 February.
When do I have to register for Self Assessment?
By 5 October after the end of the tax year in which you first had income to declare. For income in 2025-26, that was 5 October 2026.
When are payments on account due?
31 January and 31 July. The first payment on account for the next tax year is due on the same 31 January as the balancing payment, so the January bill can be larger than expected.