Missing a Self Assessment deadline is one of the easiest and most expensive mistakes in UK tax. The good news is there are only a handful of dates to remember. Here is the full timeline, from registering to paying, plus what it costs if you slip.
The short version
File your online return and pay any tax owed by 31 January following the end of the tax year. If you are new to Self Assessment, register by 5 October first. Payments on account, if they apply, fall on 31 January and 31 July.
The UK tax year, and why the dates look odd
The tax year runs 6 April to 5 April. Every deadline hangs off the end of that year. So the 2025/26 tax year ended 5 April 2026, and its deadlines fall in late 2026 and January 2027.
The key dates
Register by 5 October
If you are newly self-employed or filing Self Assessment for the first time, tell HMRC by 5 October after the end of the tax year you need to report. For 2025/26 income that means 5 October 2026. Register late and you can still file, but you risk a failure-to-notify penalty.
Paper return by 31 October
If you file on paper rather than online, the deadline is midnight on 31 October. Most people file online, which gives three more months.
Online return by 31 January
The big one. Your online return for the previous tax year is due by midnight on 31 January. For 2025/26 that is 31 January 2027.
Pay what you owe by 31 January
The same date is also the payment deadline for any tax due for the year, known as the balancing payment.
Payments on account, the part that catches people out
If your tax bill is over £1,000 and less than 80% of your tax is collected at source, HMRC asks you to pay towards next year's bill in advance. You make two payments on account, each 50% of last year's bill, due 31 January and 31 July. A balancing payment then trues up the difference the following January.
Take someone with a £2,000 bill. In January they pay the £2,000 they owe plus a £1,000 first payment on account, then a second £1,000 payment the next July. It feels like a jump the first year, because you are settling one bill and getting ahead on the next at the same time.
A full timeline for the 2025/26 return
| Task | Deadline |
|---|---|
| Register for Self Assessment | 5 October 2026 |
| Paper return | 31 October 2026 |
| Online return | 31 January 2027 |
| Pay balancing payment | 31 January 2027 |
| First payment on account | 31 January 2027 |
| Second payment on account | 31 July 2027 |
What it costs to be late
| How late | Penalty |
|---|---|
| 1 day | £100 fixed, even if you owe nothing |
| 3 months | £10 a day, up to £900 |
| 6 months | A further 5% of the tax due or £300, whichever is greater |
| 12 months | Another 5% or £300, and possibly more |
Late payment is charged separately: interest runs from 1 February, plus 5% surcharges on tax still unpaid after 30 days, 6 months and 12 months.
File early, sleep better
You can file any time after the tax year ends on 6 April, you do not have to wait for January. Filing early tells you your bill months ahead, spreads the cost, and means any refund arrives sooner. And with Making Tax Digital for Income Tax arriving from April 2026, getting into a tidy, digital habit now pays off.
Never miss a date again
Oazy tracks your deadlines for you and drafts the return from your bank data, so 31 January stops being a scramble. Connect your bank, review the few items the AI flags, and file with time to spare. Open your account.
