Miss the online Self Assessment deadline by a single day and HMRC charges you £100 — even if you owe nothing. The whole system runs on a handful of fixed calendar dates, and once you know them, staying penalty-free is mostly a matter of putting four reminders in your phone. This guide walks through every date that matters for sole traders, landlords and side-hustlers, what happens if you miss each one, and a worked example so you can see the penalty schedule in action.
Throughout, we use the deadlines for the tax year that ended 5 April 2026 (the "2025/26 return"), because that is the return most people are filing right now. The same pattern repeats every year — only the calendar shifts.
If you only remember one date, make it this: the main UK tax return deadline for filing online and paying what you owe is midnight on 31 January. Everything else below hangs off that single Self Assessment tax return deadline.
The Self Assessment tax return deadline calendar at a glance
Self Assessment hangs off the UK tax year, which runs 6 April to 5 April. For income earned in the 2025/26 tax year, the dates are:
- 5 October 2026 — deadline to register for Self Assessment if you are new to it.
- 31 October 2026 (midnight) — deadline for a paper tax return.
- 31 January 2027 (midnight) — deadline for the online return and your balancing payment.
- 31 January 2027 — first payment on account for 2026/27 (if due).
- 31 July 2027 — second payment on account for 2026/27 (if due).
Note that the year on each date moves on as you go down the list. People trip up because the return covers one tax year, but you pay it (and start paying towards the next) in the following calendar year.
| Deadline (2025/26 return) | Date | What it covers |
|---|---|---|
| Register for Self Assessment | 5 October 2026 | New filers only — triggers your UTR |
| Paper return | Midnight 31 October 2026 | If filing by post |
| Online return + balancing payment | Midnight 31 January 2027 | Almost everyone |
| First payment on account (2026/27) | 31 January 2027 | If your bill is over £1,000 |
| Second payment on account (2026/27) | 31 July 2027 | If your bill is over £1,000 |
Who must file a return
You generally need to file a Self Assessment return if, in the tax year, you were self-employed as a sole trader and your gross trading income was more than £1,000, or you were a partner in a business, or you had untaxed income such as rental profit, savings, dividends, or income from a side hustle that HMRC hasn't already taxed at source.
The £1,000 figure is the trading allowance. If your gross trading income for the year is £1,000 or less, you usually don't need to register or report it at all. Once you go over £1,000 gross, you must register. Note this is gross income before any expenses — not profit.
If you're not sure whether you're caught, read our explainer on whether you need to do a Self Assessment, and if you sell online, our guide to HMRC's side-hustle tax rules.
A word on platform sellers
Since 1 January 2024, digital platforms such as eBay, Vinted and TikTok Shop report seller income and identity to HMRC each year under the "DAC7" rules. HMRC cross-checks that data against the returns it receives. If your platform sales push your gross trading income over £1,000, you're in Self Assessment — and HMRC already has the numbers.
5 October: the registration deadline
If this is your first time, you must register for Self Assessment by 5 October following the end of the tax year in which you had the untaxed income. So for income earned in 2025/26, register by 5 October 2026.
Registering is what triggers HMRC to issue your Unique Taxpayer Reference (UTR), which you need before you can file online. The UTR can take a couple of weeks to arrive by post, so don't leave registration to the last day. Our step-by-step walkthrough of registering as self-employed covers exactly what you'll need.
31 October vs 31 January: paper or online
There are two return deadlines, and which one applies depends on how you file:
- Paper return: midnight on 31 October.
- Online return: midnight on 31 January.
Almost everyone files online, which buys you three extra months and does the tax calculation for you automatically. The paper deadline only matters if you file by post — and if you miss the 31 October paper deadline, you can still avoid a late-filing penalty by filing online instead, as long as you do so by 31 January. If you're filing for the first time, our guide to filing your first Self Assessment return is the place to start.
Payments on account: 31 January and 31 July
This is the bit that catches people out. If your Self Assessment tax bill is more than £1,000 and less than 80% of your tax was collected at source, HMRC asks you to make payments on account — advance instalments towards next year's bill.
Each instalment is half of your previous year's tax bill. They're due on:
- 31 January — alongside your balancing payment for the year just filed.
- 31 July — the second instalment.
That January payment can feel brutal because you settle last year's bill and pay the first half of next year's at the same time. We explain why in why your first tax bill feels so high, and the mechanics in full in our payments on account guide.
Late-filing penalties: the exact schedule
The late-filing penalties are automatic and they stack. They apply to the return, not the payment — so you can be penalised for filing late even if you owe no tax or have already paid it.
- The moment you miss the deadline: a £100 fixed penalty. This applies even if no tax is due or the tax was paid on time.
- After 3 months: £10 per day for up to 90 days — a maximum of £900, on top of the £100.
- After 6 months: a further penalty of the greater of £300 or 5% of the tax due.
- After 12 months: another penalty of the greater of £300 or 5% of the tax due.
Left for a full year, a return with no other tax to pay can rack up £100 + £900 + £300 + £300 = £1,600 in penalties. Our dedicated post on the Self Assessment late filing penalty covers appeals and reasonable excuse.
Late-payment penalties and interest
Separate from filing penalties, there are penalties for paying late. These are charged on the tax you still owe:
- 5% of the unpaid tax at 30 days after the due date.
- A further 5% at 6 months.
- A further 5% at 12 months.
On top of those penalties, HMRC charges interest on everything you pay late, running from the due date until the day you pay. Interest is not a penalty you can appeal — it's the cost of HMRC's money while you hold onto it. See our breakdown of HMRC interest and late-payment penalties for the detail.
Worked example: a side-hustle that tips over £1,000
Priya sells handmade candles on a marketplace alongside her PAYE job. In 2025/26 her gross sales are £6,000. Because that's over the £1,000 trading allowance, she must register and file.
She can choose to deduct the £1,000 trading allowance instead of her actual expenses. Her taxable trading profit is therefore £6,000 − £1,000 = £5,000. Her PAYE wages already use up her personal allowance, so this £5,000 is taxed as a basic-rate taxpayer: 20% × £5,000 = £1,000 of income tax. She pays no Class 4 National Insurance on it, because Class 4 only bites on trading profits above £12,570 and her £5,000 profit is below that threshold. (For the full picture of how the bill is built, see how your Self Assessment bill is calculated.)
Now suppose Priya forgets to file. Here's what that £1,000 tax bill becomes if she does nothing:
- Day after 31 January: £100 fixed late-filing penalty (charged even if she'd already paid the tax).
- At 30 days late: a late-payment penalty of 5% × £1,000 = £50 on the unpaid tax.
- From 3 months late: £10/day late-filing penalties kick in, up to a maximum of £900 over 90 days.
- Plus interest running on the £1,000 from 1 February until she pays.
So a £1,000 tax bill she could have paid on time becomes a £100 fixed penalty, a £50 late-payment charge, mounting daily penalties and interest — before the 6- and 12-month penalties even arrive. Filing and paying on time would have cost her nothing extra.
Can't pay? Set up Time to Pay
If you've filed but can't pay the bill, don't ignore it — interest and late-payment penalties build fast. HMRC offers Time to Pay, an instalment arrangement you can set up online if you owe less than £30,000, your return is filed, and you meet the eligibility criteria. Setting it up before the penalty triggers can stop further late-payment penalties accruing while you pay it off.
Our guide to HMRC Time to Pay for Self Assessment walks through the online process, and how to pay your Self Assessment bill covers every payment method and how long each takes to clear.
Your action plan
Reduce the whole system to four reminders:
- New to Self Assessment? Register before 5 October.
- Filing online (almost certainly): submit and pay before 31 January.
- Payments on account due? Pay the second instalment by 31 July.
- File early. You can file from 6 April; doing so tells you the bill months ahead so you can budget for January.
Sources
- gov.uk — Self Assessment deadlines
- gov.uk — Self Assessment penalties
- gov.uk — Payments on account
- gov.uk — Register for Self Assessment
- gov.uk — Pay your Self Assessment tax bill
- gov.uk — If you cannot pay your tax bill on time
Frequently asked questions
What is the Self Assessment tax return deadline?
For almost everyone, the Self Assessment tax return deadline is midnight on 31 January, the date for filing your return online and paying any tax you owe. If you file a paper return by post instead, the UK tax return deadline is earlier — midnight on 31 October. For the 2025/26 tax year that means 31 October 2026 for paper and 31 January 2027 for online.
Do I get a £100 penalty even if I owe no tax?
Yes. The £100 fixed late-filing penalty applies the moment you miss the deadline, even if no tax is due or the tax was already paid on time. It is charged for filing the return late, not for paying late.
What is the difference between the 31 October and 31 January deadlines?
31 October at midnight is the deadline if you file a paper return by post. 31 January at midnight is the deadline if you file online, which is what almost everyone does because it gives you three extra months and calculates the tax for you automatically.
When do I have to register for Self Assessment?
By 5 October following the end of the tax year in which you had untaxed income. For example, if you had untaxed income in the 2025/26 tax year, you must register by 5 October 2026 so HMRC can issue your UTR in time to file.
What can I do if I can't pay my Self Assessment bill?
File the return on time and set up a Time to Pay arrangement with HMRC. You can usually do this online if you owe less than £30,000, your return is filed and you meet the criteria. It spreads the cost in instalments and can stop further late-payment penalties building up.
