You register for Self Assessment by telling HMRC that you have income it does not already know about, and the deadline is set by statute: 5 October after the end of the tax year in which the income arose. For the 2025/26 tax year (6 April 2025 to 5 April 2026) that date was 5 October 2026, so anyone who has not yet registered for last year is now late. Registering late is not fatal. The tax itself is still due by 31 January 2027, and if it is paid in full by then, the failure-to-notify penalty is normally avoided. What follows covers who has to register, how the process works, and what to do if you have missed the date.
Who has to register
HMRC's own list of people who must send a return for a tax year includes anyone who:
- Was a sole trader with income over £1,000. The test is gross income before expenses, not profit. Below £1,000, the trading allowance generally means there is nothing to report;
- Was a partner in a business partnership. This applies whatever the size of your share;
- Has Capital Gains Tax to pay on a disposal that is not reported and settled some other way;
- Owes the High Income Child Benefit Charge and does not pay it through PAYE;
- Is an off-payroll worker repaying a student or postgraduate loan.
Untaxed income can also bring you into the system: rent, tips and commission, foreign income, and investment income above certain levels. For investment income, HMRC draws a clear line. Dividends over £10,000 in a tax year, or savings interest over £10,000, must go on a tax return. Below those amounts, tax can usually be collected through your tax code or a calculation letter. Some people register voluntarily, for example to pay voluntary National Insurance or to prove self-employed status for Tax-Free Childcare.
If you are not sure whether an activity counts as trading at all, the badges of trade are the test HMRC applies.
When to register: the 5 October rule
The obligation sits in section 7 of the Taxes Management Act 1970. A person who is chargeable to income tax or capital gains tax and has not been sent a notice to file must notify HMRC "within the period of 6 months from the end of the year of assessment". The tax year ends on 5 April, so the deadline is 5 October. HMRC's guidance says the rule bites if you have either never sent a return before, or were registered but did not need to send a return for the tax year 2024 to 2025.
There is nothing to stop you registering earlier. You can register as soon as you start trading. That is the better course, because registration produces your Unique Taxpayer Reference (UTR, a 10-digit number) and lets you file as soon as the tax year ends.
How to register
Registration is online through HMRC's "check how to register" service. You will need your National Insurance number. The route differs depending on why you are registering:
- Self-employed (sole trader). You register for Self Assessment and, in the same step, for Class 2 National Insurance. If you are already registered for Self Assessment for some other reason, for example rental income, you still need to register again as a sole trader so that the self-employment is recorded. Our guide to registering as self-employed covers this route in detail;
- Not self-employed. This covers landlords, people with large investment income, and those with a capital gain or the child benefit charge. You register as an individual with income to report;
- Partners. Each partner registers individually, and the partnership itself must also be registered by the nominated partner;
- Previously registered. If you have a UTR from an earlier period, you may need to reactivate the existing record rather than register from scratch. HMRC warns that a return filed without reactivation may be delayed.
Once registered, HMRC sends your UTR and then a notice to file. HMRC's "check when you can expect a reply" tool shows current processing times.
Registered after 5 October? What happens now
For 2025/26, anyone registering from 6 October 2026 onwards is late, but the consequences are narrower than most people fear. Three separate questions arise.
When the return is due. The normal deadlines are 31 October 2026 for a paper return and 31 January 2027 online. A late registrant's notice to file is issued late, and section 8 of the Taxes Management Act moves the deadline accordingly. If the notice is dated after 31 July but on or before 31 October, a paper return is due within 3 months of the notice, and an online return is still due by 31 January. If the notice is dated after 31 October, any return is due within 3 months of the notice. HMRC's guidance gives the short version: the letter or email will state your deadline. Use the date on the notice, not the standard calendar.
When the tax is due. Payment does not move. The balance for 2025/26 is due by 31 January 2027 whenever you registered, and interest runs from that date on anything unpaid.
Whether there is a penalty. The failure-to-notify penalty in Schedule 41 to the Finance Act 2008 is a percentage of the "potential lost revenue". In practice that means tax still unpaid at 31 January after the tax year. HMRC's guidance puts it plainly: if you register after 5 October and do not pay all of your tax by 31 January, you may get a failure-to-notify penalty. For a non-deliberate failure involving UK income or gains the maximum is 30% of the unpaid tax (higher percentages apply to offshore matters). That can fall to 0% where you come forward unprompted and HMRC learns of the failure within 12 months of the tax becoming unpaid. So the practical rule is to register now, estimate the liability, and pay it by 31 January 2027, even if the return itself follows later.
After registration: the obligations that follow
Registration starts a cycle that repeats each year until you tell HMRC you no longer need to file. The key points:
- Filing. Missing the filing deadline costs an initial £100. After 3 months, daily penalties of £10 apply, up to £900. Further penalties of 5% of the tax or £300, whichever is greater, follow at 6 and 12 months. The initial £100 is a fixed amount, whatever tax is due;
- Payments on account. Once your bill passes the de minimis tests, you will be asked for advance payments towards the next year. Our guide to payments on account explains how they are calculated and how to reduce them;
- Making Tax Digital. Sole traders and landlords whose qualifying income exceeds the thresholds must keep digital records and send quarterly updates. You must be registered for Self Assessment before you can sign up;
- Stopping. If your circumstances change, tell HMRC as early as possible. A return already issued must otherwise be filed, and HMRC needs time to withdraw it before the 31 January deadline.
Common mistakes
The errors we see most are timing errors. One is waiting for the UTR before working out the bill. The money is due on 31 January regardless, so estimate it from your records. Another is assuming a side income is too small to matter without checking the £1,000 gross test. A third is treating registration as a one-off. A UTR from years ago does not mean you are still "in" Self Assessment, and a dormant record usually needs reactivating.
Acumon's Self Assessment tax return service handles registration, reactivation and filing, including late-registration cases where the priority is getting the 31 January payment right. For a new business, our accountants for sole traders can set up the records from day one.