Tell HMRC by 5 October if you had untaxed income last year

Most people have heard of 31 January. Far fewer have heard of 5 October, which comes first and catches a different group.

Two things have to be true. If you received more than £2,500 of untaxed income in the 2025 to 2026 tax year, and you have not sent a tax return before, you must tell HM Revenue and Customs (HMRC) by 5 October 2026. The same applies if you registered in the past but did not need to send a return for 2024 to 2025. Untaxed income here means things like rent, tips, commission, savings interest, dividends and foreign income. Telling HMRC is registering. It is not filing, and filing is not due until 31 January 2027.

What you have to do	When
Tell HMRC you need a return for 2025 to 2026	5 October 2026
File the return online and pay the tax	31 January 2027

Untaxed income is only one reason to be in this group. HMRC’s standing list names five more. Anyone self-employed as a sole trader who earned more than £1,000 before taking off expenses. Anyone who was a partner in a business partnership. Anyone who owes Capital Gains Tax. Anyone paying the High Income Child Benefit Charge who does not pay it through PAYE. And any off-payroll worker repaying a student or postgraduate loan. The £2,500 line comes from HMRC’s 9 September announcement; the standing guidance on who must send a return lists untaxed income without putting a figure on it.

It is a bigger group than it sounds. More than 640,000 people registered for Self Assessment in the twelve months to 31 March 2026.

The catch is what “penalty” means here. Missing 5 October is not a flat fine of the kind charged for filing late. It is a failure to notify penalty, and HMRC sets it as a percentage of the tax you should have paid — what it calls the potential lost revenue.

For a non-deliberate failure that percentage turns on two things, not one. Whether you came forward or HMRC had to prompt you. And whether that happened within twelve months of the tax being due. Coming forward inside twelve months, the range is 0% to 30%. Coming forward later, 10% to 30%. Being prompted inside twelve months, 10% to 30%. Being prompted later, 20% to 30%. Deliberate failures top out at 70%, and deliberate and concealed ones at 100%.

Two things follow. If you owe no tax, a percentage of nothing is nothing. And the gap between telling HMRC yourself and waiting to be asked is real money. Come forward inside the twelve months and the band starts at zero. A prompt in that same window starts at 10%.

Registering also got faster this month. HMRC launched a rebuilt registration service on 9 September 2026. Forms arrive part-filled with what HMRC already holds, and you can save your progress and come back to it. A Unique Taxpayer Reference, the ten-digit number you cannot file without, now appears in your online account within 72 hours. By post the wait was up to 15 days. At this time of year that matters, because a reference that takes a fortnight is a fortnight of the window gone.

The thing people get wrong: registering does not commit you to owing anything. If the £1,000 trading allowance covers what you earned on the side, you may not need to register at all. The check costs nothing. The penalty does.

Where these figures come from

• Improved Self Assessment registration service launched — HM Revenue and Customs
• Self Assessment tax returns: Who must send a tax return — GOV.UK
• Compliance checks: penalties for failure to notify — CC/FS11 — HM Revenue and Customs

Every figure traces to a dated document — the line this section is named after. If one here is wrong, tell us and it gets fixed publicly. Work out your own figures.