Missed the 5 October Self Assessment Deadline? What Happens Next

If you need to file a UK Self Assessment tax return for the 2025 to 2026 tax year and have never filed one before, or did not have to file for 2024 to 2025, you must tell HMRC by registering by 5 October 2026. If you miss that date, register anyway, as soon as you can: if you still pay all the tax you owe by 31 January 2027, GOV.UK says the late registration penalty, known as a "failure to notify" penalty, should not apply.

The real risk of registering late is not the penalty itself. It is the time you lose waiting for your Unique Taxpayer Reference (UTR) with the 31 January payment date still fixed.

Who has to register for Self Assessment by 5 October?

GOV.UK says you must send a tax return if, in the last tax year (6 April 2025 to 5 April 2026), any of these applied:

• you were self-employed as a sole trader and earned more than £1,000 (before taking off anything you can claim tax relief on)
• you were a partner in a business partnership
• you had to pay Capital Gains Tax when you sold or disposed of something that increased in value
• you had to pay the High Income Child Benefit Charge and do not pay it through PAYE (the threshold is income over £60,000 from 2024 to 2025)
• you are an off-payroll worker repaying a student or postgraduate loan

You may also need a return for untaxed income, such as money from renting out property, tips and commission, savings interest, dividends or foreign income. If you are not sure, GOV.UK has a "Check if you need to send a tax return" tool.

The registration deadline applies to two groups. In GOV.UK's words, you must tell HMRC by 5 October if you have either "not sent a tax return before" or "registered before but did not need to send a tax return for the tax year 2024 to 2025". If you are in the second group, you may need to reactivate your existing account rather than register again, and GOV.UK warns: "Your tax return may be delayed if you file it without reactivating an existing account."

What happens if I miss the 5 October deadline?

Three things, according to GOV.UK.

You get a different filing deadline. "HMRC will send you a letter or email with a different deadline to send your tax return by - this will be 3 months from the date on the letter or email."

The payment deadline does not move. "You must still pay the tax you owe by 11:59pm on 31 January 2027 or you'll get a penalty."

A penalty is possible, but only in one case. "If you register after 5 October and do not pay all of your tax bill by 31 January, you may get a 'failure to notify' penalty. This penalty is based on the amount still left to pay."

So the outcome you are aiming for after a late registration is straightforward: register now, get your UTR, work out the bill, and have it all paid by 31 January 2027.

How much is the failure to notify penalty?

HMRC's factsheet CC/FS11 explains that the penalty is a percentage of the "potential lost revenue", which for a late registration is the tax still unpaid. The percentage depends on whether the failure was deliberate and whether you told HMRC before they found it.

Failure was not deliberate, and you told HMRC...	Penalty range
Unprompted, within 12 months of the tax being due	0% to 30%
Unprompted, 12 months or more after	10% to 30%
Prompted (HMRC found it), within 12 months	10% to 30%
Prompted, 12 months or more after	20% to 30%

Deliberate failures carry higher ranges: up to 70%, or 100% if concealed. The factsheet also says HMRC will not charge a penalty at all if you had a reasonable excuse, the failure was not deliberate, and you "told us without unreasonable delay after your reasonable excuse ended."

How long does it take to get a UTR?

GOV.UK says: "You'll usually get your UTR by post around 15 days after you register. It takes longer if you live overseas." The UTR is a 10-digit number, sometimes just called your tax reference. You need it before you can file.

If you are registering in October, that puts your UTR some time in late October or November, which still leaves time to file online and pay before 31 January. If you register in January, it does not.

What are the other Self Assessment deadlines for 2025 to 2026?

Date	Deadline
5 October 2026	Tell HMRC you need to file (register)
31 October 2026	Paper tax return must reach HMRC
30 December 2026	Online return, if you want the bill collected through your tax code
31 January 2027	Online return, and payment of the tax you owe
31 July 2027	Second payment on account, if you make them

All of these are 11:59pm on the day. If the 31 July date is news to you, our post on why your January tax bill is bigger than you calculated explains payments on account.

What are the penalties for filing or paying late?

From GOV.UK:

• Late return: an initial £100 penalty; after 3 months, £10 a day up to £900; after 6 months, a further 5% of the tax due or £300, whichever is greater; after 12 months, another 5% or £300.
• Late payment: 5% of the tax unpaid at 30 days, at 6 months and at 12 months, plus interest.

Missing the registration date is the least expensive mistake on this list, as long as it does not turn into a late return or a late payment.

When should I set reminders?

If you registered late, or are about to register for the first time:

1. Today. Register, or reactivate your account.
2. About 15 days later. Check the post for your UTR. If nothing has arrived, use GOV.UK's "check when you can expect a reply from HMRC" tool.
3. The day your letter arrives. Note the filing deadline it gives you, if it is different.
4. Early January. File online, so you know the bill with time to pay it.
5. A few days before 31 January. Pay, leaving room for a bank transfer to clear.

How ReminderIt can help

ReminderIt places a real phone call at the exact time you choose and reads your reminder in a natural voice, for example "File your tax return this weekend. Payment due 31 January." If you miss the call, you get a WhatsApp message.

You can set it by sending a WhatsApp message (text, voice note or photo of your HMRC letter), such as "Call me on 10 January at 7pm to file my tax return", or on the website. There is no app to install. You can also set reminders for a family member, such as a parent who files a return for rental income; they confirm first.

There is a 7-day free trial with 7 free reminder calls (3 if your number is in a country where calls cost more). After that it is one plan: $7 a month, or ₹299 a month in India, with annual and 2-year options.

The short answer

You must register for Self Assessment by 5 October 2026 if you need to file for 2025 to 2026 and have not filed before, or did not file for 2024 to 2025. If you miss it, register straight away: HMRC will give you three months from its letter to file, the tax is still due by 31 January 2027, and a failure to notify penalty applies only if tax is left unpaid after that. Allow around 15 days for your UTR.

This is general information, not tax advice; check GOV.UK or an accountant for your own situation.

Sources: GOV.UK, Register for Self Assessment; GOV.UK, Self Assessment tax returns (Who must send a tax return; Deadlines; Penalties); GOV.UK, Find your UTR number; GOV.UK, High Income Child Benefit Charge; HMRC, Compliance checks: penalties for failure to notify - CC/FS11 (updated March 2026).