Missed the 5 October Self Assessment Deadline? What Happens Now
If you started as a sole trader in 2025/26 and did not register by 5 October 2026, you are late, but this is usually easy to put right. What matters most is whether the tax is paid by 31 January 2027.
Written and reviewed by James Whitfield · Last reviewed 3 November 2026 · Checked against 2026/27 HMRC rates · Editorial standards · Methodology
Register for Self Assessment now. HMRC will send you a letter or email with a filing deadline 3 months from the date on it, but the 2025/26 tax is still due by 11:59pm on 31 January 2027.1 GOV.UK says a failure to notify penalty may apply if you register after 5 October and do not pay all your tax by 31 January; it is based on the amount still left to pay.2 In the law, that amount is the tax unpaid on 31 January following the tax year, so if you pay it all by 31 January 2027 there is nothing to base a penalty on.3
- →Deadline for 2025/26: tell HMRC by 5 October 2026 if you had not sent a return before.4
- →Register late: HMRC sets a filing deadline 3 months from the date of its letter or email.1
- →Pay by 31 January 2027 even if your filing deadline is later.1
- →Penalty, if any, is a percentage of the tax unpaid on 31 January 2027: 0% to 30% if it was not deliberate and you told HMRC within 12 months.35
- →No penalty if you had a reasonable excuse, the failure wasn't deliberate and you told HMRC without unreasonable delay.5
Related: How to register as a sole trader · Your 2025/26 return: what's due 31 January 2027 · Self Assessment penalty calculator
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Who had to register by 5 October 2026
You had to tell HMRC by 5 October 2026 if you need to send a return for 2025/26 (6 April 2025 to 5 April 2026) and you had either never sent a return before, or registered before but did not need to send one for 2024/25.4 A sole trader must send a return if they earned more than £1,000 from self-employment, before taking off expenses.6
If your income was £1,000 or less, the trading allowance means you usually do not need to tell HMRC at all.9 If you have filed returns every year, you did not need to register again.
What happens when you register after 5 October
Register online the same way as anyone else. HMRC will send you a letter or email with a different deadline for your return: 3 months from the date on it.1 That replaces the 31 January 2027 filing deadline for you, so you are not charged the £100 late filing penalty for using the time HMRC gives you.
The payment deadline does not move. You must still pay the tax you owe for 2025/26 by 11:59pm on 31 January 2027 or you will get a late payment penalty.1 If your bill is £1,000 or more, the first payment on account for 2026/27 is due the same day.7 Work out the bill now so the money is ready, even if the return comes later.
How the failure to notify penalty works
The penalty is a percentage of what HMRC calls potential lost revenue.5 For Income Tax, Finance Act 2008 Schedule 41 defines that as the tax for the year that, because of the failure, is unpaid on 31 January following the tax year.3 For 2025/26 that date is 31 January 2027. HMRC's factsheet CC/FS11 sets out the ranges:
| Not deliberate, you told HMRC within 12 months of the tax being due | 0% to 30% |
| Not deliberate, you told HMRC 12 months or more after | 10% to 30% |
| Not deliberate, HMRC found it first (within 12 months / later) | 10% to 30% / 20% to 30% |
| Deliberate (you told HMRC / HMRC found it) | 20% to 70% / 35% to 70% |
| Deliberate and concealed (you told HMRC / HMRC found it) | 30% to 100% / 50% to 100% |
Telling HMRC yourself, before you have any reason to think it is about to find out, is an unprompted disclosure, and the minimum penalty is lower.5 Registering as soon as you realise is the strongest position you can be in.
Worked example: registered in November 2026
Ali started as a self-employed gardener in June 2025, made £20,000 profit in 2025/26 and registered on 20 November 2026. With no other income, Ali's 2025/26 bill is £1,486.00 Income Tax plus £445.80 Class 4 National Insurance: £1,931.80.
| Ali pays all 2025/26 tax by 31 January 2027 | Tax unpaid on 31 January: £0 |
| Failure to notify penalty (a percentage of £0) | £0.00 |
| Also due 31 January 2027: first payment on account for 2026/27 (half of £1,931.80) | £965.90 |
| If Ali pays only £1,000 by 31 January 2027 | Unpaid: £931.80 |
| Possible failure to notify penalty (0% to 30%, unprompted, within 12 months) | £0 to £279.54 |
If you had a reasonable excuse
HMRC will not charge a failure to notify penalty if you had a reasonable excuse, the failure was not deliberate and you told HMRC without unreasonable delay after the excuse ended.5 A reasonable excuse is something that stopped you meeting the obligation even though you took reasonable care, often circumstances outside your control. If health or personal circumstances made it hard to tell HMRC, say so: HMRC asks you to tell the officer so it can be taken into account.5
If you are charged a penalty you disagree with, you can appeal. You usually have 30 days.10
Missed it by years, not months?
The same rules apply to every year you should have told HMRC about, but the lowest penalty rises to 10% once you tell HMRC 12 months or more after the tax was due, and late payment penalties and interest build up on each year.258 Register, work out the tax for each year, and tell HMRC about all of them at once. Disclosing everything yourself, and helping, keeps the percentage at the low end of the range.5
Frequently asked questions
What happens if I miss the 5 October deadline to register for Self Assessment?+
Register as soon as you can. HMRC will give you a filing deadline 3 months from the date of its letter or email. You must still pay your 2025/26 tax by 31 January 2027. If you do, there is no tax unpaid on 31 January for a failure to notify penalty to be based on.
Is there a fixed fine for registering late for Self Assessment?+
No. The failure to notify penalty is a percentage of the tax left unpaid on 31 January after the tax year, from 0% to 30% if it was not deliberate and you told HMRC within 12 months. Pay the tax on time and it is nothing.
Can I still file my 2025/26 tax return by 31 January 2027 if I register late?+
Yes, if you have your UTR in time, and filing early lets you check the bill. If HMRC's letter gives you a later filing deadline, you can use that, but the tax is still due 31 January 2027.
I earned less than £1,000 from self-employment. Did I need to register?+
Usually not. With gross self-employment income of £1,000 or less, the trading allowance means you normally don't need to tell HMRC. Above £1,000 you must register by 5 October after the tax year.
Do I pay a payment on account if I register late?+
The same rules apply as for anyone else. If your 2025/26 bill is £1,000 or more and less than 80% was taxed at source, the first payment on account for 2026/27 is due on 31 January 2027 and the second on 31 July 2027.
Sources & references
The rates, thresholds and rules in this guide are drawn from the official HMRC and GOV.UK sources below, using the confirmed 2026/27 figures. Each link opens the relevant official page in a new tab.
- Self Assessment tax returns: deadlines www.gov.uk/self-assessment-tax-returns/deadlines
- Self Assessment tax returns: penalties www.gov.uk/self-assessment-tax-returns/penalties
- Finance Act 2008, Schedule 41: penalties for failure to notify www.legislation.gov.uk/ukpga/2008/9/schedule/41
- Register for Self Assessment www.gov.uk/register-for-self-assessment
- HMRC factsheet CC/FS11: penalties for failure to notify www.gov.uk/government/publications/compliance-checks-penalties-for-failure-to-notify-ccfs11/compliance-checks-penalties-for-failure-to-notify-ccfs11
- Self Assessment tax returns: who must send a tax return www.gov.uk/self-assessment-tax-returns/who-must-send-a-tax-return
- Understand your Self Assessment tax bill: payments on account www.gov.uk/understand-self-assessment-bill/payments-on-account
- HMRC interest rates for late and early payments www.gov.uk/government/publications/rates-and-allowances-hmrc-interest-rates-for-late-and-early-payments/rates-and-allowances-hmrc-interest-rates
- Tax-free allowances on property and trading income www.gov.uk/guidance/tax-free-allowances-on-property-and-trading-income
- Disagree with a tax decision or penalty: penalties www.gov.uk/tax-appeals/penalty
Estimate your sole trader tax
The sole trader tax calculator turns this guidance into a concrete monthly take-home and tax reserve estimate, based on 2026/27 HMRC rates. Enter taxable profit — not turnover.