What Happens If You Miss the Self-Assessment Deadline?

If you miss the self-assessment deadline, HMRC charges an automatic £100 penalty the moment your online return is late, even if you owe no tax, and further penalties and interest build the longer you leave it. The online filing and payment deadline is 31 January following the tax year end, so the 2025/26 return is due by 31 January 2027. Late filing and late payment are penalised separately, so a return that is both filed and paid late attracts two sets of charges. This self-assessment tax return guide, written by Blue Tick Accountants, a Guildford-based UK tax advisory and accountancy practice, explains what a missed deadline costs, how penalties escalate, when you can appeal, and how to limit the damage if you are already late.

Key Takeaways

• Missing the online self-assessment deadline of 31 January triggers an automatic £100 penalty, payable even if you owe no tax or are due a refund.
• After three months late, daily penalties of £10 apply for up to 90 days, adding a maximum of £900 to the initial £100.
• At six months and again at twelve months late, a further penalty of 5% of the tax due or £300, whichever is greater, is charged each time.
• Late payment penalties of 5% of the unpaid tax apply at 30 days, six months, and twelve months, and HMRC charges interest on overdue tax from 1 February.
• A penalty can be cancelled if you have a reasonable excuse, and you have 30 days from the penalty notice to appeal.

How Much Is the Penalty for Filing a Late Tax Return?

The penalty for a late online self-assessment return starts at a fixed £100 and rises in stages the longer the return remains outstanding. The £100 is automatic and applies the day after the 31 January deadline, regardless of whether any tax is due. If the return is still not filed three months later, HMRC adds daily penalties of £10 for up to 90 days, a maximum of £900. At six months late, a further penalty of 5% of the tax due or £300, whichever is greater, is charged, and the same applies again at twelve months. A return left unfiled for a year can attract £1,600 or more in filing penalties before any tax or interest is counted. These charges relate only to filing; paying the tax late is penalised separately, as set out below.

What Are the Penalties for Paying Your Tax Late?

Paying your self-assessment tax after 31 January triggers late payment penalties on top of any filing penalty, plus daily interest. If tax remains unpaid 30 days after the due date, HMRC charges a penalty of 5% of the outstanding amount, with a further 5% at six months and another 5% at twelve months. Interest also accrues on the unpaid tax from 1 February until the balance is cleared, calculated daily at HMRC's published rate. Because filing and payment penalties run in parallel, the combined cost of leaving everything late is significant.

Worked example. A sole trader owes £5,000 for 2025/26 and both files and pays six months late. They face the £100 fixed filing penalty, £900 in daily filing penalties, a six-month filing penalty of £300 (the greater of 5% of the tax and £300), and late payment penalties of £250 at 30 days and a further £250 at six months. That is £1,800 in penalties, plus interest on the £5,000, before the original tax bill is settled.

Can You Appeal a Self-Assessment Penalty?

Yes. You can appeal a self-assessment penalty if you have a reasonable excuse for missing the deadline, and you normally have 30 days from the date of the penalty notice to do so. HMRC accepts a reasonable excuse as something that genuinely stopped you meeting your obligation despite taking reasonable care, such as a serious illness, a bereavement close to the deadline, a fire or flood, or a prolonged failure of HMRC's online service. Excuses HMRC generally rejects include finding the system too difficult, not receiving a reminder, or relying on someone else who let you down.

To appeal, file the outstanding return first, then submit the appeal online or in writing, explaining the circumstances and dates. If the excuse is accepted, the penalty is cancelled. For a full overview of how the return itself works, see our guide to self-assessment tax returns.

What Should You Do If You Have Already Missed the Deadline?

If you have already missed the deadline, file the return and pay what you owe as quickly as possible, because most penalties and all interest keep growing until you act. Filing immediately stops the three-month daily penalties from ever starting and limits the damage to the initial £100. If you cannot pay in full, apply to HMRC for a Time to Pay arrangement, which spreads the bill over monthly instalments and, if agreed early, can prevent some late payment penalties from arising.

Keeping digital records also matters now that Making Tax Digital for Income Tax is live. From 6 April 2026, sole traders and landlords with qualifying income above £50,000 must keep digital records and submit quarterly updates plus a final declaration by 31 January, so staying organised through the year is the surest way to avoid a missed deadline.

A late return also delays the Class 2 and Class 4 figures that build your state pension record, which our guide to National Insurance for the self-employed explains.

Frequently Asked Questions

How much is the fine for a late self-assessment tax return?

The fine for a late online self-assessment return starts at an automatic £100, charged the day after the 31 January deadline even if no tax is owed. After three months, daily penalties of £10 apply for up to 90 days, adding up to £900. At six and twelve months, a further penalty of 5% of the tax due or £300, whichever is greater, is charged each time.

Do I still get a penalty if I do not owe any tax?

Yes. HMRC charges the £100 late filing penalty even if you owe no tax or are due a refund, because the penalty is for failing to file the return on time, not for unpaid tax. The fixed penalty stands regardless of your tax position, so you should always file by 31 January even when your bill is nil.

Can I set up a payment plan if I cannot pay my tax bill?

Yes. HMRC offers Time to Pay arrangements that let you settle a self-assessment bill in monthly instalments rather than a single payment. You can often set one up online if you owe under a set threshold and apply soon after the deadline. Agreeing a plan promptly can stop further late payment penalties, though interest still applies to the outstanding balance.

How long do I have to appeal a self-assessment penalty?

You normally have 30 days from the date on the penalty notice to appeal a self-assessment penalty. To appeal you must first file the outstanding return, then submit your appeal online or in writing, setting out the reasonable excuse and the relevant dates. If HMRC accepts your excuse, the penalty is cancelled in full.

How Blue Tick Can Help

Blue Tick Accountants helps self-employed people across the UK who have missed the self-assessment deadline get back on track quickly, filing the outstanding return, calculating the exact tax and penalty position, and dealing with HMRC on appeals and Time to Pay arrangements. The practice also keeps clients ahead of every future deadline and prepares higher earners for Making Tax Digital. Head to our website and book a meeting now.

Conclusion

A missed self-assessment deadline is rarely a disaster if you act fast. The automatic £100 penalty is fixed, but the daily charges, 5% penalties, and interest only start once the return drifts months past 31 January. File the return, pay or arrange to pay what you owe, and appeal if you have a genuine reasonable excuse. The sooner you move, the smaller the bill.

About the Author

This guide was written by Leon, founder of Blue Tick Accountants, a Guildford-based UK tax advisory and accountancy practice that helps the self-employed, landlords, and limited company owners across the UK. It was last reviewed for the 2026/27 tax year.

This article is intended for general informational purposes only and does not constitute tax, legal, or financial advice. Tax rules are subject to change and their application will depend on your individual circumstances. You should always seek advice from a qualified professional before taking action. Blue Tick Accountants accepts no liability for decisions made on the basis of this content.

Related reading: Self-Assessment Tax Refunds: How to Claim Money Back from HMRC.

Related reading: Self-Assessment for Beginners: Everything You Need Before You Start.