What happens if I miss the Self Assessment deadline?

Every year, millions of people in the UK must file a Self Assessment tax return with HMRC to declare income outside regular employment. The key deadlines are 31 October for paper returns and 31 January for online submissions. The payment for any tax owed is also due by 31 January.

If you miss either the filing or payment deadline, HMRC automatically issues penalties and may charge interest on the unpaid amount. Acting quickly can reduce costs and prevent further problems.

The immediate £100 late filing penalty

If you miss the 31 January deadline, HMRC will issue an automatic £100 fixed penalty, even if you have no tax to pay or have already paid what you owe. This penalty applies as soon as the deadline passes, so it is best to file as soon as possible to avoid additional charges.

Penalties after three months

If your return is more than three months late, HMRC adds daily penalties of £10 per day, up to a maximum of 90 days (£900 in total). This means that by the time your return is three months overdue, you could owe £1,000 in penalties just for filing late.

If your return is six months late, HMRC can charge an additional 5% of the tax due or £300, whichever is higher.

If it is more than 12 months late, you may face another 5% or £300 penalty, and in serious cases where HMRC believes you deliberately withheld information, penalties can reach up to 100% of the tax owed.

Penalties for late payment

If you file your return on time but fail to pay your tax bill by 31 January, you will not receive the £100 late filing fine, but HMRC will still charge penalties for late payment. These are:

• 5% of the unpaid tax after 30 days.

• Another 5% after six months.

• A further 5% after 12 months.

Interest also accrues daily from 1 February until the balance is paid in full. The rate is variable but usually set a few percentage points above the Bank of England base rate.

Example of missed deadline penalties

Imagine you owed £2,000 in tax for the 2023 to 2024 tax year but filed your return five months late and paid your bill six months late.

You would face:

• £100 fixed penalty for late filing.

• £10 daily penalties for 90 days (£900).

• £300 penalty after six months (the higher of 5% or £300).

• 5% late payment penalty (£100).

• Plus interest on the unpaid £2,000.

In total, you could owe more than £3,400 once all penalties and interest are included.

Reasonable excuse for late filing or payment

HMRC recognises that sometimes genuine circumstances prevent taxpayers from meeting deadlines. You can appeal a penalty if you have a reasonable excuse, such as:

• Serious illness or hospitalisation around the deadline.

• A close family bereavement.

• HMRC system failures that prevented submission.

• Unexpected postal delays for paper returns.

• Fire, flood, or theft affecting business records.

Each case is judged individually, and you must file your return as soon as possible once the problem is resolved. Forgetfulness or being too busy is not considered a valid excuse.

If your appeal is accepted, HMRC may cancel or reduce the penalty. However, interest on late payments still applies.

What to do if you have missed the deadline

If you have missed the Self Assessment deadline, it is important to act quickly.

1. File your return immediately – The sooner you file, the sooner penalties stop increasing.

2. Pay what you owe – If you cannot pay in full, make a part payment to reduce interest.

3. Contact HMRC – You may be able to set up a Time to Pay arrangement to spread payments over several months.

4. Appeal any penalties – If you have a valid reason, submit an appeal using HMRC’s online service or by post within 30 days of receiving the penalty notice.

5. Keep future records organised – Make sure you have accurate financial records for next year to avoid the same issue again.

How accountants help if you have missed the deadline

An accountant can make the process of catching up and appealing penalties much smoother. They can:

• Prepare and file overdue tax returns accurately and quickly.

• Calculate your outstanding tax and penalties.

• Negotiate payment plans with HMRC on your behalf.

• Help you submit an appeal if you have a reasonable excuse.

• Ensure your records are compliant for future tax years.

They also help identify any overpaid tax or allowable expenses that could reduce your total bill.

Preventing future problems

Once you are back on track, you can avoid future missed deadlines by:

• Setting reminders for tax dates.

• Using cloud accounting software to track income and expenses.

• Filing your return well before 31 January.

• Working with an accountant to monitor your tax obligations year round.

HMRC also offers an online personal tax account where you can view deadlines, payments, and correspondence at any time.

Example in practice

A self employed tradesperson missed the January deadline by four months and owed £5,000 in tax. With help from an accountant, they filed their return, appealed the daily penalties due to a medical emergency, and arranged a payment plan through HMRC’s Time to Pay service. Their penalty was reduced, and they avoided further interest by paying monthly instalments.

This proactive approach helped them protect their credit rating and stay compliant going forward.

Conclusion

Missing the Self Assessment deadline leads to automatic penalties and interest, but quick action can limit the damage. File as soon as possible, pay what you can, and contact HMRC if you need help spreading payments.

If you have a valid reason for missing the deadline, you can appeal to have penalties reduced or cancelled. Working with an accountant ensures your tax affairs are handled professionally, helping you avoid future fines and maintain good standing with HMRC.