If you're self-employed in the UK, you must submit a Self Assessment tax return every year to HMRC.
Missing the deadline is more common than you might think — but it can quickly lead to automatic penalties and additional charges.
Here's what actually happens if you don't file your tax return on time.
The Self Assessment Deadline
For most self-employed people, the key deadline is:
31 January
This is the deadline to:
- submit your online tax return
- pay any tax you owe
For example:
The 2025–2026 tax year ends on 5 April 2026, and the tax return must be filed by 31 January 2027.
The First Penalty: £100
If you miss the 31 January deadline, HMRC automatically issues a £100 penalty.
This fine applies even if you don't owe any tax.
For example:
- Tax owed: £0
- Late filing: Yes
- Penalty: £100
After 3 Months: Daily Penalties
If your tax return is more than 3 months late, additional penalties begin.
HMRC may charge:
- £10 per day
- up to 90 days
This means the maximum additional penalty can reach:
£900
After 6 Months: Further Penalties
If the return is still not submitted after 6 months, another penalty applies.
This is the greater of:
- £300, or
- 5% of the tax owed
After 12 Months: Even Higher Penalties
If the return remains outstanding after 12 months, another penalty can be charged.
Again, this is the greater of:
- £300, or
- 5% of the tax owed
In serious cases, HMRC can charge even higher penalties if they believe information was deliberately withheld.
Interest on Unpaid Tax
If you also owe tax, HMRC will charge interest on the unpaid amount from the original deadline until the balance is paid.
This means the longer you wait, the more the total amount increases.
What If You Can't Pay?
Even if you cannot pay your tax bill immediately, it is still important to submit your tax return on time.
Filing the return avoids the late filing penalties.
If you cannot pay, you may be able to arrange a Time to Pay plan with HMRC, allowing you to spread payments over several months.
The Bottom Line
Missing the Self Assessment deadline can become expensive very quickly. A single late return can lead to hundreds or even thousands of pounds in penalties.
Submitting your tax return on time — and keeping accurate records throughout the year — helps avoid unnecessary stress and extra costs.