What if I miss the 31 January deadline? Penalty Breakdown.

The tax deadline is around the corner… 31 January. But what happens if the clock strikes midnight and your Self Assessment is still sitting in your "to-do" pile? Many assume the damage is limited to a simple £100 fine. If you bury your head in the sand, that initial £100 can snowball into a debt of over £1,600 in penalties. HMRC’s penalty regime is designed to be progressive; the longer you delay, the more severe the financial punishment becomes. Here’s how it works.

1. Late Filing Penalties: The Clock is Ticking

The immediate sting comes from failing to submit your return. It is crucial to understand that filing and paying are two separate issues in the eyes of HMRC. Even if you owe nothing at all, you will still be fined for submitting the tax return late. Here is the timeline of escalating costs for the 2024/25 tax year return (due 31 Jan 2026):

• 1 Day Late: An immediate £100 fixed penalty. This applies automatically, even if you have no tax to pay or have already paid the tax due.
• 3 Months Late: If you still haven’t filed by 1 May, daily penalties kick in. You will be charged £10 per day for up to 90 days. This can add a further £900 to your bill.
• 6 Months Late: If your return is still outstanding, you will be hit with a further penalty of £300 or 5% of the tax due, whichever is greater.
• 12 Months Late: Another £300 or 5% of the tax due (whichever is greater) is added on top. In total you could owe over £1,600 in penalties for late filing.

2. Late Payment Penalties: The "Double Whammy"

If you have not filed, you are likely late to pay too. HMRC charges separate penalties for late payment, which run alongside the filing penalties mentioned above.

• 30 Days Late: If you haven’t paid by early March (30 days after the deadline), a penalty of 5% of the tax due is charged.
• 6 Months Late: An additional 5% penalty is charged on any tax still unpaid.
• 12 Months Late: A final 5% penalty is charged on any remaining unpaid tax.

Interest accrues from 1 February…

As of January 2026, the late payment interest rate sits at a painful 7.75%. This interest is charged daily too.

Conclusion

Missing the 31 January deadline is an expensive mistake to make. If you need to use provisional figures, it’s worth doing to get your tax return filed, it can always be updated in the future. If you have genuine mitigating circumstances, your penalties can be appealed - but don’t rely on HMRC cancelling your penalty in the future. If you can’t pay HMRC, you can also negotiate a "Time to Pay" arrangement with HMRC to spread the cost of the tax bill itself.

Photo by Morgan Housel on Unsplash