What to Do If You’ve Missed the Self Assessment Deadline

Calendar showing a missed deadline, representing a late Self Assessment tax return

The 31 January deadline for Self Assessment is one of the most important dates in the tax calendar — but life happens, and sometimes it gets missed. If you’ve not filed or paid on time, the most important thing is to act quickly. The longer you leave it, the more expensive it becomes.

What Happens Immediately After the Deadline

HMRC imposes an automatic £100 penalty the moment your return is late — even if you owe no tax at all. The full penalty structure is detailed on HMRC’s Self Assessment penalties page. This applies from day one of being late, with no grace period.

After that, the penalties escalate the longer the return remains unfiled:

After 3 months — additional £10 per day, up to a maximum of £900
After 6 months — a further penalty of 5% of the tax owed (or £300 if greater)
After 12 months — another 5% of the tax owed (or £300 if greater)

On top of filing penalties, if any tax is unpaid after 31 January, interest accrues on the outstanding amount — currently at 7.75% per annum. Additional surcharges of 5% apply on unpaid tax at 30 days, 6 months, and 12 months after the deadline.

Step 1: File Your Return as Soon as Possible

The first thing to do is file your return — right now. The sooner you submit, the sooner the daily penalties stop accumulating. Don’t wait until you’ve gathered every last receipt; file with the best information you have and amend it later if needed. HMRC allows amendments to submitted returns.

Step 2: Pay What You Owe

Once you know what you owe, pay it as quickly as possible. Interest accrues daily on unpaid tax, so even a partial payment reduces the amount interest is charged on. If you cannot pay the full amount, pay what you can immediately.

Step 3: If You Can’t Pay in Full, Contact HMRC

If you’re unable to pay your tax bill in full, contact HMRC as soon as possible to discuss a Time to Pay arrangement. This allows you to spread the payment over monthly instalments. HMRC is generally willing to agree to these if you contact them proactively — they would rather receive the tax over time than not at all.

You can set up a Time to Pay arrangement online through your HMRC account if you owe less than £30,000, or by calling HMRC’s Payment Support Service for larger amounts.

Can You Appeal the Penalties?

Yes — if you have a reasonable excuse for filing or paying late, you can appeal to HMRC to have the penalties cancelled. HMRC accepts genuine emergencies, serious illness, bereavement, and other circumstances outside your control. However, simply forgetting, being too busy, or not knowing about the deadline are unlikely to be accepted.

Appeals must be made promptly and in writing. See our guide to Self Assessment penalties for more detail on what HMRC will and won’t accept.

How to Avoid It Happening Again

The best way to avoid future late filing is to prepare your return well before the deadline — not in the last few days of January. Good bookkeeping throughout the year means your return can be prepared quickly once the tax year ends in April.

Many sole traders who use a bookkeeper find their returns are submitted by October or November, giving them over two months’ notice of their tax bill before it’s due — and plenty of time to have the funds ready.

Need Help Sorting This Out?

PBAS can help you get your return filed quickly and accurately, even if it’s already late. We can also advise on penalty appeals if you believe you have a reasonable excuse. Get in touch as soon as possible — every day counts.

Contact PBAS Now

Need Help with This?

PBAS provides affordable bookkeeping and accountancy services for sole traders and small businesses across East Lothian, Edinburgh, Midlothian and throughout Scotland. If you’d like a hand with your accounts, self assessment or any tax matter, get in touch for a free, no-obligation chat.

This article provides general guidance only. For specific advice on your circumstances, consult HMRC directly via the GOV.UK Self Assessment deadlines or speak to a qualified bookkeeper.