You missed the Self Assessment deadline. The 31 January deadline has come and gone, and you haven’t filed your tax return. Take a breath — this is fixable, and you’re not alone. Hundreds of thousands of people miss the deadline every year.
The most important thing: file your return now. Not tomorrow, not next week — now. Every day you delay, the penalties increase. Here’s exactly what happens, what it costs, and what to do.
What Happens When You Miss the Deadline
According to HMRC’s guidance on Self Assessment deadlines, the 31 January deadline is for both filing your return and paying your tax bill. Missing it triggers two separate consequences:
- Late filing penalties — automatic, escalating fines for not submitting your return
- Late payment interest — interest charged on any unpaid tax from 1 February
These are independent. You can be penalised for late filing even if you owe no tax. And you can be charged interest on late payment even if you filed on time. Let’s break them down.
The Late Filing Penalty Scale
The penalty structure for late Self Assessment filing is set out in HMRC’s penalty guidance:
| When | Penalty |
|---|---|
| 1 day late | £100 (automatic, even if you owe no tax or are due a refund) |
| 3 months late | £10 per day (up to 90 days = maximum £900) |
| 6 months late | £300 or 5% of the tax due (whichever is higher) |
| 12 months late | £300 or 5% of the tax due (whichever is higher) — plus further penalties for deliberate failure |
So if you’re 12 months late, you could face:
- £100 initial penalty
- Up to £900 in daily penalties
- £300 or 5% of tax due (6-month penalty)
- £300 or 5% of tax due (12-month penalty)
That’s a minimum of £1,600 in penalties, before any tax or interest you owe.
The £100 penalty applies even if you owe nothing
This catches people off guard. The initial £100 late filing penalty is automatic — it applies even if:
- You owe £0 in tax
- You’re due a refund
- You overpaid and HMRC owes you money
The penalty is for failing to file, not for owing tax. As one Reddit user discovered: “I was due a refund but still got the £100 penalty because I didn’t file on time.”
Late Payment Interest and Penalties
Separately from filing penalties, if you owe tax and haven’t paid by 31 January, HMRC charges interest on the unpaid amount.
The late payment interest rate is currently 7.25% (as of early 2025 — check HMRC’s current rate). Interest is calculated daily from 1 February until the date you pay.
Additionally, there are late payment penalties:
| When | Penalty |
|---|---|
| 30 days late | 5% of the tax unpaid |
| 6 months late | 5% of the tax unpaid (additional) |
| 12 months late | 5% of the tax unpaid (additional) |
So if you owe £5,000 in tax and haven’t paid after 12 months:
- Interest: £5,000 × 7.25% = £362.50 (approximately, for one year)
- Late payment penalties: 5% × 3 = £750
- Late filing penalties: £1,600 (minimum)
- Total: £7,712.50 — on a £5,000 tax bill
This is why you need to act fast. Check the penalty tables above to estimate your charges.
Step 1: File Your Return Immediately
Don’t wait. The longer you delay, the more it costs. Here’s how to file:
If you already have a Government Gateway account
- Go to HMRC Online
- Log in with your Government Gateway user ID and password
- Navigate to Self Assessment
- Complete and submit your return
The online system will calculate your tax automatically. You’ll see your tax bill immediately after submission.
If you don’t have an account yet
- Go to HMRC’s registration page
- You’ll need your National Insurance number
- HMRC will send you a UTR (Unique Taxpayer Reference) by post — this takes up to 10 working days
- Once you have your UTR, set up your Government Gateway account and file
Problem: If you missed the deadline because you didn’t register in time, you’re in a tough spot — HMRC posts the UTR and can’t expedite it. File as soon as you receive it. See our guide on what to do if you’ve lost your UTR number for help tracking it down.
If you can’t file online
You can file on paper, but the paper deadline is 31 October — three months earlier than the online deadline. If you’re reading this after 31 January, you’re already past the paper deadline too. You must file online.
Step 2: Pay Your Tax Bill
Once you’ve filed, you’ll know exactly how much you owe. Pay it as soon as possible to stop the interest clock. According to HMRC’s payment guidance:
- Online by debit card, corporate credit card, or bank transfer — fastest method
- By bank transfer (Faster Payments, CHAPS, or Bacs) — use your UTR as the payment reference
- By cheque — slower, and interest continues until the cheque clears
If you can’t afford to pay the full bill, file anyway. The late filing penalty is separate from your tax bill, and you can arrange a payment plan with HMRC. See our guide on what to do if you can’t afford your tax bill and our HMRC Time to Pay arrangement guide.
Step 3: Appeal the Penalty (If You Have a Reasonable Excuse)
HMRC may cancel late filing penalties if you have a reasonable excuse. According to HMRC’s reasonable excuse guidance, valid reasons include:
- Serious illness — hospitalisation, serious injury, or mental health crisis that prevented you from filing
- Bereavement — death of a close relative shortly before the deadline
- HMRC system failure — if HMRC’s online system was down on the deadline date
- Postal delay — if you filed on paper and it was delayed by Royal Mail (you need proof of posting)
- Fire, flood, or theft — events beyond your control that prevented filing
What does not count as a reasonable excuse:
- “I forgot” — not valid
- “I was too busy” — not valid
- “I didn’t know the deadline” — not valid
- “My accountant was on holiday” — generally not valid
- “I found the form too complicated” — not valid
You must file the return first, then appeal the penalty. HMRC won’t cancel a penalty for a return that still hasn’t been filed. See our full guide on how to appeal an HMRC penalty for the step-by-step process.
What If You Owe No Tax?
Many people who miss the deadline actually owe no tax — or are due a refund. This happens when:
- You had employment income with tax deducted via PAYE, and your side income was below the £1,000 trading allowance
- You overpaid tax during the year and are due a refund
- Your expenses exceeded your income (a loss)
You still get the £100 late filing penalty. The penalty is for not filing, not for owing tax. However, if you’re due a refund, HMRC will usually offset the penalty against the refund. So if you’re due a £300 refund, the £100 penalty reduces it to £200.
File anyway. Even if you owe nothing, filing protects you from the escalating penalties (daily £10 charges after 3 months, etc.) and allows you to claim your refund.
What If You’ve Already Filed But Paid Late?
If you filed on time but couldn’t pay your tax bill by 31 January, you won’t face late filing penalties — but you will face:
- Interest on the unpaid tax from 1 February
- Late payment penalties (5% at 30 days, 6 months, and 12 months)
In this case, pay as much as you can now, and consider a Time to Pay arrangement for the remainder. See our guide on filing late and what happens for the full breakdown of late payment penalties.
Common Mistakes People Make After Missing the Deadline
1. Not filing at all
The biggest mistake is assuming that because you missed the deadline, there’s no point filing. Wrong. The penalties escalate the longer you leave it. File now — even if it’s months late — to stop the daily penalties from accumulating.
2. Filing but not paying
Filing stops the late filing penalties, but late payment interest and penalties continue until you pay. If you can’t afford the full amount, contact HMRC about a payment plan. Ignoring the bill makes it worse.
3. Not appealing when you have a genuine excuse
If you had a serious illness or bereavement, HMRC may cancel your penalty. Many people don’t know this and just pay. See our appeal guide for how to do this.
4. Waiting for HMRC to contact you
HMRC will eventually send you a penalty notice, but by the time it arrives, you may already be into the daily penalty period (£10/day after 3 months). Don’t wait for the letter — file proactively.
How to Avoid Missing the Deadline Next Year
Once you’ve dealt with the immediate crisis, set up systems so this doesn’t happen again:
- File early — you can file as soon as the tax year ends on 5 April. There’s no reason to wait until January.
- Set a reminder — put a recurring calendar alert for 1 December (gives you 2 months buffer)
- Keep records throughout the year — don’t leave it all to January. See our receipt sorting guide for a practical system.
- Consider using an accountant or filing service — if you find the process stressful, a service like TaxScouts or Crunch (use code CostanAQk) can handle it for you. See our Get Help pillar for comparisons.
- Prepare for Making Tax Digital — from April 2026, you’ll need to keep digital records and submit quarterly updates. See our MTD for income tax guide to understand what’s changing.
The Bottom Line
Missing the Self Assessment deadline is stressful but fixable. The key actions:
- File your return now — stop the penalties from escalating
- Pay your tax bill — or set up a payment plan if you can’t afford it
- Appeal if you have a reasonable excuse — HMRC may cancel the penalty
- Set up systems to avoid missing it next year — file early, keep records, set reminders
The £100 initial penalty is annoying but manageable. The real damage comes from the daily penalties (£10/day after 3 months) and the 6-month and 12-month penalties. File today to stop the clock.
For the full breakdown of late filing and late payment penalties, see our guide on what happens when you file late. If you can’t afford your tax bill, see what to do when you can’t afford your HMRC tax bill. If you want to appeal a penalty, see how to appeal an HMRC penalty.