Quick answer
If you miss the Self Assessment deadline, HMRC charges an automatic £100 penalty for filing late, even if you owe no tax. Daily penalties, percentage-based charges and interest on unpaid tax build up the longer you wait, so file as soon as you can.
Updated October 2026 · Figures based on 2026/27 rates
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Late filing penalties
The penalty starts at £100 the day after the 31 January online deadline. After 3 months, HMRC adds £10 a day for up to 90 days (a maximum of £900). At 6 months a further charge applies of 5% of the tax due or £300, whichever is higher, and the same again at 12 months.
Late payment penalties and interest
Tax not paid by 31 January attracts a 5% penalty if still unpaid after 30 days, a further 5% after 6 months and another 5% after 12 months. HMRC also charges interest from the due date until you pay.
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What to do now
File the return first, because that stops the filing penalties growing. Then pay what you owe or ask HMRC about a Time to Pay arrangement. If you had a genuine reason for being late, you can appeal the penalty.
How the penalties build up over time
The charges for a missed deadline come in stages, and each stage is triggered by how long the return stays outstanding. The first, a flat £100, applies the day after the 31 January online deadline. From three months late, HMRC adds £10 a day for up to 90 days, which caps this stage at £900. At six months and again at twelve months, a further charge applies of 5% of the tax due or £300, whichever is higher. Separate late payment penalties then run alongside these, at 5% of the unpaid tax at 30 days, six months and twelve months, with interest on top.
Filing and paying are separate problems
Many people assume that filing on time but paying late is the same as missing the deadline altogether. It is not. Filing late triggers the filing penalties above. Paying late triggers the payment penalties and interest. You can be late on one, the other, or both, and the cost differs accordingly. For the payment side in more detail, read our guide to late payment penalties explained.
Worked example: a return filed seven months late
This is an illustrative example. Priya has a tax bill of £3,000 for the year and does not file or pay until about seven months after the 31 January deadline.
| Charge | Working | Amount |
|---|---|---|
| Initial late filing penalty | Flat charge, day one | £100 |
| Daily penalties | £10 x 90 days (maximum) | £900 |
| Six-month filing charge | 5% of £3,000 = £150, so the £300 minimum applies | £300 |
| Late payment, 30 days | 5% x £3,000 | £150 |
| Late payment, six months | 5% x £3,000 | £150 |
| Total penalties | £1,600 |
That is £1,600 in penalties on a £3,000 tax bill, before interest is added. Had Priya filed and paid in the first week of February, the cost would have been £100 plus a small amount of interest. The earlier you act, the more of the cost you can still avoid.
Common mistakes
- Not filing because you cannot pay. The filing penalties apply regardless, and filing at least stops the daily penalties growing.
- Assuming a nil-tax return does not need filing on time. The £100 penalty still applies.
- Waiting for HMRC to send a reminder. Penalties accrue automatically.
- Giving a vague appeal. A reasonable excuse needs a clear explanation and dates. See our guide on reasonable excuse for a late tax return.
- Assuming an extension exists. In practice there is no general extension; read whether you can get a deadline extension.
Practical checklist
- Check whether you have a Self Assessment record and your Unique Taxpayer Reference.
- Gather income and expense records for the tax year.
- File the return online as soon as possible to stop further daily penalties.
- Pay what you can immediately. If you cannot pay in full, contact HMRC about Time to Pay, which is generally available for debts under £30,000. Our guide to paying in instalments explains the process.
- If you have a genuine reason for lateness, submit a penalty appeal with dates and evidence.
- If you only registered late, see our note on registering after 5 October.
How SATR can help
Our qualified UK accountants can prepare and file overdue returns online, including for earlier years, from a fixed fee from £199 + VAT. Visit our late Self Assessment tax return service to get started.
This is general information, not personal tax advice.
Why acting quickly matters
HMRC penalties are largely mechanical, which means the cost depends on dates rather than on how you feel about the situation. Filing a return that is only a few days late costs far less than one that is months late, and filing at any point stops the daily penalties from continuing to grow. If you have received a penalty notice, read it carefully, note the dates, and check that the amounts match the timetable above. If something looks wrong, such as a penalty charged for a year when you had no obligation to file, you can ask HMRC to review it. Our guide to HMRC letters explained can help you understand what each notice means.
Frequently asked questions
Do I pay a penalty if I owe no tax?
Yes. The £100 late filing penalty applies even when the return shows nothing to pay, although you can appeal if you have a reasonable excuse.
Can I get a penalty cancelled?
HMRC can cancel penalties where you had a reasonable excuse, such as serious illness or a bereavement, if you file promptly once the problem ends.
Does filing late affect payments on account?
Late filing does not change the payments on account themselves, but a late return can delay HMRC working out your final bill, and any tax paid late attracts interest and possible penalties.
How far back can HMRC charge penalties for old returns?
Penalties can apply to any overdue return, including earlier years, and the charges build up as described above. Filing as soon as possible stops further daily penalties. Check current HMRC guidance for your particular years.
What if I only realised I needed to register after 5 October?
You should register and file as soon as you can. Penalties for the late return are measured from the filing deadline, so delay only increases the cost.
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The Bottom Line
Missing the Self Assessment deadline is expensive: the £100 penalty lands the day after 31 January, and daily penalties, 5% charges and interest pile on the longer you wait. The cheapest move is to file now, pay what you can, and appeal only if you had a genuine reasonable excuse.
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