Miss the Self Assessment deadline by even a day and HMRC charges an automatic £100 penalty — whether or not you owe any tax. It escalates from there: daily charges after 3 months, then percentage-of-tax penalties at 6 and 12 months, on top of separate penalties and interest for paying late.
Late filing penalties, in order
| How late | Penalty |
|---|---|
| 1 day | £100 fixed penalty |
| 3 months | £10 a day, up to a maximum of £900 (90 days) |
| 6 months | A further 5% of the tax due, or £300 — whichever is greater |
| 12 months | Another 5%, or £300 — whichever is greater |
This is HMRC's own penalty structure, taken directly from its Self Assessment tax returns: penalties page. The daily and 6/12-month charges are cumulative — they stack on top of the initial £100, and on top of each other, the longer a return stays outstanding.
The £100 penalty is charged even if you owe no tax at all, or are due a refund. There is no "nil return" exemption. If you registered for Self Assessment and HMRC expects a return from you, filing late costs £100 regardless of the numbers inside it.
Late payment penalties are separate — and stack with interest
Filing late and paying late are assessed independently, so it's possible to be charged for both at once. If tax is still unpaid after the 31 January deadline, HMRC's penalty schedule is:
| How late paying | Penalty |
|---|---|
| 30 days | 5% of the tax unpaid |
| 6 months | A further 5% of the tax still unpaid |
| 12 months | A further 5% of the tax still unpaid |
On top of these penalties, interest is charged on the amount owed from the day after the payment deadline until it's paid in full — this accrues regardless of whether a late payment penalty has kicked in yet. All of this is set out on the same gov.uk penalties page.
Because filing and paying are assessed separately, someone who files a day late but pays on time is charged the £100 filing penalty only — no late payment penalty applies. Someone who files on time but can't pay for two months faces late payment penalties and interest, but not the filing penalties above.
What counts as a "reasonable excuse"
HMRC will consider cancelling a penalty if you had a reasonable excuse and you filed or paid as soon as you reasonably could once the problem was resolved. According to gov.uk's guidance on reasonable excuses, examples HMRC accepts include:
- Your partner or another close relative died shortly before the filing or payment deadline.
- You had an unexpected stay in hospital that prevented you dealing with your tax affairs.
- A fire, flood or theft prevented you completing your return.
- Delays connected to a disability or mental illness you have.
- A failure in HMRC's own online services, or a computer failure while you were preparing your return.
- Postal delays that you couldn't have predicted.
The same page is explicit about what doesn't count: a cheque that bounced or a payment that failed because you didn't have enough money, not receiving a reminder from HMRC, a mistake on your return, or simply finding HMRC's system difficult to use. If your circumstances match one of the accepted examples, you still need to act — HMRC expects the return or payment to follow "as soon as you're able to" once the excuse no longer applies.
How to appeal
You have 30 days from the date the penalty was issued to appeal, per gov.uk's guidance on disagreeing with a tax decision. Appealing doesn't pause the underlying obligation to file or pay — if you haven't filed yet, do that regardless of the appeal, since further daily and percentage penalties keep accruing on an outstanding return whether or not an appeal against an earlier penalty is pending.
A worked example of how the penalties stack
The table below isn't a real HMRC figure — it's an illustration of how the late filing scale in the first table compounds over a year, so the numbers are for explanation only, not a quote from gov.uk.
| Time since deadline | Filing penalty charged so far |
|---|---|
| Day 1 | £100 |
| 3 months | £100 (daily penalties haven't started until day 91) |
| 4 months (30 days into the daily charge) | £100 + £300 (30 days × £10) = £400 |
| 6 months+ | £100 + £900 (daily cap) + at least £300 = £1,300 or more |
| 12 months+ | £1,300+ plus a further 5%-or-£300 charge = £1,600 or more |
Two things this illustrates: the daily penalty is capped at £900 (90 days × £10), so it stops growing after month 6 regardless of how much longer the return stays outstanding; and the 6-month and 12-month charges are each the greater of a flat £300 or 5% of the tax due, so for a large tax bill the total can be far higher than the £1,600 shown here.
If HMRC doesn't accept your appeal
If HMRC rejects an appeal made under a reasonable excuse, gov.uk's disagree with a tax decision or penalty guide sets out what comes next: you can ask HMRC for an internal review of the decision, and if that doesn't resolve it, take the case to the tax tribunal. Both of those are separate, later steps from the initial 30-day appeal window above, and neither pauses the return itself — the safest position, whatever stage an appeal is at, is still to have filed.
If you genuinely can't pay
The filing penalty and the payment penalty are triggered independently, so the single highest-leverage thing you can do if money is the problem is file on time anyway — that alone avoids the £100 fixed penalty and everything that escalates from it. For the payment itself, gov.uk's guidance on help if you cannot pay your tax bill points to setting up a payment plan before the 31 January deadline passes, which is a materially different position than defaulting into penalties and then trying to negotiate afterwards.
How ac-co does this
ac-co's Self Assessment product is built around the 31 January date, not around catching up after it: your return is drafted from connected accounts well ahead of the deadline, so filing on time doesn't depend on finding a free evening in the last week of January. If a deadline has already passed, ac-co still files as soon as your return is ready — the sooner it's submitted, the sooner the daily and percentage penalties above stop accruing, even though the £100 fixed penalty itself is unavoidable once the first deadline is missed.
Related reading
- Self assessment deadline 2026/27 — every date these penalties are measured against.
- How to get your SA302 — needed once your return is filed, penalties or not.
- How to find your UTR number — required before you can register or file at all.