Missing the Self Assessment deadline is one of the most avoidable financial mistakes a sole trader or limited company director can make. HMRC's penalty system is automatic, escalating, and applies regardless of whether you actually owe any tax. A return filed one day late triggers a £100 penalty. A return still outstanding after a year can result in over £1,600 in filing penalties alone — before late payment charges and daily interest are added.
This post explains exactly what happens when you miss the 31 January deadline: the full penalty structure, how interest is calculated, the important distinction between filing penalties and payment penalties, and what options you have if you find yourself already late.
The key deadlines for 2025/26
| Deadline | What it covers |
|---|---|
| 5 October 2026 | Register for Self Assessment if filing for 2025/26 for the first time |
| 31 October 2026 | Paper tax return deadline for 2025/26 |
| 31 January 2027 | Online return deadline and payment of tax owed for 2025/26, plus first payment on account for 2026/27 |
| 31 July 2027 | Second payment on account for 2026/27 |
The deadline that most people are aware of — and most commonly miss — is 31 January. This is the date by which your online return must be submitted and any tax owed paid in full. There is no grace period. A return received on 1 February is late.
Filing penalties — the escalating structure
Filing penalties and payment penalties are two separate systems that run in parallel. You can be charged both simultaneously. Filing penalties apply based on how late your return is — regardless of whether you owe any tax at all.
Day 1 late — £100 automatic penalty
An immediate fixed penalty of £100 applies from the first day after the deadline. This applies even if your tax bill is zero, even if you are due a refund, and even if you have already paid your tax in full. The penalty is for the late return, not the late payment.
3 months late — £10 per day, up to £900
If the return is still outstanding after three months (from 1 May for a 31 January deadline), HMRC adds daily penalties of £10 per day for up to 90 days — a maximum additional charge of £900. These accrue automatically and are not separately notified until HMRC issues a penalty notice.
6 months late — greater of £300 or 5% of tax owed
At six months, a further penalty applies: whichever is higher between £300 and 5% of the tax due. The £300 minimum means this penalty applies even on a nil-tax return. A taxpayer with a £10,000 bill faces a further £500 at this stage.
12 months late — a further £300 or 5%, up to 100% if deliberate
The same charge repeats at 12 months. In serious cases — where HMRC determines information was deliberately withheld — penalties can reach 100% of the tax due. By 12 months, a nil-tax return has accumulated £1,600 in filing penalties (£100 + £900 + £300 + £300) without a single penny of tax being involved.
Common misconception: "I don't owe any tax so there's no penalty." This is wrong. The £100 day-one penalty and all subsequent filing penalties apply regardless of the tax position. The only charges that do not apply on a nil-tax return are the percentage-based payment surcharges — because those are calculated on the unpaid amount.
Payment penalties — separate and additional
If tax is owed and not paid by 31 January, HMRC also applies payment penalties. These are calculated as a percentage of the unpaid balance and are entirely separate from the filing penalties above.
| How late | Payment penalty |
|---|---|
| 30 days after deadline | 5% of unpaid tax |
| 6 months after deadline | Further 5% of unpaid tax |
| 12 months after deadline | Further 5% of unpaid tax |
The cumulative payment penalty over 12 months is therefore 15% of the original unpaid amount — applied on top of the filing penalties and interest. A £5,000 tax bill left unpaid for a year would attract £750 in payment penalties alone, before interest.
Interest — charged daily from 1 February
Separate from both penalty systems, HMRC charges interest on any unpaid tax from the day after the payment deadline until the amount is paid in full. From 6 April 2025, the late payment interest rate changed to the Bank of England base rate plus 4 percentage points. As of early 2026, with the base rate at 3.75%, the late payment interest rate is 7.75% per annum, applied daily.
Interest is not a fixed charge — it accumulates every day on the outstanding balance. It cannot be appealed and is charged even where a Time to Pay arrangement is in place. On a £5,000 bill, the daily interest is approximately £1.06 per day at the current rate — modest initially, but significant over months.
What to do if you are already late
If you have already missed the deadline, the most important thing is to act immediately. Every day you wait adds to the cost.
- File the return as soon as possible. Filing stops the daily £10 penalties from continuing to accrue. Even if you cannot pay the tax, filing the return is always the first priority. Late filing and late payment are separate — filing now limits one set of charges regardless of your ability to pay.
- Pay what you can. Payment penalties and interest are calculated on the outstanding balance. Any partial payment reduces the amount on which those charges accumulate.
- Contact HMRC about a Time to Pay arrangement. If you cannot pay in full, HMRC's Time to Pay scheme allows you to spread payments over typically up to 12 months. You can self-serve online if you owe less than £30,000. Once a TTP is in place, payment penalties stop accruing — though interest continues until the balance is cleared.
- Consider appealing if you have a reasonable excuse. HMRC can waive filing penalties where there is a genuine reasonable excuse — serious illness, bereavement, a fire or flood destroying records, or HMRC system failures. The appeal must be made within 30 days of the penalty notice, in writing or through your HMRC online account, and the excuse must have genuinely prevented you from filing. Financial difficulty, forgetting the deadline, and reliance on an advisor who failed to file are not accepted.
The new points-based system — MTD filers from April 2026
From April 2026, sole traders and landlords with income over £50,000 are required to join Making Tax Digital for Income Tax, submitting quarterly updates instead of a single annual return. For these taxpayers, HMRC operates a points-based penalty system rather than the immediate fixed fine structure. Each missed quarterly submission earns one penalty point. Once points reach the threshold (four points for annual filers, two for quarterly), a fixed £200 penalty applies for that submission and each subsequent late one. Points can be cleared by maintaining a full period of compliance and submitting all outstanding returns.
The £100 day-one penalty and the escalating structure described above continues to apply to the final annual return (end of period statement and final declaration) for MTD filers — the soft landing applies to quarterly updates only.
In summary — the worst-case scenario for a nil-tax return filed 12 months late: £100 (day one) + £900 (daily penalties, 90 days) + £300 (six months) + £300 (twelve months) = £1,600 in filing penalties. No tax owed. No interest. Just penalties for not filing a piece of paperwork.
Already behind with your tax return?
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Get in TouchThis article is for general information purposes and does not constitute tax advice. Penalty rules and interest rates can change. Always check current HMRC guidance or speak to a qualified accountant before making decisions.