Missing the Self Assessment deadline triggers automatic penalties from day one — even if you owe no tax. The penalties escalate the longer you wait. The single best action you can take right now is to file immediately. Here is exactly what happens and what you owe in 2026/27.
The Penalty Tiers: What You Owe
| How late | Penalty |
|---|---|
| 1 day late (1 February onwards) | £100 automatic fixed penalty |
| 3 months late (from 1 May) | £10/day for up to 90 days — maximum £900 on top of the £100 |
| 6 months late (from 1 August) | 5% of the tax owed, OR £300 — whichever is higher |
| 12 months late (from 1 February of the following year) | A further 5% of tax owed OR £300 — whichever is higher |
These penalties stack. A return filed 12 months late with £5,000 of tax owed could face:
- £100 (day 1)
- £900 (daily penalties)
- £250 (5% of £5,000 at 6 months)
- £250 (5% of £5,000 at 12 months)
- Total penalties: £1,500 — plus interest on the unpaid tax
Interest on Unpaid Tax
HMRC charges daily interest on unpaid tax from 1 February (the day after the deadline). The current rate is 7.75% per annum (as of April 2026). On a £5,000 tax bill, this costs approximately £1.06/day — adding £388 per year in interest.
Interest accrues even if you file on time but do not pay. Filing is a separate obligation from paying — do both by 31 January.
The 2025/26 Tax Year: Key Deadlines
| Deadline | What it applies to |
|---|---|
| 5 October 2026 | Register for Self Assessment (if new income source in 2025/26) |
| 31 October 2026 | Paper Self Assessment return for 2025/26 |
| 31 January 2027 | Online Self Assessment return for 2025/26 |
| 31 January 2027 | Pay tax owed for 2025/26 plus second payment on account |
| 31 July 2027 | First payment on account for 2026/27 |
What Counts as a Reasonable Excuse
HMRC will cancel a penalty if you have a reasonable excuse — meaning something that genuinely prevented you from filing on time, beyond your control.
Accepted reasonable excuses:
- Serious illness, hospitalisation, or death of a close relative shortly before the deadline
- Your computer or internet service failed on the deadline day
- HMRC’s own online service was unavailable on 31 January (HMRC publishes confirmed outages)
- Fire, flood, or theft destroying your tax records
- Postal delays preventing delivery of paper returns (last working day before 31 October)
Not accepted as reasonable excuses:
- Forgetting the deadline
- Being too busy or having too much work
- Pressure from work or family
- Relying on an accountant who failed to file (although you may have a claim against the accountant)
- Not receiving a reminder from HMRC (you are responsible for knowing the deadline)
How to Appeal a Penalty
- File your return first — you cannot appeal a penalty for a return that has not yet been filed
- Pay any tax owed — unpaid tax continues to accrue interest regardless of an appeal
- Appeal within 30 days of the penalty notice: online via your Personal Tax Account, or by writing to HMRC
- State your reasonable excuse clearly — be specific and provide dates and evidence where possible
If HMRC rejects your appeal, you can escalate to the Independent Tax Tribunal — this is free and many taxpayers win on reasonable excuse grounds when they have genuine evidence.
Worked Example: James, Filed 5 Months Late
James had self-employment income in 2024/25 but missed the 31 January 2026 deadline. He filed his return on 1 July 2026 (5 months late) with £3,200 of tax owed.
Penalties incurred:
- £100 (automatic, day 1)
- £900 (daily £10 × 90 days = £900 cap — 3 months to 6 months)
- 5% of £3,200 at 6 months = £160 (above the £300 minimum? No — £160 < £300, so penalty is £300)
- Total penalties: £1,300
Interest: 5 months × (7.75%/12) × £3,200 ≈ £103
Total cost of being 5 months late: £1,403 on top of the £3,200 tax — a 44% surcharge.
Lesson: Filing even 1 day after 1 February costs £100. Filing and paying on 1 February would have cost nothing.
If You Cannot Pay the Tax
If you owe tax you cannot pay in full, you can set up a Time to Pay arrangement with HMRC — call 0300 200 3822. HMRC will usually agree a monthly payment plan. Interest continues to accrue during the arrangement, but penalties stop once you have filed and engaged with HMRC.
Do not avoid filing because you cannot pay — filing late and not paying is always worse than filing on time and arranging to pay.
See our Self Assessment guide, I forgot to declare income to HMRC, and backdating PAYE tax relief.