For UK Self Assessment, the four key dates each tax year are: 5 October (register if first-time), 31 October (paper filing deadline), 31 January (online filing AND balancing payment AND first payment on account), and 31 July (second payment on account). Miss the 31 January and the £100 penalty applies immediately regardless of whether tax is owed.
These dates don''t move. Every year, thousands of UK taxpayers get caught out by the 31 January deadline. The fix is calendar discipline — they''re predictable to the day.
The complete calendar
For the tax year that ends 5 April 2026 (called the "2025/26 tax year"):
| Date | Event |
|---|---|
| 5 October 2026 | Register for Self Assessment if first time |
| 31 October 2026 | Paper return deadline |
| 31 January 2027 | Online return deadline |
| 31 January 2027 | Balancing payment for 2025/26 due |
| 31 January 2027 | First payment on account for 2026/27 due |
| 31 July 2027 | Second payment on account for 2026/27 due |
5 October — first-time registration
If you''re newly self-employed, a new landlord, or otherwise newly need to file Self Assessment, you must register with HMRC by 5 October following the tax year in which you first need to file.
Registration process:
- Visit GOV.UK and register online for Self Assessment.
- HMRC sends a 10-digit UTR (Unique Taxpayer Reference) by post within 2-3 weeks.
- You also get an HMRC online account.
Don''t leave registration until December — the UTR arrives by post and the process can''t be rushed.
Missing the 5 October deadline triggers a "failure to notify" penalty, calculated as a percentage of the tax owed, scaled by how late and whether it was deliberate.
31 October — paper return deadline
If you''re filing on paper (form SA100), it must reach HMRC by 31 October.
Realistically, almost no one files on paper any more. HMRC strongly encourages online filing. Paper filers are typically:
- Older taxpayers without internet access.
- Trustees and personal representatives where the paper format is more convenient.
- People with very complex affairs whose accountant prefers paper.
If you''re going to file on paper, plan around the earlier deadline — the 31 October deadline is 3 months earlier than the online deadline.
31 January — the big one
This is the deadline that catches most people. Three things are due on 31 January:
1. Online tax return filing
The completed Self Assessment return for the tax year ending in April of the previous calendar year.
For 2025/26 (year ending 5 April 2026): file by 31 January 2027.
2. Balancing payment
The tax owed for that tax year, MINUS any payments on account already made.
If your liability is £8,000 and you''ve made payments on account of £4,000 in January and £4,000 in July, your balancing payment is £0.
If your liability is £10,000 and you''ve paid £8,000 on account, balancing payment is £2,000.
3. First payment on account for the NEXT tax year
If your previous year''s liability was over £1,000 and less than 80% was collected via PAYE, you must also make a payment on account equal to half the previous year''s liability.
So in January 2027, you''re paying:
- Balancing payment for 2025/26.
- PLUS first payment on account for 2026/27.
This is what creates the "January shock" — you''re effectively paying 1.5 years of tax in one go in your first year of Self Assessment.
31 July — second payment on account
Half the previous year''s liability, paid as the second instalment.
If you''re no longer self-employed (or income has dropped), you can apply to reduce the payments on account. But if you reduce them too far and the actual liability is higher, HMRC charges interest on the underpayment.
The penalty regime for late filing
The penalties compound. For the 31 January online deadline:
- 1 day late: £100 fixed penalty, regardless of whether tax is owed.
- 3 months late (after end of April): £10 per day, up to 90 days = up to £900 more.
- 6 months late (after end of July): another £300 OR 5% of the tax owed (whichever is higher).
- 12 months late (after end of January following): another £300 OR 5% of tax owed.
A return filed 12 months late with no tax owed costs £1,600 in penalties. With tax owed, the percentages scale.
The penalty regime for late payment
Separate from filing penalties. Even if you file on time but don''t pay:
- 30 days late: 5% of unpaid tax.
- 6 months late: another 5%.
- 12 months late: another 5%.
Plus interest from the original due date at the HMRC rate (currently around 7.75% — base rate plus 2.5%).
Reasonable excuse appeals
Penalties can be appealed if you have a "reasonable excuse" — typically defined narrowly:
- Serious illness, hospitalisation, or bereavement.
- Fire, flood, or theft destroying records.
- HMRC system unavailability on the day.
Not accepted as reasonable excuse:
- "I forgot."
- "I was too busy."
- "My accountant failed me."
- "Companies House didn''t remind me."
Appeals must be in writing within 28 days, with evidence.
How to actually hit the dates
Most UK SME owners and accountants run on this rhythm:
- April-May: gather records for the prior tax year (just ended on 5 April).
- June-September: prepare the return — early enough that issues can be resolved without panic.
- October: review and finalise. File well before Christmas if possible.
- November-December: filing window.
- January: chasing the laggards and dealing with last-minute filings.
The single most useful habit: file before 1 December. The system is faster, HMRC support is less stretched, and any errors discovered have time to be corrected.
What to do this month
- Note your filing deadlines for the current tax year on your calendar with reminders at 60, 30, and 7 days.
- If this is your first year filing: register now if you haven''t. The UTR arrives by post.
- Set up direct debit for the tax payment (faster than bank transfer, no risk of forgetting).
- If you use an accountant, agree the document-handover deadline so they have time to file early.