Your 2025/26 Self Assessment return is due online by 31 January 2027, and you must pay any tax you owe by the same date. Miss it and HMRC charges an automatic £100 penalty even if you owe nothing, then adds daily penalties, further percentage charges, and interest at 7.75% a year. This guide lays out every deadline for the current tax year, the full penalty ladder, and the moves that keep you clear of both.
Self Assessment deadlines for the 2025/26 tax year
The 2025/26 tax year ran from 6 April 2025 to 5 April 2026. Every date below applies to that return. HMRC works to the minute, so 11:59 pm on the deadline day is the real cut-off.
| Deadline | Date |
|---|---|
| Tell HMRC you need to file (register) | 5 October 2026 |
| Paper tax return reaches HMRC | 31 October 2026 |
| Ask HMRC to collect tax through your tax code | 30 December 2026 |
| Online tax return reaches HMRC | 31 January 2027 |
| Pay the tax you owe (balancing payment) | 31 January 2027 |
| Payments on account fall due | 31 January and 31 July |
Register early if this is your first return. HMRC has to issue your Unique Taxpayer Reference before you can file, and that takes time you will not have in late January.
What happens if you file your return late
HMRC issues an automatic £100 penalty the moment you miss the filing deadline, even when you owe no tax or you have already paid. The longer the return stays outstanding, the more it costs, and the charges stack rather than replace each other.
| How late the return is | Penalty |
|---|---|
| 1 day | £100 fixed penalty |
| 3 months | £10 a day, for up to 90 days (maximum £900) |
| 6 months | £300, or 5% of the tax due, whichever is higher |
| 12 months | A further £300, or 5% of the tax due, whichever is higher |
A return that is a full year late attracts at least £1,600 in filing penalties before HMRC adds a penny for paying late. If HMRC decides you withheld information deliberately, the 12 month charge can climb to 100% of the tax due.
What happens if you pay your tax late
Late payment carries its own penalties on top of anything you owe for filing late, and interest runs alongside them.
| How late the payment is | Charge |
|---|---|
| 30 days | 5% of the unpaid tax |
| 6 months | A further 5% of the unpaid tax |
| 12 months | A further 5% of the unpaid tax |
HMRC also charges late payment interest on everything you owe from 1 February until you clear the balance. The rate is 7.75% a year from 9 January 2026, set at the Bank of England base rate plus four percentage points. Interest applies to the penalties too, so the bill grows every day it stays open.
A worked example: how a £5,000 bill becomes £7,700
Numbers make the risk concrete. Say you owe £5,000 for 2025/26 and you file and pay nothing until 1 February 2028, a full year late.
- Filing penalties: £100 + £900 + £300 + £300 = £1,600
- Late payment penalties: 5% three times over = £750
- Interest: roughly £390 on the tax at 7.75% for a year, plus interest on the penalties
Your £5,000 bill turns into about £7,740. The penalties and interest add more than half again on top of the tax itself, and none of it is negotiable once the deadlines pass.
The failure to notify penalty
There is a separate trap for people who should have registered but did not. If you needed to file, missed the 5 October registration date, and still owe tax on 31 January, HMRC can charge a failure to notify penalty on top of everything above. It is based on the tax at stake and on how HMRC reads the reason for the delay. Coming forward before HMRC contacts you, an unprompted disclosure, keeps this penalty as low as it can go. Waiting for HMRC to find you does the opposite.
Payments on account, and why your January bill can shock you
Once your Self Assessment bill passes £1,000, HMRC usually asks for payments on account: two advance instalments towards next year’s tax, each equal to half your latest bill. One falls due on 31 January alongside your balancing payment, the other on 31 July. Miss either and interest applies in the same way as any late tax. People filing for the first time get caught out here more than anywhere else, because their opening January bill covers the year just gone plus the first instalment for the year ahead. Budget for both, not just the balance.
Making Tax Digital for Income Tax changes the rhythm from April 2026
A bigger shift is already underway. Making Tax Digital for Income Tax started on 6 April 2026 for sole traders and landlords with qualifying income above £50,000. The threshold drops to £30,000 from April 2027 and £20,000 from April 2028. If you are in scope, you keep digital records and send HMRC quarterly updates through compatible software, then a final declaration after the tax year ends. The 31 January payment date stays. What changes is the reporting rhythm, and a separate points system applies to late quarterly submissions. If your income sits near any of these thresholds, plan the switch now rather than in the week before a quarterly deadline.
How to appeal a Self Assessment penalty
You can appeal a penalty if you have a reasonable excuse, something that stopped you meeting the deadline despite taking reasonable care. HMRC accepts examples such as a serious illness, a bereavement close to the deadline, or a failure by HMRC systems or your software provider. It does not accept that you found the system too hard, that no reminder arrived, or that your agent let you down. You normally have 30 days from the date on the penalty notice to appeal, so act quickly and keep evidence.
Five ways to stay clear of penalties
- Register early. If this is your first return, tell HMRC well before 5 October so your reference arrives in time.
- File in autumn, not January. An early return tells you the exact bill months ahead, so you can budget for it calmly.
- Set money aside as you earn. Keep a fixed share of income in a separate account for the January bill.
- Use the tax code option. Submit online by 30 December if you want HMRC to collect a smaller balance through your PAYE code.
- Get help before the deadline, not after. Fixing a late or wrong return costs far more than getting it right the first time.
Frequently Asked Questions
What is the Self Assessment deadline for 2025/26?
31 January 2027 for online returns and payment, or 31 October 2026 if you file on paper.
Is there a penalty if I file late but owe no tax?
Yes. The £100 penalty applies even when your tax bill is zero or already paid in full.
How much is HMRC interest on late tax?
7.75% a year from 9 January 2026, charged daily until you pay the balance.
Can I get the £100 penalty cancelled?Can I get the £100 penalty cancelled?
Only if you have a reasonable excuse and appeal within 30 days. HMRC reviews each case on its own facts.
What is a payment on account?
An advance instalment towards next year’s tax, due on 31 January and 31 July, each equal to half your previous bill.
Facing a deadline or a penalty you think is wrong?
IBISS & CO files Self Assessment for sole traders, landlords, directors and expats across our Tooting, Barking and Wimbledon offices. We file on time, deal with HMRC for you, and challenge penalties where there is a case to make. Book a call before your next deadline.
Muhammad Sufyan Moavia
Muhammad Sufyan Moavia, Chartered Tax Adviser and Fellow Chartered Certified Accountant at IBISS & CO.
He has 15 years experience in accounting and tax, advising individuals and business owners on personal tax, HMRC matters, IHT, CGT and tax planning.
Last review: July 2026
