What's due on 31 January
By 11:59pm on 31 January 2027, HMRC needs two things from you for the tax year that ran from 6 April 2025 to 5 April 2026:
- Your online tax return. The paper deadline, 31 October 2026, has already passed, so it has to go online now.
- The tax you owe. That's income tax and Class 4 National Insurance on your profit, plus any student loan repayments, less anything already paid.
If you also have a job and wanted a smaller bill collected through your tax code instead, the return had to be in by 30 December 2026.
Why January can cost more than you expect
If your bill is over £1,000, and less than 80% of your tax was already taken at source, HMRC also asks for a payment on account towards the next year. It's half of this year's bill, and it's due the same day.
So in your first January after going self-employed you can be paying a full year's tax plus half of the next. The second half is due on 31 July. From the year after, the payments on account cover most of the bill and January is lighter.
If you think your income has dropped, you can ask HMRC to reduce your payments on account when you file.
What it costs to be late
Filing and paying are penalised separately. A late return is penalised even if you owe nothing or are due a refund.
| Return late by | Penalty |
|---|---|
| 1 day | £100 |
| 3 months | £10 a day, up to £900 |
| 6 months | 5% of the tax due or £300, whichever is more |
| 12 months | Another 5% or £300, whichever is more |
Paying late adds 5% of the unpaid tax at 30 days, again at 6 months and again at 12 months. Interest is charged on top, at 7.75% a year from 9 January 2026, from the day after the deadline until you pay.
If something serious stopped you filing on time, such as a stay in hospital, you can appeal with a reasonable excuse.
If you work under CIS
The 20% contractors deducted counts towards your bill. Add up your monthly payment and deduction statements and enter the total on the return. Many subcontractors find the deductions cover more than their tax, and the return is how you get the difference back.
Getting it done
You can file yourself through your HMRC online account. You'll need your figures for income, expenses, any CIS deductions and other income such as a job or savings.
If you'd rather not, an accountant or a tax return service can do it for you. Leave time: in January they're at their busiest.
Common questions
I've never sent a return. Is it too late?
You should have registered by 5 October 2026. If you register now, HMRC gives you three months from their letter to send the return, but the tax is still due by 31 January 2027.
Can I pay in instalments?
If you can't pay in full, you can set up a payment plan with HMRC online. Interest still runs on what's unpaid, but setting one up before the deadline can stop the late payment penalties.
I made a loss. Do I still need to file?
Yes, if HMRC has asked you to send a return. Filing also records the loss, which you may be able to set against other income or carry forward.
Sources
- Self Assessment tax returns: deadlines, GOV.UK. Checked 2 Nov 2026
- Self Assessment tax returns: penalties, GOV.UK. Checked 2 Nov 2026
- Understand your Self Assessment tax bill: payments on account, GOV.UK. Checked 2 Nov 2026
- HMRC interest rates for late and early payments, GOV.UK. Checked 2 Nov 2026
Spotted something out of date? Email figures@toolboxmoney.co.uk and we'll check it within two working days.