Self Assessment is how self-employed people in the UK report their income and pay income tax and National Insurance. The deadlines are fixed, and a late return carries an automatic penalty, so it pays to know the dates. This guide covers the main ones and gives you a checklist. It is general information. Always confirm dates on GOV.UK.
The tax year
The UK tax year runs from 6 April to 5 April. The 2025/26 tax year ended on 5 April 2026, so the return for that year is the one you file in the current cycle.
Key dates for the 2025/26 tax year
- 5 October 2026: the deadline to tell HMRC you need to complete a return if you have not filed one before.
- 31 October 2026: the deadline for a paper tax return.
- 31 January 2027: the deadline to file your return online and to pay any tax you owe for 2025/26. It is also the date of the first payment on account for 2026/27 if you are asked to make them.
- 31 July 2027: the second payment on account for 2026/27.
What are payments on account?
If your Self Assessment bill is above a certain amount, HMRC asks you to pay the next year’s tax in advance in two instalments, on 31 January and 31 July. Each is usually half of the previous year’s bill. This catches many new sole traders out, because the first January bill can include the tax for the year just ended plus the first instalment for the next one. Set money aside through the year so the bill does not hurt.
Penalties for being late
If you miss the filing deadline you receive an automatic penalty, currently £100 even if you owe no tax, and further penalties build up the longer the return is outstanding. Late payment also brings interest and additional penalties. Filing early removes most of the stress, and you can file any time after the tax year ends.
A simple checklist
- Register for Self Assessment and get your Unique Taxpayer Reference if you are new.
- Gather your income records. Total your sales and any other taxable income for the year.
- Gather your expenses and receipts. Check them against our list of allowable expenses.
- Reconcile your bank for the whole year so nothing is missing.
- Run your profit and loss for 6 April to 5 April.
- File the return online and check the tax calculation.
- Pay what you owe by 31 January and note any payments on account.
- Keep your records for the period HMRC requires.
Looking ahead: Making Tax Digital
If your qualifying income is above HMRC’s threshold, you will move from a yearly return to quarterly updates. Read our plain-English guide to Making Tax Digital to see whether it affects you and when.
How AvrioBooks helps
With invoices, expenses and your bank in one place, the figures for your return are a report away. Run a profit and loss report for the tax year, export it to PDF, Excel or CSV and use it to complete your return or send it to your accountant. A tax audit report shows the tax on every transaction if you also handle VAT.
Frequently asked questions
When is the Self Assessment deadline for 2025/26?
The online return and the payment are both due by 31 January 2027. A paper return is due by 31 October 2026.
When do I need to register for Self Assessment?
By 5 October after the end of the tax year in which you started self-employment.
What happens if I file my return late?
You receive an automatic penalty, currently £100, with more added the longer it is outstanding. Paying late also adds interest and penalties.
What are payments on account?
Advance payments towards next year's tax, due on 31 January and 31 July, each usually half of the previous year's bill.
Keep reading
Making Tax Digital for Income Tax: a plain-English guide for sole traders
Making Tax Digital for Income Tax replaces the once-a-year return with regular digital updates. Here is who it affects, when, and what to do now.
22 October 2026 · 9 min read
VAT registration: the threshold, when to register and how
Once your taxable turnover passes the threshold you must register for VAT. Here is when, how, and what it means for your business.
21 October 2026 · 7 min read
Allowable expenses for sole traders: what you can claim in the UK
Claiming every legitimate business cost lowers your tax bill. Here is what counts, what does not and how to keep the proof.
20 October 2026 · 8 min read