Self Assessment Deadlines for UK Sole Traders: A 2025/26 Calendar with Penalties

Self Assessment has a rhythm. For sole traders, the beat is not the 5 April year-end or the January rush alone. It runs from October registration deadlines through to the following July, with payments on account sitting in between. If you started trading in 2024/25, your first return is due by 31 January 2026. If you have been trading for years, the same dates still apply. Miss them and HMRC adds penalties and interest, often before you have had a chance to explain. The system is unforgiving on dates, even when your profits are modest or your records are straightforward. A return can be required even if no tax is due, so the deadline matters as much as the bill.

The calendar below covers the 2025/26 Self Assessment cycle: the return for 2024/25, the payments due during 2025/26, and the deadlines that follow. It is written for sole traders, not limited companies, though the principles of filing and paying on time apply across the board. Dates are not moved simply because they fall on a weekend or bank holiday, so it is safer to treat each one as fixed.

The 2025/26 Self Assessment calendar

Most sole traders can ignore the finer points and remember two dates: 31 January and 31 July. The 31 January 2026 deadline is the big one. Your 2024/25 online return must be filed, and any tax owed for that year must be paid. That same date also brings the first payment on account for 2025/26, if you make them. The second payment on account is due by 31 July 2026. If you file on paper, the return deadline is 31 October 2025, but the payment deadline stays 31 January 2026. For example, a sole trader who started in June 2024 must register by 5 October 2025, file the 2024/25 return by 31 January 2026, and pay any balancing amount plus the first payment on account by the same date.

1. 6 April 2025: The 2025/26 tax year begins.
2. 5 October 2025: Deadline to register for Self Assessment if you need to file a return for 2024/25 and have not registered before.
3. 31 October 2025: Paper return deadline for 2024/25.
4. 31 January 2026: Online return and payment deadline for 2024/25. Balancing payment due, plus first payment on account for 2025/26 where applicable.
5. 5 April 2026: The 2025/26 tax year ends.
6. 31 July 2026: Second payment on account for 2025/26.
7. 31 January 2027: Online return and payment deadline for 2025/26.

Payments on account: the July bill explained

If your previous year's tax and Class 4 National Insurance bill was more than £1,000, HMRC usually asks for payments on account. Each is half of that previous bill. They are not a penalty or an extra tax; they are advance payments towards the current year. Suppose your 2024/25 bill was £6,000. You would normally pay £3,000 by 31 January 2026 and another £3,000 by 31 July 2026. If your profits drop, you can apply to reduce the payments. Do so carefully. Reduce them too far and you may face interest if the final bill is higher. If you are unsure, speak to an accountant before changing them. A common mistake is to treat the July payment as optional because no return is due then. It is not optional; it is a payment on account of the current year's liability.

What happens if you miss a deadline

Filing penalties

• One day late: £100 fixed penalty. This applies even if you have no tax to pay, unless you have a reasonable excuse.
• Three months late: £10 per day, up to a maximum of £900.
• Six months late: 5% of the tax due or £300, whichever is greater.
• Twelve months late: another 5% or £300, whichever is greater.

Payment penalties

• 30 days late: 5% of the unpaid tax.
• Six months late: another 5%.
• Twelve months late: another 5%.

Interest is charged daily on unpaid tax from the due date until it is paid. Filing late and paying late are treated separately, so a late return can trigger penalties even if you pay what you owe on time. The penalties are cumulative in the sense that missing another milestone adds another charge, so a return left unfiled for a year can attract several separate penalties on top of interest.

A “reasonable excuse” is a specific test. Cash flow problems alone usually do not count. If you cannot pay, file the return anyway and contact HMRC to set up a payment plan. Time to pay arrangements can stop some collection action, though interest continues to run. For legal or financial advice about your specific position, seek professional advice.

If you cannot pay, file first. A filed return with an unpaid bill is almost always better than an unfiled return with an unpaid bill. The late-filing penalty clock starts the day after the deadline, and it does not pause because you are waiting to find the money.

Reasonable excuse and appeals: what HMRC accepts

HMRC decides reasonable excuse case by case, but the bar is higher than many sole traders expect. Serious illness, bereavement, a fire or flood at your premises, or an HMRC system failure can qualify. “I forgot”, “I was too busy”, and “my accountant did not remind me” generally do not. If you receive a penalty notice, check the date and reason. You can appeal online or by post, usually within 30 days. Include evidence such as medical letters, insurance reports, or correspondence. If HMRC rejects the appeal, you can ask for a review or take the matter to tribunal. Responsibility for filing remains yours even if an agent acts for you, so keep an eye on the dates yourself.

Cash flow planning and common diary traps

Self Assessment is easier when you treat the tax as a monthly cost rather than a January surprise. Estimate your tax and Class 4 National Insurance as profits come in, and move a percentage into a separate savings account. A rough rule of thumb is to set aside 20% to 30% of profit, depending on your bracket and other income. Then check your HMRC account in November and December, not on 30 January. Filing early does not bring the payment date forward, but it gives you time to query errors and arrange a payment plan if needed. Common traps include missing the 5 October registration deadline, assuming a paper return is still possible in January, and forgetting that a payment on account is due in July even though no return is filed then.

If you are already late

Act immediately rather than waiting for the next deadline. File the outstanding return, even if you cannot pay in full. Then contact HMRC to discuss a Time to Pay arrangement. Interest will continue, but agreeing a plan can reduce the risk of enforcement action. Keep copies of everything you send. If the delay was caused by something outside your control, appeal the penalty with evidence. If there was no reasonable excuse, paying the penalty and moving on is often cheaper than a long dispute. The key is not to let one missed date become three.

Final thought

The Self Assessment year is a cycle, not a single event. The 5 October registration date, 31 October paper deadline, 31 January online and payment date, and 31 July payment on account are fixed points. Put them in your calendar now, set reminders two weeks before each one, and send records to your accountant early if you use one. The cost of missing a deadline is rarely just the penalty; it is the interest, the stress, and the hours spent fixing something that could have been avoided. For legal or financial advice about your specific position, seek professional advice.

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