Self Assessment Deadlines & Penalties (2026)

Filing a Self Assessment tax return is one of those tasks that’s easy to delay—until it suddenly becomes urgent. Missing deadlines can be expensive, but with the right understanding and a bit of planning, it doesn’t have to be stressful.

Source: HMRC Self Assessment deadlines. Updated 6 September 2026.

When Is the Self Assessment Deadline?

For the 2025/26 tax year (6 April 2025 to 5 April 2026), the usual online filing and payment deadline is 31 January 2027. Tell HMRC by 5 October 2026 if you need to register, and submit a paper return by 31 October 2026 if using that route. Check any different deadline HMRC has notified to you.

By midnight on 31 January, you must:

• Submit your online Self Assessment tax return, and
• Pay any tax owed for that tax year

For a complete overview of all HMRC submission dates, our UK tax dates and deadlines page is a helpful reference throughout the year.

Paper Return Deadline (Earlier Than You Think)

If you submit a paper tax return, HMRC must receive it by 31 October following the end of the tax year. That’s three months earlier than the online deadline.

Because postal delays are outside your control, paper filing carries more risk. If your return arrives late—even by a single day—you may still be penalised.

What Happens If You Miss the Filing Deadline?

HMRC penalties start automatically. You do not need to owe tax to be fined—late filing alone is enough.

Late filing penalties:

• Up to 3 months late: Automatic £100 penalty
• More than 3 months late: £10 per day for up to 90 days (maximum £900)
• 6 months late: Additional £300 or 5% of the tax due (whichever is higher)
• 12 months late: Another £300 or 5% of the tax due (whichever is higher)

If HMRC believes information was deliberately withheld, penalties can increase further.

Key point: Filing something is always better than filing nothing. Even if your figures are not final, submit the return using the best information you have—you can amend it later.

Penalties for Paying Tax Late

The standard penalties below apply to the 2025/26 return; MTD rules for later tax years are explained separately.

Source: HMRC Self Assessment penalties. Updated 6 September 2026.

Filing the return is only half the job. You must also pay the tax due by 31 January.

Late payment penalties:

• 30 days late: 5% of the unpaid tax
• 6 months late: Additional 5%
• 12 months late: Further 5%

Interest continues to accrue until the tax is fully paid.

If you cannot pay in full, contacting HMRC early is critical. In many cases, a Time to Pay arrangement can prevent penalties from escalating.

Can You Still File a Paper Self Assessment?

Yes, paper returns are still allowed. However, there are some drawbacks:

• Earlier deadline (31 October)
• Slower processing times
• Higher risk of postal delays
• Penalties still apply even if you later file online

If HMRC processes your late paper return first, a £100 penalty may still apply—even if you later submit an online return.

For most taxpayers, online filing is simply more practical.

Making Tax Digital: Which Penalty Rules Apply?

For taxpayers joining MTD for Income Tax in 2026/27, new penalty rules apply from that tax year. The 2025/26 return due on 31 January 2027 remains under the previous rules. Late-submission penalties use points; late-payment penalties do not.

For 2026/27, HMRC will not impose penalties for missed quarterly-update deadlines, but digital records and quarterly updates are still required. See HMRC’s MTD penalty guidance for the applicable rules and transition arrangements.

Our MTD for Income Tax guide explains the reporting process.

Watch Out for Payments on Account

Payments on account often surprise first-time Self Assessment taxpayers.

If most of your tax is collected through Self Assessment, HMRC may require:

• Two advance payments towards the next tax year
• Each payment equal to 50% of the previous year’s tax bill

Example: If your tax bill is £6,000, you may need to pay £6,000 for the current year plus £3,000 as the first payment on account.

Can You Appeal or Avoid a Penalty?

HMRC may cancel penalties if you have a reasonable excuse, assessed on a case-by-case basis.

Examples that may be accepted:

• Serious illness
• Bereavement shortly before the deadline
• Major IT failures
• Problems with HMRC’s own systems

HMRC will not usually accept:

• Not understanding the system
• Forgetting the deadline
• Not receiving a reminder

You can appeal online or by submitting form SA370.

How to Avoid Problems in Future Years

Once you’re registered for Self Assessment, staying organised makes everything easier.

Understanding your current UK tax rates and allowances also helps you plan contributions and income timing more effectively throughout the year.

Good habits include:

• Keeping HMRC login details secure
• Maintaining clear income and expense records
• Filing early rather than waiting until January
• Reviewing your tax position during the year

How Long Must You Keep Records?

You must keep records for at least five years after the 31 January deadline of the relevant tax year.

Example: If you filed your 2024/25 return by 31 January 2026, you must keep records until at least January 2031.

You can submit your tax return as soon as the tax year ends on 5 April—giving you plenty of time to plan and budget.

Take the Stress Out of Self Assessment

Self Assessment does not need to be confusing or time-consuming. With the right support, it becomes a routine task rather than a yearly headache.

At SRZ Accountancy, we handle Self Assessments from start to finish—ensuring deadlines are met, penalties are avoided, and there are no surprises. Our chartered certified accountants support thousands of clients each year, helping them stay compliant and in control.

If you’d rather focus on running your business or enjoying your time, we’re here to handle the numbers. Find out more about our Self Assessment tax return service, or get in touch with SRZ Accountancy for a free initial consultation.