Self Assessment Deadlines and Penalties

Missing Self Assessment deadlines can be costly. This article explains when you need to register, file your return, and pay your tax—plus what penalties you'll face if you're late and how to challenge them if you have a reasonable excuse.

Key deadlines you need to know

If you need to complete a Self Assessment tax return, you must meet three important deadlines:

5 October – You must register for Self Assessment (or reactivate your account) by this date if you need to complete a tax return for the previous tax year and you either haven't sent one before, or you registered previously but didn't need to send one for the 2023 to 2024 tax year. You can be fined if you don't tell HMRC by 5 October.

31 October – This is the deadline for paper tax returns. If you're filing on paper, your return must reach HMRC by midnight on 31 October following the end of the tax year.

31 January – This is the deadline for online tax returns and for paying any tax you owe. For the 2024/25 tax year (which runs from 6 April 2024 to 5 April 2025), both your online return and payment are due by 31 January 2026.

You can submit your return any time after 5 April (the end of the tax year). Filing early means you'll know what you owe and have time to budget or set up a payment plan if needed.

Who needs to register for Self Assessment

You must send a Self Assessment tax return if, in the last tax year, you were:

• Self-employed as a sole trader and earned more than £1,000 (before deducting expenses)
• A partner in a business partnership
• Required to pay Capital Gains Tax when you sold or disposed of something that increased in value
• Required to pay the High Income Child Benefit Charge and don't pay it through PAYE
• An off-payroll worker repaying a student or postgraduate loan

You may also need to send a return if you have untaxed income from property rental, tips and commission, savings and investments, dividends, or foreign income.

Even if you're not required to submit a return, you can choose to do so to claim certain Income Tax reliefs, prove you're self-employed (for example, to claim Tax-Free Childcare or Maternity Allowance), or pay voluntary National Insurance contributions.

Late filing penalties

HMRC will send you a penalty notice if you miss the deadline for submitting your tax return. The penalties increase the longer your return is overdue.

You'll be charged a penalty if you file your return late, even if you have no tax to pay or you've already paid the tax you owe.

Late payment penalties

You'll also face penalties if you don't pay your Self Assessment tax bill by 31 January. These are separate from late filing penalties, so you could be charged both if you file late and pay late.

If you don't pay on time, you'll be charged interest on the outstanding amount from the date it was due until you pay it in full.

How to appeal a penalty

If you disagree that a penalty is due, you can appeal to HMRC. You'll need:

• The date the penalty was issued
• The date you filed your Self Assessment tax return (if you've submitted it)
• The date you paid your tax (if you've paid it)
• Details of your reasonable excuse for filing late or not paying on time

A reasonable excuse is something that prevented you from meeting the deadline despite taking reasonable care to do so.

You should consider paying the penalty even if you appeal. If you don't pay and your appeal is rejected, you'll have to pay interest on the penalty from the date it was due until the date you paid it. If HMRC agrees with your appeal, they'll repay what you've paid with interest (provided you don't have any other outstanding tax to pay).

To appeal, you can either make your appeal online or download form SA370 or SA371 to appeal by post. HMRC provides an online tool to check how to appeal an individual or partnership Self Assessment penalty.

After you submit your appeal

Once you've submitted your appeal, HMRC will review it and send you a decision. If you disagree with the outcome, you can ask HMRC to review their decision.

If you don't need to send a return

If you've received a penalty notice but you don't actually need to send a Self Assessment tax return, you can ask HMRC to cancel the penalty.

Avoiding penalties in the 2025/26 tax year

For the current tax year (2025/26, running from 6 April 2025 to 5 April 2026), remember:

• Register for Self Assessment by 5 October 2026 if you're new to Self Assessment or need to reactivate your account
• File a paper return by 31 October 2026
• File an online return and pay any tax due by 31 January 2027

Once you've registered, you can estimate your tax bill, set up weekly or monthly payments to help you budget, and submit your return early to give yourself more time to arrange payment.

Sources

• Self Assessment tax returns
• Appeal a Self Assessment penalty for late filing or late payment

This article provides general guidance based on current HMRC rules. For advice specific to your situation, speak to your accountant.