What happens if you miss the 2024-25 self-assessment deadline?

The self-assessment deadline for 2024-25 tax returns has passed.

Of the 12.1 million customers expected to file this year, the latest figures show 10 million had filed ahead of the deadline on Saturday. 

In a bid to aid later filers, HMRC kept its phone lines open on the Saturday and extended its web chat services to 10 times its usual weekend capacity.

If you filed after midnight on 31 January, you will face an immediate £100 penalty, and if you still haven't submitted your return, the penalties can escalate.

Here Which? explains the consequences of a late submission, what you can do now, and the steps you can take to formally appeal an HMRC penalty.

What are the penalties if you miss the deadline?

Unless you have a 'reasonable excuse' or have made an arrangement with the tax office, the fees for filing late are as follows:

• One day late (1 February 2026): £100 fine
• Up to three months late (30 April 2026): £10 per day, capped at 90 days (plus the initial £100 fine)
• Six months late (31 July 2026) 5% of the tax due or £300, whichever is higher.

In addition to filing penalties, there are also fines for paying late. These are 5% of the unpaid tax at 30 days, 6 months, and 12 months. Interest accumulates on any unpaid balance from day one, and this is currently set at 7.75% –  the Bank of England base rate plus 4%.

• Find out more: how to fill in a self-assessment tax return.

What if you can't pay?

If you can’t afford to pay right now, you should contact HMRC as soon as possible and request a ‘time to pay’ arrangement. These allow you to spread the cost of what you owe over monthly instalments – although interest is still charged on the outstanding bill. 

You can get a time to pay arrangement if your tax bill is under £30,000, do so within 60 days of the 31 January deadline, and have no other existing HMRC debts.

In all other cases, you must contact the Payment Support Service at 0300 200 3835. HMRC evaluates these plans on a case-by-case basis rather than using a standard timeframe. It's important to note that ignoring the debt can lead to severe consequences, including debt collection or court action.

• Find out more: What is a UTR number?

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Can you appeal?

If you believe a penalty is incorrect or you have a 'reasonable excuse', you can formally appeal a penalty to HMRC. Reasonable excuses can include: 

• Bereavement: The death of a close relative or partner shortly before the deadline
• Serious illness: An unexpected hospital stay or a serious illness 
• Mental health: A condition that affected your capacity to comply at the time of the deadline
• System failures: HMRC’s online services being unavailable or failing to accept your return
• Software issues: Your computer or tax software failing while you were preparing your return
• Registration delays: Applying for a Pin, User ID, or password before the deadline, but receiving it too late to file on time
• Lost records: Theft, fire, or flooding that destroyed records which could not be replaced in time
• Postal disruption: Unforeseen delays such as a Post Office fire, flood, or prolonged industrial action, provided you posted the return in good time.

Recent data published by HMRC shows that between 31 March and 30 September 2025, it received 32,258 appeals against late filing and payment penalties. Of these, taxpayers won 20,076 cases. This means the tax office won 37.8% of the appeals lodged against it.

• Find out more: late tax returns and penalties for mistakes.

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How to appeal a HMRC penalty 

Generally, you have 30 days from the date the penalty notice was issued to submit your challenge. If you miss this window, you must provide an additional explanation for the delay.

1. File your return first

HMRC will not consider an appeal until the outstanding tax return has been submitted. Even if you cannot pay the tax bill immediately, you must send the return to stop the clock on further late-filing penalties.

2. Gather your evidence

You need to prove you have a reasonable excuse to successfully appeal. Collect any documentation that supports your claim, such as medical records or hospital letters if you were ill, death certificates in the case of bereavement, and screenshots of error messages if you experienced technical issues with HMRC’s website. You should try to gather as much evidence as you can, as this is the main way HMRC will decide on your case. 

3. Submit your appeal 

The easiest way to appeal against a late submission or late payment penalty is through your Government Gateway account. 

To submit your appeal, you’ll need to include the following: 

• Date the penalty was issued
• Date you filed your Self Assessment tax return
• Details of your reasonable excuse.

After submitting your appeal online, stay on the page until you see a confirmation message. This will tell you that it’s received and when HMRC will give you its decision.

You can also appeal by post. If your penalty comes in a letter, then you will also receive an appeal form with it. You can then follow the instructions in the letter. Alternatively, you can download and complete the SA370 appeal form and send it to HMRC with a letter outlining the details of your appeal. 

If you don’t have an appeal form, you can send a signed letter to HMRC instead, explaining the reason behind the delay. Appeal letters and forms can then be sent to: Self Assessment, HM Revenue and Customs, BX9 1AS.

• Find out more: HMRC adds new features to its app – here’s what you can do.