How to Amend Your Self-Assessment Tax Return If You Made a Mistake

Have you ever submitted your Self-Assessment return, felt relieved it was finally done, and then spotted a mistake later? It happens to thousands of small firms, sole traders and individuals every year. A small typing error, a figure in the wrong box or forgetting to include income can all lead to confusion and worry.

The good news is that you can Amend Self-Assessment Tax Return entries if you realise something is not correct. HMRC allows you to fix mistakes as long as you follow the steps properly and stay within the deadlines.

In this blog we will explain how amendments work, the deadlines you need to know, what happens to your tax bill and how Myiva can support you with stress free corrections. You will also learn when you can amend things online and when paper forms are needed.

When Do You Need to Amend Your Self-Assessment Tax Return?

It’s really important to know when you need to amend your Self Assessment tax return, even if it’s just to sort out a small mistake. A tiny error can mess with your tax bill, so keeping everything accurate is a big deal. Here are some common situations when you might need to make some changes:

You suddenly remember income you missed off

If you’ve forgotten to tell HMRC about some income – whether that’s from freelancing, renting out a property or getting dividends – you’ll want to update your return to get the numbers right. Leaving off income can land you with penalties or interest if HMRC notices the difference later on.

You spot a few numbers that are wrong or a typo or two

Even a tiny slip-up on the keyboard can shift a number into the wrong box or transpose some expenses. If you spot any errors in your income or what you’re allowed to deduct, it makes sense to go back and fix them.

Your personal or business situation changes

Things like getting a new pension, selling some assets or sorting out some business expenses might change your tax situation. If any of these things happened during the tax year and you didn’t flag them up, you’ll need to go back and make some adjustments.

You get some new tax documents late on

Sometimes, you might get some more information after you’ve already submitted your return – like a revised payslip or a form from an investment. If that happens, you’ll want to make the necessary changes so your tax record is up to date.

You want to claim some reliefs or allowances that you missed

If you’ve got some expenses, tax credits or reliefs that you forgot to claim, you can go back and get them sorted – which might even cut down on your tax bill.

In short, if anything in your original Self Assessment return isn’t right, or has changed, it’s a good idea to go back and sort it out as soon as you can. Doing so keeps your tax record straight, avoids getting hit with unnecessary fines and helps to keep your mind at ease.

How Do I Amend My Self-Assessment Tax Return?

The way you amend your return depends on how you originally filed it. If you submitted it online, you can Amend Self-Assessment Tax Return details through your HMRC online account. Log in, select the tax year you want to change and update the figures. Once you send the amendment, your account will show the revised calculation when HMRC processes it.

If the online option is not available, you can still make changes using a paper amendment. This is common when the return was filed through software or by a previous accountant. To amend by post, complete the SA100 form and any extra schedules that apply to you. Mark the form clearly as an amendment and send it to HMRC for review.

Paper amendments take longer than online changes, but they are still a valid way to Amend Self-Assessment Tax Return information. Whichever method you use, make sure the figures are correct before you submit the updates so HMRC can process them without delay.

What Are the Key Deadlines for Amending My Self-Assessment Tax Return?

You get twelve months from the usual Self-Assessment deadline to Amend Self-Assessment Tax Return details. So, if you submitted your 2023 to 2024 return by 31 January 2025, you have until 31 January 2026 to make any corrections.

If you miss this time frame, you cannot use the normal amendment option. You may still be able to correct the mistake, but it has to be done through other routes, such as asking HMRC for overpayment relief if you paid too much tax. These requests need more proof and are only accepted in certain situations.

It is still possible to ask HMRC to change your return after twelve months, but they will look at your case one by one. You will need to show why the original figures are wrong and include any evidence that supports your request. This is one area where professional help from Myiva can make things clearer and increase your chances of success.

Will Amending My Tax Return Affect My Tax Bill

Every amendment has the potential to change your tax calculation. If your correction means you owe more tax, HMRC will issue an updated bill and set a new payment deadline. It is important to pay this balance quickly to avoid interest charges.

If the amendment shows that you overpaid tax, you can receive a refund. HMRC usually sends refunds directly to your bank account or adjusts your next year’s tax bill. You can claim your refund within your HMRC online account or wait for HMRC to process it automatically.

This is another reason many people choose to Amend Self-Assessment Tax Return entries as soon as possible. The faster you fix mistakes, the faster you receive the correct tax position.

How Long Does It Take for HMRC to Process an Amendment?

HMRC processing times vary. Online amendments are normally reviewed within seventy-two hours, although some cases may take longer if HMRC needs additional checks. Paper amendments take significantly more time because they must be reviewed manually.

You will know your amendment has been processed when your HMRC account updates with a new calculation. If a refund is due, it is usually paid within one to two weeks after the update. Updated tax bills may take the same amount of time.

If you are waiting longer than expected, you can contact HMRC or ask Myiva to check the status for you.

What Should I Do If I Miss the Amendment Deadline?

Missing the deadline can feel stressful, but you still have some choices. HMRC will not let you use the standard amendment process after twelve months, but other routes may apply depending on your situation.

Here are the main options if you miss the deadline:

• You can request overpayment relief if you paid too much tax
• You can write to HMRC with evidence showing why your tax calculation is wrong
• You can correct certain mistakes through your next year’s tax return
• You can ask for HMRC’s help if you were unable to amend earlier due to illness or another valid reason
• You can appoint a tax adviser like Myiva to prepare a formal appeal
• You can provide additional documents to support your case
• You can challenge HMRC’s decision if they refuse your correction

These options can take longer and may require more paperwork. Making sure the request is clear and properly supported increases your chances of success. This is why many small firms prefer Myiva to handle amendments once the standard window has closed.

Will Amending My Self-Assessment Tax Return Affect My Tax Bill

If your amendment shows that you owe more tax, HMRC will add the extra amount to your new calculation and send an updated bill. It is always better to pay the balance quickly to avoid interest on late payments.

If your amendment reveals that you have paid too much tax, you can receive a refund. This may be paid directly to your bank account or applied to your future tax bill if you prefer. You can claim the refund in your HMRC online account or wait for HMRC to issue it automatically.

This is another reason to Amend Self-Assessment Tax Return figures early. The sooner you update your return, the sooner your correct tax position is confirmed.

FAQs: Frequently Asked Questions

Conclusion

Correcting mistakes does not have to be stressful. Once you know how to Amend Self-Assessment Tax Return entries the right way, the process becomes far more manageable. Whether you need to fix an income figure, adjust an expense or update your details, HMRC gives you clear routes to make things right.

If you want accurate corrections, fast support and a simple online process, Myiva is here to help. From amendments to full Self-Assessment filing, our team makes tax easier for small firms and individuals.

Need help correcting your return. Get in touch with Myiva today and let our experts fix it for you.