They say ‘the best laid plans of mice and men often go awry’, but why is it always the plans that you really don’t want to go wrong that somehow find a way to?

Filing your Self Assessment is an important task; it’s the biggest tax filing of the year after all, and making a mistake could leave you in hot water with HMRC. Check out our guide on how to complete your first Self Assessment.

So you go through all the hard work to make sure that this year’s filing is absolutely iron-clad, you proudly hit submit, and five days later, your heart sinks: you’ve made a mistake, and now you need to work out how to make an amend to your Self Assessment – if you even can, that is.

Well, steady your heart rate and wipe the sweat from your brow; it is possible to amend your Self Assessment, and even better, it’s easier than you might think.

Why might I need to amend my Self Assessment?

There are several reasons you may need to change your tax return after you have already filed it:

• You forgot to include something, such as an invoice or untaxed income.
• You changed your name or address during the same tax year and it needs updating.
• You made a typing error, for example a figure in the wrong box or a misplaced decimal point.
• You realise you want to claim overpayment relief because you believe you paid the wrong amount of tax.

If you spot a mistake on your return don’t panic. It happens, and the important thing is that you noticed it. It is much better to amend the return yourself rather than waiting for HMRC to spot the issue first, which can raise unnecessary concerns.

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Will there be any changes to my tax bill if I amend my Self Assessment?

That depends on what you are changing. If you are simply updating personal details, such as your name or address, your tax bill will stay the same.

If you are correcting financial information, such as adding a missing invoice, untaxed income, or adjusting expenses, your tax bill is likely to change. HM Revenue and Customs (HMRC) will update your account once the amendment has been processed and let you know if you need to pay more tax or if you are due a refund.

How long do I have to amend a Self Assessment?

You can make an amendment to your Self Assessment within 12 months of the submission deadline (January 31^st). Amendments made within these 12 months are fairly easy to make.

It is possible to make a correction to a Self Assessment beyond the 12 month window, but the process is a little more complicated. 

The easiest rule to live by is to update your Self Assessment as quickly as possible if you’ve spotted a mistake. It’s not like the washing up or the recycling; it can’t wait until tomorrow, no matter how many times you tell yourself it can. 

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How to amend a Self Assessment within 12 months of filing

So, let’s say you’ve filed your 2024/25 tax return ahead of the January 2026 deadline – that means you’ve got until January 2027 to make any amends, if they’re required.

The process of amending a Self Assessment within this 12 month window is actually pretty simple:

1. Sign into your HMRC tax account.
2. Click on “Self Assessment account”.
3. Select “At a glance” on the left side of the screen.
4. Choose “Tax return options”.
5. Select the tax year you want to make amends for.
6. Click the tax return you’d like to amend.

From there, it’s as simple as making the changes you need to make, then re-submitting your newly-updated Self Assessment!

If you filed your Self Assessment through an accountant like Crunch, be sure to alert them at the nearest possible opportunity and let them know what needs changing. 

How to amend a Self Assessment 12 months after filing

It gets a little more complicated here, but hey, better safe than sorry, right?

Whether you filed your own Self Assessment, or hired an accountant to handle it for you, making an alteration to a Self Assessment that was submitted over 12 months ago requires a hand-written request.

In your letter, you’ll need to outline:

• Which Self Assessment needs amending.
• An explanation of the error that needs correcting.
• The reason why you believe you’ve over- or underpaid on tax for that tax year.
• How much you believe you’ve been over- or underpaid.

You’ll also need to sign your letter with your signature, not just your printed name.

When you’re done, send your letter and corrected tax return to:

Self Assessment

HM Revenue and Customs

BX9 1AS

United Kingdom

If you’re sending your letter by courier, you’ll need to use a different address.

What if I’m making a claim on a refund?

If you believe the mistake on your Self Assessment entitles you to a refund, your letter needs to include a few other details:

• You need to confirm that you’re making a claim because you’ve overpaid on tax for that tax year.
• You need to submit proof that you’ve already paid the tax bill in question.
• You need to outline how you wish to receive your refund (i.e. bank transfer, cheque, etc.) and the relevant details HMRC will need to pay you.
• You need to confirm that you haven’t already tried to claim this refund in the past.
• You need to submit a signed declaration, confirming everything you’ve said in your letter is correct to the best of your knowledge.

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When will I hear back from HMRC?

If you’ve made your amendment online, you can expect to see an update to your tax bill within three days.

If your amendments result in you owing more money, HMRC will provide you with a new deadline to pay the remainder of your bill. Generally speaking though, we’d recommend paying the new bill as soon as possible.

If your changes entitle you to a refund, you can head to the “Request payment” section of your account and request a refund right then and there – though it may take up to a month before the money lands in your account.

For those of you that submitted your requests by post, you can expect to receive a response within four weeks. If you owe more, HMRC will include instructions on the next steps; if you’re owed money, you can expect it to arrive with or fairly shortly after your letter, depending on whether you asked for a bank transfer or a cheque.

Important deadlines that you need to know as someone self employed

Registering as self employed (you do not need to do this every year) - October 5th

Paper tax returns - October 31st

Online tax returns - January 31st

Deadline for paying tax owed - January 31st**

** If you want any tax you owe to be automatically taken from your PAYE wage or from a pension, then you must submit your tax return by December 30ths. Please note that this payment method is only for those who are eligible. 

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How Crunch can help

An ounce of prevention is worth a pound of cure, especially when it comes to mistakes on your Self Assessment. So, if you want to avoid mistakes next year, talk to Crunch!

We can take the stress and complexity of Self Assessment submission of your hands for as little at £140+VAT. All you need to do is upload your documents to the software, and we’ll take care of the rest!

For more information, head over to our Pricing page, or give one of our friendly experts a call for a free consultation.