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We all mess up sometimes, especially when it comes to tax returns. Whether you’ve entered the wrong number, missed out an expense, or sent it off before spotting something odd, it’s not the end of the world.
In this post, we’ll walk you through how to fix mistakes on your tax return, what the deadlines are, what to do if you’re already past them, and how to avoid common errors in the future. No panic needed.
Can you fix your tax return after filing?
Yes, you can. If you submitted your Self Assessment online, you can change it up to 12 months after the original deadline. For the 2024 to 2025 tax year (which ended on 5 April 2025), that means you have until 31 January 2027 to make corrections.
If you filed a paper return, you’ll need to send an updated version by post before the same deadline. That’s another good reason to file online next time. It’s quicker, easier, and lets you fix mistakes in just a few clicks.
Common mistakes you can fix
You’re not alone. These are the sorts of things people regularly go back and correct:
Missing income, like freelance jobs or bank interest
Expenses that should have been claimed but were forgotten
Typos in numbers (adding or missing a zero is more common than you’d think)
Entering the wrong Unique Taxpayer Reference (UTR)
Claiming deductions you’re not entitled to
How to fix mistakes online
If you filed your return through your HMRC online account, follow these steps:
Log in to your HMRC account
Go to Self Assessment and then select More Self Assessment details
Choose Amend Self Assessment return
Make your changes and resubmit it
Once you’ve sent the correction, HMRC will send you a new calculation. If you’ve overpaid tax, you’ll get a refund. If you owe more, they’ll tell you what you need to pay and when.
What if the deadline has passed?
If the 12-month amendment window has closed, you can’t change your return through your account. But you’re not stuck.
You can write to HMRC and ask them to make a correction. In your letter, include:
The tax year the mistake relates to
What the mistake was
What the correct information should be
If the correction means you paid too much tax, HMRC might issue a refund. But they are not obliged to. You have four years from the end of the tax year to claim back overpaid tax.
For example, to claim a refund for the 2021 to 2022 tax year, you must contact HMRC by 5 April 2026.
What happens if HMRC finds the mistake?
Sometimes HMRC spots errors before you do. If they open an enquiry or send you a letter, don’t panic. If the mistake was accidental and you work with them quickly, you’ll usually just need to pay the correct amount, plus some interest if it’s overdue.
However, if HMRC thinks the mistake was careless or deliberate, you could face penalties. These depend on:
How serious the mistake was
Whether you told HMRC first
How helpful you are during the process
Being open and proactive goes a long way. It can even reduce or remove the penalty.
Tips to avoid tax return mistakes
Making a correction is fine, but let’s help you avoid needing to in the first place. Here are a few practical steps:
Keep clear records all year round. Tools like Xero make this easy and cut down on errors
Check your figures twice before you file. Small mistakes can make a big difference
Use an accountant or tax adviser if things get confusing
Look at your return with fresh eyes or ask someone else to review it before submitting
Final thoughts
Mistakes on your tax return are not the end of the world. HMRC understands that people make honest errors, and there are clear ways to fix things quickly and simply. The key is to act as soon as you spot the problem and be upfront about what happened.
If you’ve found something wrong on a return you’ve already filed, or you just want to make sure your next one is done right first time, we’re here to help. We’ll sort the numbers, deal with HMRC, and make sure you claim everything you’re entitled to.
Need help fixing a tax return or getting set up properly for next year? Get in touch today.
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