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5 Min. Read

Can You Change Your Tax Return After Filing It With HMRC?

Can You Change Your Tax Return After Filing It With HMRC?

You are able to make changes to your self-assessment tax return after you've already filed it with HMRC. How you do this depends on HMRC's time limits and how you originally filed it.

Here’s What We’ll Cover:

Is There a Deadline for Making Changes to My Self-Assessment Tax Return?

How Do I Make Changes to My Tax Return?

I Need to Write to HMRC, What Do I Include?

Will I Need to Pay More Tax if I Change My Tax Return?

Is There a Deadline for Making Changes to My Self-Assessment Tax Return?

Yes, there are deadlines for making changes to your self-assessment tax return. This is so that your tax bill or tax relief is updated according to your new information. HMRC maintains deadlines for amendments so that the whole process stays aligned.

If you need to make a change to your 2019-20 self-assessment tax return, you need to do it by 31st January 2022. And the deadline for the following 2021-21 tax year is January 31st 2023. Missing these deadlines means that you'll need to write to HMRC, you won't be able to amend through your online tax account.

If you happen to notice mistakes in a previous 'closed' year's self-assessment tax return, you'll also need to write directly to HMRC to get that sorted out. The list of things you need to include is detailed below.

How Do I Make Changes to My Tax Return?

The way you make any changes to a filed tax return depends on how you initially submitted it. UK taxpayers have 2 tax return options - on paper tax returns and digital self-assessment returns.

Paper Returns

Submitting a paper tax return is increasingly uncommon, but HMRC still accepts them. Your deadline was earlier, the 31st October after the end of the tax year, rather than the 31st January online filing deadline. If you then discover a mistake that you need to correct, you'll need to download a whole new tax return. Correct the necessary pages and write 'amendment' at the top of each new page. Make sure you put your Unique Taxpayer Reference (UTR) number and full name with your accompanying letter.

Send your amended paper return to the same address as your other self-assessment paperwork. If you've lost this, address it to the general self-assessment inquiries team at:

Self Assessment

HM Revenue and Customs


United Kingdom

Online Returns

If you've made a mistake on your tax return online, you can fix it online as long as you're within the deadlines above. You'll need to go into your self-assessment account, through your Government Gateway account. Because you filed your return online, you already have your Government Gateway user ID and password. So there's no waiting for anything to come through the post again.

Then it's just a few clicks to make your amendments.

  • Log into your Government Gateway account
  • Click 'Your Tax Account'
  • Choose 'Self Assessment account'
  • Go to 'At a glance' in the left-hand menu
  • Click on 'Tax return options'
  • Pick the relevant tax year for the return you want to change
  • Make your changes
  • Refile your amended return

Once you know where to start, HMRC has developed a very intuitive process that's explained and labelled in plain English.

I Need to Write to HMRC, What Do I Include?

If you've made a mistake in tax returns that are before 2019-20, then you need to write to HMRC. Even if it's your 2018-19 tax return and you submitted it online. Because your tax bill was settled, these are considered to be 'closed' tax years. So any subsequent amendments need to be looked at carefully and HMRC prefers your application to alter any self-assessment details to be in writing.

By definition, changing your tax return means that you'll either owe HMRC more tax, or you'll be owed a refund from HMRC. So it's important to clearly state why you want to make an amendment. Your letter should also include:

  • The exact financial year you want to correct
  • Why you think you haven't paid enough tax, or why you think you paid too much
  • Calculation of how much you think you've underpaid or are owed

If you're also including a tax refund state that you're claiming 'overpayment relief'. Remember that these can be backdated for 4 tax years. You need to prove that during the relevant tax year you paid tax through the self-assessment process. In your letter you should also make it clear that you haven't submitted a prior claim for this tax refund and tell HMRC how you'd like them to repay you.

Finally, your letter needs to have a formal declaration of integrity that says, "the details you've given are correct and complete to the best of your knowledge".

Will I Need to Pay More Tax if I Change My Tax Return?

You might need to pay more tax, depending on what new information you're giving to HMRC. If you forgot to declare part of your income and you're now making that amendment, then your income tax bill will go up. But it also means that you're telling HMRC and being proactive about correcting your mistake. Which means you're likely to avoid any penalties that you can get if HMRC discover undeclared income themselves.

But if your amended return gives HMRC new information which helps you claim a refund, then you'll be due a nice cheque in the mail, or a reduction of your next tax bill.

The most important thing is to be completely honest with HMRC, pay your tax liabilities in full and get any tax refunds you're eligible for. Tax efficiency is what everyone's aiming for - including FreshBooks. Use our capacity for digital record-keeping to keep you right from now on. Then you'll never need to worry about amending a tax return again!