What happens if I make a mistake on my UK tax return?

Kirsty Young Personal Tax, Tax

Doing your own tax return can be pretty scary. There are hefty penalties if you get your tax return wrong, which is why we are writing this blog to help you avoid mistakes.

Who is ultimately responsible?

We do need to be clear on one thing from the start.

Even if you appoint someone to complete your tax return on your behalf (such as an accountant), you still remain responsible.

If there is an error on your tax return, it’ll be you that will be held responsible. Not your tax-savvy beer buddy, your accountant or your agent who submits your return. This is why it’s so important if you do ask someone else to prepare your returns, you are satisfied they are professional, competent and up to date!

As the Gov website guide for professionals “Penalties: an overview for agents and advisers” says:

“When you are acting on behalf of a client, they still have responsibility for their returns, calculations and payments. Your authorisation as an agent allows HMRC to deal with you on your client’s behalf, but any liability for penalties for late returns, late payments or any errors on paperwork legally remains with your client.”

In other words, the buck stops with you. Pretty surprising right?


What are the penalties for getting my tax return wrong?

If you make a mistake on your UK tax return and submit it, you can be charged a penalty by HMRC. The penalties can be pretty heavy-duty too, depending on the nature of the error. (Just to say, these penalties are on top of any penalties issued for not actually submitting your return on time.)

HMRC will impose penalties of an additional percentage of the extra tax due as a result of your tax return mistake. How much hinges on the intent behind the error:

  • A lack of reasonable care = between 0% and 30% of the extra tax due
  • A deliberate error = between 20% and 70% of the extra tax due
  • A deliberate and concealed error = between 30% and 100% of the extra tax due

Whether you are charged at the minimum or towards the higher end depends on whether:

  • You told HMRC about it (‘unprompted disclosure’)


  • HMRC told you about it (‘prompted disclosure’)

HMRC will decide on the exact percentage based on the facts.

The good news is that penalties are generally negotiable. The really good news is that if you’ve made a genuine mistake and you’ve taken ‘reasonable care’, there is unlikely to be a penalty. The challenge, of course, is proving that you took reasonable care…


How to tell HMRC about a mistake on your tax return

If you’ve noticed an error on your most recent tax return, the chances are you can just amend the return online.

Within 12 months of submission

You get 12 months from the filing deadline to amend your return online. For most people, their tax return submission deadline is 31st January each year. Presuming you spot the error in less than 12 months from the date where you had to file that last return by, you can log in online and amend the return.

Beyond 12 months from submission

If you are outside the deadline, then writing HMRC a letter is usually the best method. Make sure the letter is clear, concise, and includes your own ‘Unique Tax Reference’ number. Your letter should be posted using a trackable postage system such as Special Delivery to ensure it is “signed for”, so you have proof of delivery.


Wot no tax return?

As a side note, if you have income to tell HMRC about and have not done a tax return at all, you will need to contact them to get a registration to allow you to submit a tax return. As part of the government initiative Making Tax Digital, this usually involves setting up online access at the Government Gateway.

You can generally submit tax returns for the last 4 years in this way. Income going back further than this requires a different route outside the scope of this blog! Contact your accountant for advice or book a consultation with us.



For most errors on your most recent / last return, you can adjust the tax return online. The chances are there will be minimal (if any) penalties, outside the interest that would be charged on any overdue tax.

For more historic errors, you can write to HMRC.

If the penalties are down to carelessness, or HMRC have to come to you (‘prompt’ you) and say your return is wrong, the penalties can be pretty harsh.


Yikes this sounds scary!

It is – and it isn’t. What you need to ensure is that you and your accountant get it right first time. Ask your accountant to help you understand your accounts, so you know what you are reading and can spot errors more easily.

If you don’t have an accountant, or feel your current accountant hasn’t got a grip on your tax return the way you’d like, we’d love a chat about how we can help.