What Happens If You Make a Mistake on Your Tax Return?
Submitting your Self Assessment tax return and then realising something is wrong can be worrying.
Perhaps you forgot some income, entered an expense incorrectly, used the wrong figure from a P60 or simply typed a number into the wrong box.
A mistake does not automatically mean you will receive a penalty.
HMRC distinguishes between genuine mistakes where reasonable care was taken and inaccuracies caused by careless or deliberate behaviour. HMRC states that where someone took reasonable care but their return was still incorrect, it will not charge an inaccuracy penalty. GOV.UK
The important thing is to deal with an error once you discover it.
Can you change a Self Assessment tax return after submitting it?
Yes.
Submitting your return does not permanently lock every figure.
HMRC allows taxpayers to amend a Self Assessment return after it has been submitted. Under the Self Assessment rules, you can generally amend your return within 12 months of the statutory filing deadline. GOV.UK
For a 2025/26 online Self Assessment return, the normal filing deadline is 31 January 2027.
That generally means you can amend that return until 31 January 2028.
If you discover the problem after the amendment window has closed, different procedures may be required.
What sort of mistakes can be corrected?
There are plenty of reasons a submitted return might need changing.
Common examples include:
- Forgetting some self-employed income
- Missing rental income
- Omitting savings interest
- Forgetting dividends
- Entering an incorrect P60 figure
- Claiming the wrong business expense
- Missing an allowable expense
- Using the wrong pension contribution figure
- Entering numbers in the wrong section
- Accidentally entering the same income twice
Some errors increase your tax bill.
Others can mean you’ve actually paid too much tax.
Either way, correcting the return ensures HMRC has the appropriate information.
Will HMRC fine you for making a genuine mistake?
Not necessarily.
HMRC says everyone has a responsibility to take reasonable care over their tax affairs.
But reasonable care does not mean that no human error can ever occur.
HMRC’s own guidance gives the example of somebody accidentally transposing figures on their Self Assessment return as a situation that can amount to an inaccuracy despite reasonable care. GOV.UK
Where reasonable care was taken, HMRC says no inaccuracy penalty is due. GOV.UK
That is very different from knowingly submitting incorrect information.
What does HMRC mean by “reasonable care”?
There isn’t one identical standard for every taxpayer.
HMRC says reasonable care depends on the individual’s abilities and circumstances. GOV.UK
Practical examples of taking reasonable care can include:
- Keeping adequate financial records
- Checking figures before filing
- Using the correct documents
- Asking HMRC when you’re uncertain
- Seeking advice from a suitably qualified tax adviser
HMRC specifically identifies keeping accurate records and seeking appropriate advice where necessary as ways taxpayers can demonstrate reasonable care. GOV.UK
What if the mistake was careless?
The position becomes different where HMRC considers an inaccuracy to have resulted from a failure to take reasonable care.
HMRC may charge an inaccuracy penalty where an incorrect return results in tax being:
- Unpaid
- Understated
- Over-claimed
and the underlying behaviour was careless or deliberate. GOV.UK
The penalty is linked to the additional tax resulting from correcting the error rather than simply being a fixed charge for making any mistake.
Genuine mistake vs careless mistake
This distinction matters.
Imagine you carefully prepare your return using your records but accidentally transpose two digits in a figure.
That could potentially be an innocent error despite taking reasonable care.
Now imagine you have several bank accounts but make no attempt to check any of them before estimating your taxable income.
HMRC could potentially take a different view of whether reasonable care was taken.
The circumstances surrounding the mistake matter.
What if you deliberately leave income off?
That is much more serious.
HMRC distinguishes between:
Careless inaccuracies
Deliberate inaccuracies
Deliberate and concealed inaccuracies
A deliberate inaccuracy occurs where you knew information was wrong when you submitted it.
A deliberate and concealed inaccuracy goes further and involves actively trying to hide the incorrect position. HMRC gives deliberately understating income or overstating business expenses as examples of deliberate inaccuracies. GOV.UK
These behaviours can result in substantially more serious penalties than an innocent mistake.
Correct mistakes when you find them
Suppose you submit your return in October.
Then in December you discover a savings account you forgot to include.
Don’t simply ignore it because the return has already been filed.
HMRC’s guidance emphasises the importance of putting inaccuracies right. HMRC also distinguishes between taxpayers who voluntarily disclose an error before HMRC discovers it and those who only disclose after HMRC has identified the issue. GOV.UK
Correcting an error promptly is therefore important.
Telling HMRC yourself can matter
HMRC describes a disclosure made before you have reason to believe it has discovered or is about to discover the error as an unprompted disclosure.
A disclosure made after that point is generally considered prompted.
HMRC says the minimum penalty for an unprompted disclosure is lower than the minimum for a prompted disclosure where a penalty applies. GOV.UK
That provides another reason not to leave a known mistake sitting unresolved.
What if the correction means you owe more tax?
Correcting your return may change the tax calculation.
For example:
Original tax liability: £4,200
Corrected tax liability: £4,850
The amendment has created an additional £650 liability.
You would then need to deal with the additional amount due.
Depending on when the correction is made, interest or late-payment consequences can also become relevant if tax should already have been paid.
HMRC charges interest on Self Assessment tax that is paid late. GOV.UK
What if the mistake means you paid too much?
Errors can work in the opposite direction.
Perhaps you:
- Forgot an allowable expense
- Entered income twice
- Used an incorrect employment figure
- Missed an eligible relief
Correcting the return could reduce your tax liability.
Where the normal amendment deadline has already passed, HMRC’s rules also provide an overpayment relief process in qualifying circumstances, generally subject to a four-year time limit after the end of the relevant tax year. GOV.UK
So don’t assume an old overpayment is automatically lost — but there are specific rules and deadlines.
Can HMRC correct your return itself?
In some circumstances, yes.
HMRC has powers to correct certain obvious errors or omissions in a return.
Its Self Assessment guidance explains that an HMRC officer can make corrections within nine months after the completed return was delivered, including certain arithmetic errors or other issues it believes are incorrect based on the information available. GOV.UK
However, you should not rely on HMRC to spot every mistake for you.
Responsibility for providing accurate information remains with the taxpayer.
What if HMRC changes something you think was correct?
An HMRC correction does not necessarily mean you must simply accept it.
HMRC’s internal Self Assessment guidance says a taxpayer can reject an HMRC correction by giving notice within 30 days of receiving it. HMRC would then need to pursue the matter through the appropriate enquiry process if it wished to continue. GOV.UK
If you receive an unexpected correction, check the underlying figures before responding.
Using an accountant doesn’t remove your responsibility
An accountant can prepare and submit the return on your behalf, but you still need to provide complete and accurate information.
HMRC explicitly states that where you use a tax adviser, it remains your responsibility to provide them with accurate and complete information. GOV.UK
For example, your accountant cannot report a savings account you never tell them exists.
Before approving your return, check that all your relevant income and information have been included.
How can you reduce the risk of mistakes?
A large proportion of Self Assessment problems can be prevented with better records.
Before preparing your return, gather relevant documents such as:
- P60s and P45s
- Self-employment income records
- Business expense records
- Rental statements
- Bank-interest statements
- Dividend vouchers
- Pension information
- Relevant tax-relief information
- Details of taxable benefits
- Capital-gains information where applicable
Then compare the completed return against your records before submitting it.
HMRC specifically expects taxpayers to maintain records that allow them to prepare accurate returns. GOV.UK
Don’t rush your return in January
This is another reason completing Self Assessment earlier can be useful.
If you start preparing your return months before the deadline, you have more time to:
- Find missing records
- Query unusual figures
- Contact banks or investment providers
- Check expenses
- Review previous information
- Ask your accountant questions
- Correct problems before the filing deadline
Trying to reconstruct an entire tax year immediately before 31 January increases the opportunity for information to be missed.
Frequently Asked Questions
Can I correct my Self Assessment after submitting it?
Yes. You can generally amend your return within 12 months following the statutory filing deadline. GOV.UK
Will HMRC fine me for an accidental mistake?
Not automatically. HMRC says no inaccuracy penalty is due where you took reasonable care but the return was still incorrect. GOV.UK
What happens if my mistake means I owe more tax?
Correcting the return can increase the amount payable. If the tax has become overdue, interest and potentially late-payment consequences may also need to be considered. GOV.UK
Should I correct a small mistake?
If information on your return is wrong, it should be corrected appropriately rather than knowingly left inaccurate.
Is deliberately leaving income off different from making a mistake?
Yes. HMRC distinguishes between careless, deliberate and deliberate-and-concealed inaccuracies, with deliberate behaviour treated more seriously. GOV.UK
What if I discover the mistake after the amendment deadline?
Different procedures may be available depending on whether the correction increases or reduces your tax liability. For example, qualifying overpayments can potentially be addressed through overpayment relief, subject to its conditions and time limits. GOV.UK
Final thoughts
A typo or genuine mistake on a tax return does not automatically put you in serious trouble with HMRC.
What matters is why the error happened and what you do when you discover it.
If you took reasonable care but still made a genuine mistake, HMRC’s guidance says an inaccuracy penalty should not apply.
If you discover something is wrong, however, don’t knowingly leave the return inaccurate.
At Merranti Accounting, we can review your Self Assessment, identify errors or missing information, make appropriate amendments and help establish the correct tax position.
Written by Jason Fudge
Managing Director, Merranti Accounting
Jason is Managing Director and co-founder of Merranti Accounting. He works with business owners on financial management, commercial strategy, business planning and the financial decisions involved in running and growing a business.
