Correcting Your Tax Return

Amending Within 12 Months

You can amend your Self Assessment return at any time within 12 months of the filing deadline. For example, a 2025/26 return (deadline 31 January 2027) can be amended until 31 January 2028.^[1]

To amend online:

Sign in to your HMRC online account
Go to your Self Assessment return for the relevant year
Select “Amend your return”
Change the relevant figures
Submit the amended return

HMRC will recalculate your tax. If you owe more, you’ll need to pay the difference plus any interest. If you’ve overpaid, you can claim a refund. Filing with HMRC-recognised software in the first place reduces the chance of arithmetic and wrong-box errors.

After 12 Months

Once the 12-month amendment window has closed, you can’t amend online. However, if you’ve overpaid tax due to an error, you can write to HMRC to claim overpayment relief. This must be done within 4 years of the end of the relevant tax year.^[1]

Common Mistakes

The most frequent errors HMRC sees include:^[2]

Entering figures in the wrong box (e.g. turnover instead of profit)
Missing income — forgetting bank interest, a second employment, or dividend payments
Claiming wrong expenses — personal costs mixed with business costs
Using the wrong UTR or mixing up personal and business references
Arithmetic errors — less common with online filing, but can happen with manual entries

What If HMRC Spots the Mistake?

HMRC can open a compliance check (formerly called an enquiry) into your return. They have 12 months from the date you filed to open a check into a return filed on time. For late returns, they have 12 months from the date you actually filed.^[2]

If HMRC discovers errors after the normal enquiry window, they can issue a discovery assessment — going back up to 4 years for careless errors, 6 years for deliberate errors, and 20 years for fraud.

Penalties for Errors

HMRC classifies errors by severity:^[4]

Type of Error	Unprompted Penalty	Prompted Penalty
Reasonable care taken	No penalty	No penalty
Careless	0–30%	15–30%
Deliberate	20–70%	35–70%
Deliberate and concealed	30–100%	50–100%

“Unprompted” means you told HMRC about the error yourself. “Prompted” means HMRC found it. Telling HMRC voluntarily always results in a lower penalty.

Found an error? Correct it as soon as possible. If you tell HMRC before they find it, any penalty will be significantly reduced — and in many cases, there’ll be no penalty at all.

Frequently Asked Questions

How long do I have to amend my tax return?

You can amend your Self Assessment return within 12 months of the filing deadline. For example, a 2025/26 return filed by 31 January 2027 can be amended until 31 January 2028.

Can I correct my tax return after 12 months?

You cannot amend online after 12 months, but if you overpaid tax due to an error you can write to HMRC to claim overpayment relief within 4 years of the end of the relevant tax year.

What are the penalties for errors on a tax return?

There is no penalty if you took reasonable care. Careless errors attract penalties of 0–30%, deliberate errors 20–70%, and deliberate and concealed errors 30–100%. Telling HMRC voluntarily always results in a lower penalty.

How far back can HMRC investigate a tax return?

HMRC can go back 4 years for careless errors, 6 years for deliberate errors, and 20 years for fraud. For returns filed on time, HMRC has 12 months from the filing date to open a standard compliance check.

Sources

Self Assessment tax returns: corrections — GOV.UK
Self Assessment tax returns — GOV.UK
Complain about HMRC — GOV.UK
HMRC penalties for errors — GOV.UK
Estimate your Income Tax for the current year — GOV.UK

How to Correct Mistakes on Your Tax Return