Self Assessment amendment: how do you correct a UK return?

A Self Assessment amendment lets you correct a UK tax return after submitting it. The usual window runs for 12 months after the Self Assessment deadline, rather than from the day you filed. The route depends on how you submitted the return, and a correction can increase or reduce your tax bill.

• Check the tax year and amendment deadline before changing anything.
• Use the correction route for an online return, paper return or commercial software.
• Support the revised figures with records rather than estimates made to match an expected bill.
• Review the updated statement, payment deadline and payments on account after filing again.

When can you make a Self Assessment amendment?

According to HMRC guidance checked on 24 October 2026, the ordinary correction period lasts 12 months after the Self Assessment deadline. Filing early does not start a separate amendment clock based on your submission date. This distinction matters when an old return still appears in your account.

HMRC gives the example of the 2024 to 2025 tax year, for which the usual amendment deadline is 31 January 2027. Check the applicable deadline for your own return rather than carrying that example across to another year. The date on a saved document can refer to when it was downloaded, not the tax period it covers.

A correction window is not an extension for submitting the original return or paying tax. For the separate filing decision, see the guide to UK paper tax return deadlines and filing options. Late filing and late payment have their own consequences.

What should you check before correcting the figures?

Start with a copy of the submitted return and identify the entry that needs changing. Note the original figure, the replacement and the document explaining the difference. This creates a clear record if you later need to explain the correction to HMRC or an accountant.

HMRC’s Self Assessment overview says records such as receipts and bank statements help taxpayers complete accurate returns. Depending on the entry, relevant evidence might include an employer’s statement, an interest statement or a pension contribution record. A bank transaction alone may not explain how an amount belongs on the return.

• Confirm that the evidence relates to the tax year being amended.
• Check whether an amount was omitted, duplicated or entered in the wrong section.
• Review connected entries rather than changing only the final total.
• Keep the explanation and supporting documents alongside the original submission.

If the issue concerns contributions, the guide to checking UK pension tax relief before claiming extra explains why the contribution method matters. A lower bill is not, by itself, evidence that a revised claim is correct.

How do you amend an online or paper return?

Returns filed through HMRC online services

HMRC requires a wait of 3 days, or 72 hours, after filing before an online return can be updated. Once that period has passed, enter your tax account independently rather than following a link in an unexpected message. Locate the Self Assessment area and the return options for the relevant year.

Open that year’s return, correct the entries and submit it again. Merely editing a figure is not the final filing step. Keep the revised return and any submission confirmation so that you can distinguish the amendment from your original filing.

Returns filed on paper

HMRC’s paper process uses corrected return pages. Mark each affected page as an amendment and include your name and Unique Taxpayer Reference, commonly called a UTR. Send the pages to the address shown on your Self Assessment correspondence.

Check that any supplementary pages affected by the correction are included. Keep copies of what you send and a record of posting. A revised paper return does not produce the immediate online bill update available through HMRC’s online amendment process.

Returns filed using commercial software

Contact the software provider about its amendment procedure rather than assuming the HMRC website can replace it. HMRC says to contact the department if the software cannot make corrections. If an accountant filed for you, agree who will submit the change to avoid conflicting versions.

How can a correction change the amount you owe?

An amendment may reveal extra tax, reduce the liability or leave the overall bill unchanged. According to HMRC, an online correction produces a revised bill immediately, while the statement updates within 3 days. The statement shows the difference from the previous bill and any interest.

If more tax is payable, the updated bill also explains the payment deadline and the effect on payments on account. These are advance payments towards a later bill. Do not assume that correcting a past entry affects only one balance displayed in your account.

After a paper amendment, HMRC sends an updated bill. Check the revised position before treating a lower liability as money available to spend; a tax calculation and money received are different things. Keep payment records with the amendment paperwork.

For cash-flow planning, a household bill audit can help separate regular commitments from an additional tax payment. That budgeting exercise does not alter HMRC’s payment terms or resolve a disputed calculation.

What if the ordinary amendment deadline has passed?

HMRC directs taxpayers to write to it when a return needs correcting outside the normal amendment window. This can involve reporting income left out of an earlier return or seeking overpayment relief. The fact that the online amendment route has closed does not settle the underlying tax position.

Identify the tax year, explain the error and organise the evidence before contacting HMRC. Overpayment relief is a separate claim with its own requirements, not an automatic reopening of the return. Where the position involves several years or uncertain tax treatment, professional help may be useful.

Informational content only, not financial advice.