Can I amend my Self Assessment after submitting? (2026/27 Guide)

Taxpayers frequently identify omitted expenses or income variations after they file their tax returns. To answer the question—can I amend my Self Assessment after submitting—the UK tax system provides a clear statutory mechanism for you to correct mistakes. However, for the 2026/27 tax year, the method of amendment depends heavily on whether you fall under traditional Self Assessment rules or the new Making Tax Digital (MTD) regime.

Below is an exhaustive breakdown of the taxpayer’s rights, HMRC’s reciprocal correction powers, and the penalty implications for self-correcting a return.

The Taxpayer’s Right: The 12-Month Amendment Window

For traditional filings, you possess a statutory right to amend your Self Assessment return. Under section 9ZA of the Taxes Management Act 1970 (TMA 1970), a person may amend their personal or trustee return by providing notice to an officer of the Board (HMRC).

The legislation imposes a strict time limit: an amendment may not be made more than 12 months after the filing date.

• Application to 2026/27: The standard electronic filing deadline for the 2026/27 tax year is 31 January 2028. Therefore, you have until 31 January 2029 to freely amend your return.
• Outcome: Making a valid amendment within this 12-month window formally overrides the original self-assessment.

Correcting Making Tax Digital (MTD) Updates (2026/27)

From 6 April 2026, sole traders and landlords with qualifying income over £50,000 must transition to Making Tax Digital for Income Tax Self Assessment (MTD for ITSA). This fundamentally alters how you correct your tax records, as you must maintain digital records and submit quarterly updates rather than a single annual return.

If you make a mistake when creating a digital record, or forget to record income or expenses, you must correct it in your MTD-compatible software.

• In-Year Corrections: If you discover and correct the error during the tax year, the software automatically factors the fix into your next quarterly update to HMRC.
• Post-Quarter 4 Corrections: If you have already sent your fourth quarterly update but have not yet finalised your tax position for the year, you can choose to either correct the digital record and resend the fourth update, or adjust the category totals directly in your software.

HMRC’s Reciprocal Power to Correct (The Revenue)

While you have 12 months to amend your return, HMRC (The Revenue) holds a corresponding, albeit narrower, statutory power to correct your submission.

Under section 9ZB of the TMA 1970, the Revenue may correct obvious errors in your return.  This power is generally reserved for arithmetical mistakes or obvious errors of principle (such as entering net figures where gross figures are required). HMRC can only execute this correction within 9 months of the date the return was delivered. If HMRC issues a correction notice, you have the right to reject it within 30 days, causing the correction to have no effect.

Missed the Deadline? Overpayment Relief Claims

If you discover an error after the 12-month section 9ZA amendment window has closed, you can no longer amend the Self Assessment return directly. However, if the error means you have paid too much tax, you can file a standalone claim for relief.

Under the UK tax code, claims for relief concerning the amount of a loss or adjustments for a tax year must generally be made not more than 4 years after the end of the relevant tax year.

• Application: For a 2026/27 error discovered in February 2029 (after the amendment window closes), you have until 5 April 2031 (four years from the end of the 26/27 tax year) to submit an overpayment relief claim to HMRC to recover the excess tax paid.

Penalties and Unprompted Disclosure

Correcting an error on your tax return is not just a matter of compliance; it is a critical strategy for penalty mitigation. If your original Self Assessment understated your tax liability, you risk financial penalties for submitting an inaccurate return.

Under Schedule 24 to the Finance Act 2007, HMRC must reduce the standard percentage of any inaccuracy penalty to reflect the quality of a taxpayer’s disclosure.

• Unprompted Disclosure: If you voluntarily amend your return to disclose the inaccuracy before HMRC discovers it (or announces an enquiry), the disclosure is “unprompted.” Paragraph 9 of Schedule 24 provides for specific reductions in penalties where a person discloses an inaccuracy.
• Outcome: If the original error was “careless,” an unprompted disclosure can reduce the penalty to 0%. If the error was “deliberate,” an unprompted disclosure still guarantees a substantially lower minimum penalty than if HMRC discovers the error themselves.

Summary Table: Amendment Rules and Outcomes

Next steps for research: Review the specific criteria HMRC uses to define “careless” versus “deliberate” inaccuracies under Schedule 24 FA 2007, and verify the precise procedural steps for submitting a 4-year overpayment relief claim via the digital Government Gateway portal.