How to Correct Past Self Assessment Returns: A Practical Guide

Why You Might Need to Correct a Past Return

Mistakes happen. You might have forgotten to include some freelance income, claimed an expense you weren't entitled to, or simply entered a figure incorrectly. The good news is that HMRC provides a formal process for correcting errors. The bad news is that deadlines are strict, and getting it wrong can be costly.

Amending Your Return Within the Deadline

If you need to correct a Self Assessment return, the easiest route is to amend it yourself. You have 12 months from the 31 January filing deadline of the relevant tax year to make changes online via your HMRC Personal Tax Account or using commercial software.

For example, if you want to amend your 2023/24 return (filed by 31 January 2025), you have until 31 January 2026 to do so. As of today, 8 July 2026, that window has now closed for the 2023/24 year.

• Log in to your HMRC online account and select the relevant tax year
• Make the corrections and resubmit — HMRC will recalculate your tax automatically
• If you owe more tax, a new payment will become due; if you overpaid, you can request a repayment
• Keep a clear record of what you changed and why

What If the 12-Month Window Has Passed?

Once the amendment window closes, you can no longer change the return yourself online. Instead, you must write to HMRC explaining the error and requesting an overpayment relief claim (if you overpaid) or making a voluntary disclosure (if you underpaid).

For overpayments, you can claim back tax going back four years from the end of the relevant tax year — so as of July 2026, you can claim back to the 2022/23 tax year. Use form R40 or write directly to HMRC's Self Assessment office with full details.

For underpayments discovered after the deadline, making a voluntary disclosure promptly is strongly advisable. HMRC treats unprompted disclosures more favourably when calculating penalties.

Discovery Assessments: When HMRC Finds the Error First

If HMRC identifies an error or omission before you disclose it, they may raise a discovery assessment under Section 29 of the Taxes Management Act 1970. This allows HMRC to go back and collect unpaid tax outside the normal enquiry window.

• Four years from the end of the tax year for innocent mistakes
• Six years for careless errors (for example, not keeping proper records)
• Twenty years for deliberate underreporting or fraud

A discovery assessment means HMRC believes they have found something new — information that was not reasonably available to them when your return was originally processed. If you disagree with their assessment, you have 30 days to appeal in writing, or 30 days to request a review.

Penalties and Interest

Whether you self-correct or HMRC discovers an error, you may face penalties and interest on unpaid tax. Penalty rates depend on the behaviour involved:

• Unprompted disclosure of a careless error: 0–30% of the unpaid tax
• Prompted disclosure of a careless error: 15–30%
• Deliberate errors: up to 70% unprompted, 100% prompted

Interest is charged at the current HMRC late payment rate from the original due date. Acting quickly and voluntarily always reduces your exposure.

Practical Steps to Take Now

If you suspect an error on any past return, do not wait for HMRC to come to you. Review your records, quantify the discrepancy, and either amend online if within the window or write to HMRC promptly. Consider speaking to a qualified accountant before making contact, especially if the amounts involved are significant or the error spans multiple years.

Keeping thorough records — invoices, bank statements, and expense receipts — for at least six years is your best defence against an unexpected discovery assessment.