Spotting a mistake on a tax return you’ve already submitted is more common than you might think — a missed expense, an omitted source of income, or a figure that simply doesn’t add up. The important thing is what you do next, because HMRC treats a self-corrected error very differently from one it uncovers itself.
Not sure whether you’re still within the amendment window, or how to approach HMRC about an older error? Book a free 15-minute consultation with Felix Accountants and we’ll help you work out the best next step.
Step One: Work Out Which Deadline Applies
How you correct the error depends on how long ago you filed the return in question.
Within 12 Months of the Filing Deadline
If you’re within 12 months of the normal Self Assessment filing deadline for that return, you can simply amend it yourself. For an online return, log back into your HMRC account, update the relevant figures, and resubmit — the system recalculates your bill automatically, showing whether you owe more or are due a refund. For example, a 2024/25 return filed by the 31 January 2026 deadline can generally be amended up to 31 January 2027.
More Than 12 Months After the Deadline
Once the 12-month window has closed, you can no longer amend the return online. Instead, you’ll need to write to HMRC explaining the correction, or make a formal overpayment relief claim if the error means you paid too much tax. Overpayment relief claims can generally be made up to four years after the end of the tax year the return relates to.
What to Include in a Written Correction or Overpayment Relief Claim
- The tax year the correction relates to
- A clear explanation of what was wrong and why
- The amount you believe was overpaid or underpaid
- Supporting evidence (invoices, statements, calculations)
- A signed declaration confirming the details are correct and complete to the best of your knowledge
If the Error Means You Owe More Tax
If correcting the mistake increases your tax bill, it’s best to notify HMRC and pay the difference as soon as you’re aware of it. Interest accrues from the original due date, and coming forward yourself, before HMRC identifies the discrepancy independently, generally puts you in a much stronger position on penalties than waiting to be caught out. This is the same underlying principle behind voluntary disclosure routes like the Let Property Campaign for landlords with undeclared rental income specifically.
If the Error Means You Overpaid
If you’re due a refund, amending within the 12-month window is the most straightforward route — HMRC recalculates your position and processes the repayment. Outside that window, an overpayment relief claim achieves the same result but requires a more formal written submission with supporting evidence.
Genuine Mistakes vs Careless or Deliberate Errors
HMRC distinguishes between an honest, reasonable mistake and one caused by carelessness or deliberate action, and this distinction affects whether a penalty applies at all. If you took reasonable care and made a genuine error in good faith, you’re unlikely to face a penalty for correcting it — HMRC generally responds far more favourably to taxpayers who put things right themselves. If the error was significant, spanned several years, or involved undeclared income you knew about, professional advice is worth getting before you approach HMRC, since the correct classification affects both the penalty and how many years need correcting.
What If HMRC Corrects the Error First?
HMRC can amend a return itself within nine months of the date you filed it, typically to correct obvious errors, and will notify you of any change. If HMRC identifies a more significant discrepancy through a compliance check, the process moves from a simple correction into a formal enquiry, and the potential penalties for the same underlying mistake are usually higher than if you’d corrected it proactively.
A Practical Example
Suppose you filed your 2023/24 return in January 2025 and later realise, in mid-2026, that you forgot to include some rental income. Since more than 12 months have passed since the 31 January 2025 deadline, you can’t amend the return online — you’d need to write to HMRC, or, if the omission relates to rental income specifically, consider whether the Let Property Campaign disclosure process is the more appropriate route, since it’s specifically designed for this kind of correction.
How Felix Accountants Can Help
Whether you’re inside the 12-month amendment window or need to make a formal overpayment relief claim or voluntary disclosure, we’ll help you work out the right process, prepare accurate figures, and manage the correspondence with HMRC on your behalf.
Frequently Asked Questions
How long do I have to amend a Self Assessment tax return?
You generally have 12 months from the normal filing deadline for that tax year. For example, a return with a 31 January 2027 deadline can usually be amended up to 31 January 2028.
What is overpayment relief?
Overpayment relief is a formal claim you can make to recover tax you’ve overpaid, once the standard 12-month amendment window has passed. It must generally be made within four years of the end of the relevant tax year and requires a written submission with supporting evidence.
Will I be penalised for correcting my own mistake?
If the error was a genuine mistake made despite taking reasonable care, a penalty is unlikely. Penalties are more commonly applied where HMRC considers the error careless or deliberate, and coming forward yourself before HMRC identifies the issue generally results in a lower penalty than waiting.
Can HMRC change my tax return without telling me?
HMRC can make certain corrections within nine months of your filing date, but it will notify you of any change made. Anything beyond a simple correction, such as a discrepancy found through a compliance check, involves a more formal process where you’re kept informed and can respond.
