How far back can I correct a tax return?

At some point almost every taxpayer wonders whether they can go back and fix an old tax return. Maybe you forgot to claim an allowance, missed a source of income, entered the wrong figures or have only just discovered an error. In my opinion this is far more common than people realise and the important thing is not whether you made a mistake but how quickly you deal with it. HMRC understands that errors happen, but there are strict rules on how far back you can amend a tax return and different time limits depending on whether you are correcting an honest mistake or disclosing undeclared income.

This guide explains how far back you can correct a Self Assessment tax return, how the time limits work, what happens when those limits have passed, when you need to make a formal disclosure and how HMRC treats late corrections. I also explain the difference between amending a return and making a disclosure as the two are often confused. By the end you should have a clear understanding of your options, whatever the age of the tax year involved.

The simple rule for amending a submitted tax return

HMRC allows you to amend your Self Assessment tax return for up to 12 months after the statutory filing deadline. That means you can correct mistakes for a full year after the point at which the return was due, not a year after you originally submitted it. This matters because the statutory deadline is always 31 January following the end of the tax year.

For example, if you are dealing with your 2022 to 2023 tax return, the filing deadline was 31 January 2024, which means you can amend that return up to 31 January 2025. Within that period you can log into your personal tax account, edit the figures and resubmit without needing to give a reason. HMRC treats this as a normal amendment.

In my opinion this 12 month window is generous enough for most accidental errors, especially for people who prepare their own returns and only later notice an overlooked allowance or miscalculation.

What happens once the 12 month amendment window has passed?

Once the amendment window closes you cannot simply log in and edit your tax return. The return becomes final in the normal sense, but that does not mean you are unable to correct mistakes. It simply means you must use a different process.

If you realise you have overpaid tax you can submit an overpayment relief claim. If you realise you have underpaid tax you must make a disclosure to HMRC as soon as possible. These are two separate mechanisms and the one you use depends on the type of mistake and whether any tax is due.

In my experience this is the point at which people become anxious because they worry that going back several years will trigger penalties. In reality HMRC is generally fair when you approach them proactively and provide a full explanation.

How far back can you correct an honest mistake that caused you to overpay tax?

If you believe you paid too much tax in a past year you can make an overpayment relief claim for up to four years after the end of the tax year. This is not the same as the 12 month amendment window. It is a separate statutory rule that allows you to reclaim tax if you can prove you should not have paid it.

For example, the 2020 to 2021 tax year ended on 5 April 2021. You have until 5 April 2025 to claim overpayment relief for that year. The same logic applies for other years. The claim must be made in writing, must explain why you overpaid tax and must be supported by evidence. HMRC will then review the claim.

In my opinion four years is a generous timeframe and often allows taxpayers to recover missed reliefs such as pension contributions, trading losses or incorrect capital gains calculations.

How far back can you correct a return if you underpaid tax?

If you underpaid tax because of an error or omission, HMRC expects you to disclose it as soon as you realise. The time limits here are different because HMRC can go back further depending on the nature of the mistake.

HMRC can assess tax going back:

• up to four years for a simple mistake

• up to six years if the error was careless

• up to twenty years if the error was deliberate

• and potentially further for offshore matters

These time limits do not mean you can wait that long. They simply describe how much history HMRC is legally allowed to review. If you know you underpaid tax you are expected to correct it immediately. The right route is either amending the return (if still within the 12 month period) or using HMRC’s Digital Disclosure Facility.

In my opinion the biggest advantage of disclosing voluntarily is that penalties are significantly reduced compared with waiting for HMRC to find the error through their systems or data matching.

What if you missed declaring property income or self-employment income years ago?

Undeclared income such as foreign earnings, rental income or small self employed profits must be disclosed regardless of how long ago they arose. The Let Property Campaign is available specifically for landlords and is usually the best route for correcting property-related issues. For other income types, the Digital Disclosure Facility is the correct mechanism.

Even if the income dates back many years HMRC will still process the disclosure. They may ask questions, particularly if the omission covered a long period, but voluntary disclosure shows cooperation and normally results in significantly lower penalties.

In my experience many people worry that HMRC will take a harsh approach. In reality HMRC largely focuses on whether you have come forward voluntarily and whether you can provide a full, honest explanation.

Can you correct a tax return beyond the official time limits?

You cannot amend the tax return itself beyond the 12 month window. You also cannot make an overpayment relief claim beyond the four year limit. However you can still correct errors by writing to HMRC, especially if the correction benefits HMRC rather than the taxpayer.

If the correction would reduce your tax bill and the official deadlines have passed you normally cannot recover the overpaid amount. HMRC does not make exceptions unless the error was due to HMRC’s own mistake. In those rare cases you can request an Extra Statutory Concession, although this is unusual and requires strong evidence.

In my opinion this is why it is always worth checking your figures before filing and reviewing your tax return again shortly after submission. The earlier you identify an error the more options you have.

How long does HMRC take to process corrections?

Amendments made within the 12 month window are usually processed quickly. Overpayment relief claims can take several months because they are reviewed manually. Voluntary disclosures also take time because HMRC often needs to raise assessments for several years.

From what I have seen, the most important factor is clarity. The clearer your explanation and evidence, the faster HMRC can resolve the matter.

Final thoughts

You can correct a tax return for up to 12 months after the filing deadline through the normal amendment process. Beyond that point you still have options. Overpayment relief allows you to correct overpaid tax up to four years after the end of the tax year, while the Digital Disclosure Facility allows you to correct underpaid tax for as far back as necessary depending on whether the error was simple, careless or deliberate.

In my opinion the most important thing is to act quickly once you discover an error. HMRC is always more lenient when you come forward voluntarily, and correcting a return sooner rather than later keeps penalties and interest to a minimum. With the right approach and a clear explanation you can resolve past mistakes and bring your tax affairs back up to date.