How far back can I correct a tax return?

At Towerstone Accountants we provide specialist personal tax services, for self employed, and individuals across the UK. This article has been written to explain How far back can I correct a tax return, in clear practical terms, so you understand how personal tax and Self Assessment rules apply in real situations. Our aim is to help you stay compliant, avoid costly mistakes, and make confident tax decisions.

This is a question I am asked more often than almost any other and usually it comes with a fair amount of anxiety attached. Someone has spotted a mistake, found an old letter from HMRC, realised income was missed, or been told by a new accountant that something was done incorrectly years ago. The immediate worry is always the same. Is it too late to fix it and what happens now.

From experience, most people assume there is a single simple rule. In reality, how far back you can correct a tax return depends on what kind of tax return it is, what type of correction you are making, whether HMRC is already involved, and whether the error worked in your favour or HMRC’s.

In this article I want to explain clearly and calmly how far back you can correct a UK tax return, what routes are available if you are outside the normal time limits, and what I advise clients to do in real life when historic issues come to light. Everything here is grounded in current UK rules and how HMRC actually applies them, not just how they appear on paper.

What HMRC means by correcting a tax return

Before getting into time limits, it helps to be clear about what correcting a tax return actually means.

In HMRC terms, a correction usually falls into one of three categories.

• Amending a return that was filed incorrectly

• Claiming a repayment or relief that was missed

• HMRC correcting or challenging something you filed

Each of these has different rules and deadlines and confusing them is where people often go wrong.

Most people asking this question are trying to amend a Self Assessment tax return that they submitted with an error. That is where I will start.

Amending a Self Assessment tax return within the normal deadline

If you filed your Self Assessment tax return online, you normally have 12 months from the filing deadline to amend it.

The filing deadline for most individuals is 31 January following the end of the tax year. That means you usually have until the following 31 January to make changes.

For example, if the return relates to the 2022 to 2023 tax year and was due by 31 January 2024, you normally have until 31 January 2025 to amend it online.

This is the simplest scenario. You log into your HMRC account, make the correction, resubmit the return, and HMRC recalculates the tax.

From experience, many errors are caught within this window and are resolved quickly with minimal fuss. If tax is owed, you pay it. If tax is overpaid, you get a refund or the amount is credited.

If you are within this time limit, the process is usually straightforward.

What happens if the amendment window has passed

This is where things start to feel more complicated but it does not mean you are stuck.

Once the normal amendment window has closed, you can no longer amend the return directly online. That does not mean the error cannot be corrected. It just means a different process applies.

At this point, corrections usually fall into one of two routes.

• Overpayment relief claims

• Voluntary disclosures or HMRC adjustments

Which route applies depends on whether the mistake resulted in you paying too much tax or too little.

Claiming a refund using overpayment relief

If the mistake means you paid too much tax, you may be able to claim a refund using something called overpayment relief.

Overpayment relief allows you to ask HMRC to repay tax that was overpaid due to an error or omission in a tax return.

The key point is the time limit.

You normally have four years from the end of the tax year to make an overpayment relief claim.

For example, for the 2019 to 2020 tax year, which ended on 5 April 2020, the deadline to claim overpayment relief would usually be 5 April 2024.

This is longer than the amendment window and catches many people by surprise. From experience, clients are often relieved to find that refunds can still be claimed years later.

However, there are conditions.

HMRC will not accept an overpayment relief claim if:.

• You already had the opportunity to amend the return and chose not to

• The claim is really an attempt to reopen an enquiry decision

• The overpayment arose because you misunderstood the law but applied it as HMRC intended at the time

Overpayment relief must also be made in writing and supported by clear calculations and explanations. It is not something you do casually.

Correcting underpaid tax after the deadline

If the mistake means you underpaid tax, the position is different.

There is no time limit that protects you from correcting underpaid tax. HMRC has the power to assess underpaid tax going back several years and in some cases much further.

From experience, this is where fear often creeps in. People worry they are opening a can of worms by coming forward.

In reality, voluntary disclosure is almost always the right approach.

If you discover an error that led to underpaid tax, you can disclose it to HMRC. How far back HMRC can go depends on the behaviour that led to the error.

How far back HMRC can go if tax was underpaid

HMRC’s ability to assess underpaid tax depends on whether the error was:.

• Careless

• Deliberate

• Innocent and reasonable

This distinction matters a great deal.

If the error was careless, meaning a lack of reasonable care but no intent to deceive, HMRC can usually go back up to six years.

If the error was deliberate, meaning you knowingly submitted incorrect information, HMRC can go back up to twenty years.

If the return was correct based on the information you had at the time and the mistake was genuinely unavoidable, HMRC may be limited or unable to assess further tax.

From experience, most historic errors fall into the careless category rather than deliberate. HMRC is usually more interested in behaviour and cooperation than punishment.

Coming forward voluntarily often reduces penalties significantly and in some cases removes them entirely.

Making a voluntary disclosure to HMRC

A voluntary disclosure is when you tell HMRC about a mistake before they contact you about it.

This can be done through HMRC’s disclosure facilities or by writing directly, depending on the situation.

In my experience, voluntary disclosure has several advantages.

• Penalties are usually lower

• HMRC is more cooperative

• Payment plans are easier to negotiate

• Stress is reduced compared to an investigation

The disclosure should explain what went wrong, how far back the issue goes, how the figures were calculated, and what tax is due.

This is not something I recommend doing casually or emotionally. A clear structured disclosure makes a huge difference to how HMRC responds.

What if HMRC has already opened an enquiry

If HMRC has already opened an enquiry into a tax return, your ability to amend or correct it directly may be restricted.

During an enquiry, HMRC controls the process. Corrections are made as part of the enquiry rather than through amendments or claims.

How far back HMRC can go in an enquiry depends on the same behavioural rules I mentioned earlier. Careless errors allow a six year look back. Deliberate errors allow a much longer one.

From experience, cooperation and clarity during an enquiry are critical. Trying to hide issues or drip feed information usually makes things worse.

Special cases and common scenarios

There are certain situations that come up regularly in practice where people are unsure what applies.

Missed income discovered years later

If you discover income that was not reported, even many years later, it should usually be disclosed. HMRC may assess tax going back up to six years in most careless cases.

Expenses claimed incorrectly

Incorrect expense claims are common. If they reduced tax incorrectly, HMRC can reassess within the relevant time limits. Voluntary correction usually reduces penalties.

Capital gains errors

Capital Gains Tax errors often surface late because assets were sold years ago. Overpayment relief may apply if tax was overpaid. Underpayments may still be assessable.

Self Assessment not filed at all

If a return was never filed, HMRC can go back further. Failure to file opens the door to extended assessments and penalties.

How I advise clients when historic issues come to light

When a client comes to me worried about past tax returns, I take a structured approach.

First, we establish the facts. What tax years are involved, what was filed, and what is wrong.

Second, we assess the behaviour. Was it careless, innocent, or something else.

Third, we identify the correct route. Amendment, overpayment relief, or disclosure.

Fourth, we plan the communication with HMRC carefully. Tone and clarity matter.

In my opinion, rushing or panicking leads to mistakes. Taking a measured approach almost always leads to a better outcome.

Common misconceptions I see all the time

From experience, there are a few myths that cause unnecessary stress.

• Thinking mistakes cannot be fixed after one year

• Believing HMRC will automatically apply maximum penalties

• Assuming old errors are safer left alone

• Confusing refunds with amendments

• Thinking HMRC already knows everything

None of these are reliably true. The rules are more flexible than people expect but only if handled correctly.

Key points to takeaway

So how far back can you correct a tax return. The honest answer is it depends.

Amendments usually have a one year window. Refund claims often have a four year window. Underpaid tax can be corrected going back six years in many cases and sometimes further.

From experience, the biggest risk is not correcting old mistakes. It is ignoring them and hoping they never surface.

HMRC is far more reasonable when you come forward voluntarily with a clear explanation and accurate figures. Most situations can be resolved without drama if handled properly.

If you have discovered an issue with an old tax return, the best step is to deal with it calmly and proactively. Waiting rarely improves the outcome.

You may also find our guidance on What happens if I miss the Self Assessment deadline, and How do accountants help people who have missed previous returns, helpful when reviewing related personal tax questions. For a broader overview of Self Assessment deadlines, reporting, and obligations, you can visit our self assessment guidance hub.