Taxation (UK) · The systems for self-assessment and the making of returns

HMRC Enquiries, Amendments and Discovery Assessments Explained

Updated 11 October 2026 · Fact-checked

An enquiry is HMRC's right to check a filed return, normally opened within 12 months of filing for on-time returns. An amendment corrects a return. A discovery assessment lets HMRC collect undercharged tax after the enquiry window closes, within 4, 6 or 20 years depending on taxpayer behaviour and the information HMRC had.

Understand HMRC Enquiries, Amendments and Discovery Assessments

A self-assessment return is treated as correct when filed. HMRC does not check every return first. Instead it has powers to check afterwards, and the taxpayer has the right to correct mistakes. The three tools in this topic form a timeline: amend, enquire, then discover.

Amendments. You can amend your own return within 12 months of the filing deadline. For an individual, the deadline is 31 January after the tax year, so the amendment window runs to the following 31 January. HMRC can also correct obvious errors, such as arithmetical mistakes, within 9 months of the date the return was filed. You can reject HMRC's correction within 30 days of the notice.

Enquiries. HMRC can open an enquiry without giving a reason. It can enquire into the whole return or one part of it. It must give written notice within the time limit. After that, the return is safe from enquiry, unless you amend it, which can open a new window for the amendment. An enquiry ends when HMRC issues a closure notice stating its conclusions. You can ask the tribunal to direct HMRC to issue one.

Discovery assessments. Once the enquiry window has closed, HMRC can still raise a discovery assessment if it discovers that income or gains have been undercharged. It cannot use this just because it changed its mind. One of two conditions must be met. Either the loss of tax was brought about carelessly or deliberately by the taxpayer, or HMRC could not reasonably have been aware of the problem from the information given in the return (insufficient information).

The time limits depend on the situation, measured from the end of the tax year:

20 years where the loss of tax was deliberate.
6 years where it was careless.
4 years where it was neither careless nor deliberate, but HMRC lacked sufficient information.

For companies, the period is measured from the end of the accounting period. If the error was innocent and the return gave HMRC enough information, no discovery assessment is possible at all.

Why it matters. A genuine, innocent error that was disclosed properly is protected after the enquiry window. Careless or deliberate errors stay open much longer and can also bring a penalty for errors.

Key rules to remember

Taxpayer amendment limit: Amend within 12 months of the filing deadline; For an individual, the deadline is 31 January following the tax year. Companies use 12 months from the filing due date.
HMRC correction of obvious errors: Within 9 months of the date the return was filed; The taxpayer can reject the correction within 30 days of the notice.
Enquiry limit: return filed on time: Notice within 12 months of the actual filing date; Measured from the date HMRC receives the return, not from the deadline. No reason needs to be given.
Enquiry limit: return filed late (individual): The first quarter day falling at least 12 months after the actual filing date; Quarter days are 31 January, 30 April, 31 July and 31 October. A company's late return gets 12 months from the end of the quarter in which it was filed.
Enquiry into an amended return: Limited to the amendment window after the amendment is made; The same pattern applies: 12 months, or for an individual's late amendment, the first quarter day falling at least 12 months after it is made.
Discovery assessment time limits: 4 years (not careless or deliberate, but insufficient information); 6 years (careless); 20 years (deliberate); Measured from the end of the tax year (individuals) or accounting period (companies). Conditions must be met: careless or deliberate behaviour, or insufficient information given to HMRC. An innocent error with adequate information cannot be assessed.
Penalty maximums for errors (ACCA table): Careless 30%; deliberate but not concealed 70%; deliberate and concealed 100%; Minimums: careless 0% unprompted and 15% prompted; deliberate not concealed 20% and 35%; deliberate and concealed 30% and 50%.

How to solve HMRC Enquiries, Amendments and Discovery Assessments questions

Use this order for any question on enquiries, amendments or discovery. Dates decide the answer, so lay them out first.

1Identify the taxpayer (individual or company) and the tax year or accounting period.
2Find the filing deadline and the actual filing date. Was the return on time or late?
3If the question asks about amending, add 12 months to the filing deadline (not the actual filing date).
4If it asks about an enquiry, work out the limit: 12 months from the actual filing date if on time; for a late individual return, the first quarter day falling at least 12 months after the actual filing date.
5Check whether today's date or the date HMRC acts is inside the window. If inside, an enquiry is allowed. No reason is needed.
6If outside, consider a discovery assessment. Decide the taxpayer's behaviour (careless, deliberate or neither) and whether the return gave HMRC enough information. Pick 6 or 20 years for careless or deliberate, or 4 years if neither but the information was insufficient, from the end of the tax year. If the error was innocent and the information was adequate, no assessment is possible.
7State the conclusion in one sentence with the exact date, then mention the penalty range if the behaviour was careless or deliberate.

Quickest way: Date-line shortcut

When to use it: Use this for objective test questions that give you dates and ask which action is still possible.

Write the filing deadline and actual filing date on your rough sheet.
Add 12 months to the deadline for the amendment limit.
Add 12 months to the actual filing date for the enquiry limit. If the return was late, move to the first quarter day at or after that date.
If both have passed, use the behaviour label: careless 6 years, deliberate 20 years, from the end of the tax year. If the error was neither but the return gave insufficient information, use 4 years. If it was innocent and the information was adequate, no assessment is possible.
Pick the option that matches your dates exactly. Beware options that start the clock from the wrong date.

Common mistakes in HMRC Enquiries, Amendments and Discovery Assessments

Measuring the enquiry window from the filing deadline instead of the actual filing date.
The amendment limit uses the deadline, so students use it for both.
Fix: Amendment: 12 months from the deadline. Enquiry: 12 months from the actual filing date.
Saying HMRC needs a reason to open an enquiry.
Students assume HMRC must suspect something.
Fix: HMRC can enquire without giving any reason. It can also enquire at random.
Applying 4, 6 or 20 years to enquiries.
The numbers are learned as a group.
Fix: Those limits are for discovery assessments only. Enquiries are normally 12 months.
Allowing a discovery assessment for innocent errors within 4 years automatically.
Students treat 4 years as the default limit and forget the conditions.
Fix: HMRC needs careless or deliberate behaviour, or insufficient information in the return. The 4-year limit applies only to the insufficient-information case. An innocent error with adequate information cannot be assessed.
Forgetting the quarter day rule for late individual returns.
It is a small detail inside the enquiry rule.
Fix: If the return was late, find the date 12 months after filing, then use the first 31 January, 30 April, 31 July or 31 October that falls on or after it.
Mixing up HMRC's 9-month correction of obvious errors with the 12-month enquiry window.
Both are HMRC powers with similar time limits.
Fix: Correction of obvious errors is 9 months from the filing date. An enquiry is a deeper check with its own notice.

Worked examples

Example 1

Maya files her 2025–26 self-assessment return on 20 December 2026. The filing deadline was 31 January 2027. (a) By when can Maya amend the return? (b) By when can HMRC open an enquiry? (c) By when can HMRC correct obvious errors?

Show the solution

The amendment limit is 12 months from the filing deadline: 31 January 2027 plus 12 months is 31 January 2028.
The return was filed on time, so the enquiry limit is 12 months from the actual filing date: 20 December 2026 plus 12 months is 20 December 2027.
HMRC's correction of obvious errors is within 9 months of the filing date: 20 December 2026 plus 9 months is 20 September 2027.

Answer: (a) 31 January 2028. (b) 20 December 2027. (c) 20 September 2027.

Example 2

Late in 2026, HMRC discovers that Ben, a sole trader, left out £8,000 of dividend income from his 2021–22 return. The return was filed on time, the enquiry window has closed and Ben made the omission carelessly. Can HMRC raise a discovery assessment? What if the omission was an innocent error and the return gave HMRC enough information?

Show the solution

The 2021–22 tax year ended on 5 April 2022.
Careless behaviour gives a 6-year limit: 5 April 2022 plus 6 years is 5 April 2028.
The enquiry window has closed and the loss arose through Ben's carelessness, so the condition for a discovery assessment is met.
The assessment is raised in late 2026, before 5 April 2028, so it is in time.
If the error was innocent (neither careless nor deliberate) and the return gave HMRC enough information, no condition for a discovery assessment is met, so HMRC cannot assess. Even if the information had been insufficient, the 4-year limit would have ended on 5 April 2026, which has passed, so it would be out of time.

Answer: Yes, HMRC can raise a discovery assessment for the careless omission (limit 5 April 2028) and may charge a careless-error penalty of up to 30%. If the error was innocent and HMRC had adequate information, no discovery assessment is possible. Even on insufficient information, the 4-year limit ended on 5 April 2026.

Exam tips

Write out the dates in your working. The marks, or the right option, depend on getting the start date right.
Learn the pairing: amendment from the deadline, enquiry from the actual filing date.
Decide the behaviour label from the wording: 'careless' means 6 years and 'deliberate' means 20 years. 4 years applies only where there is no careless or deliberate behaviour but HMRC lacked sufficient information. An honest mistake with adequate information in the return gives no discovery assessment.
In constructed response answers, state that HMRC does not need to give a reason for an enquiry and explain how it ends with a closure notice.
Link the topic to penalties: careless errors carry up to 30%, and the ACCA tax table gives the full range.

Practice questions from The systems for self-assessment and the making of returns

HMRC Enquiries, Amendments and Discovery Assessments: frequently asked questions

How long can HMRC open an enquiry into a tax return?

Normally 12 months from the date the return was filed, if it was filed on or before the deadline. For a late individual return, HMRC has until the first quarter day falling at least 12 months after the filing date. An amended return can open a new window for the amendment.

What is the difference between an enquiry and a discovery assessment?

An enquiry is a check on a return within a short window, and HMRC needs no reason. A discovery assessment is used after that window has closed, when HMRC finds tax has been undercharged. It has stricter conditions and longer time limits.

What is the discovery assessment time limit?

It is 20 years from the end of the tax year where the loss of tax was deliberate and 6 years where it was careless. It is 4 years where the loss was neither careless nor deliberate but HMRC lacked sufficient information from the return. An innocent error with adequate information cannot be assessed. For companies, the period runs from the end of the accounting period.

Can I amend my return 12 months after the filing date?

The limit is 12 months after the filing deadline, which for an individual is 31 January after the tax year. After that, you can usually make a claim for overpaid tax instead, which has its own time limits.