HMRC normally has 12 months from the day it receives an on time tax return to open an enquiry into it, whether the return is from an individual, a trust, a partnership or a company. If the return is filed late, or the taxpayer amends it, the window runs to the next quarter day (31 January, 30 April, 31 July or 31 October) after the first anniversary of the filing or amendment. Companies in groups that are not small count the 12 months from the filing date instead.

What is the enquiry window?

An enquiry is HMRC’s formal power to check a self assessment return. For individuals and trustees it is in section 9A of the Taxes Management Act 1970 (TMA 1970), for partnerships section 12AC, and for companies paragraph 24 of Schedule 18 to the Finance Act 1998. Each rule says HMRC may enquire only if it gives notice “within the time allowed”, and that period is the enquiry window.

The enquiry can cover anything in the return or that should have been in it, including claims and elections (section 9A(4)). A return that has been the subject of one notice of enquiry cannot normally be the subject of another, except following an amendment (section 9A(3)). Once the window has closed, HMRC can only reopen the year through a discovery assessment, which has its own conditions.

How long does HMRC have for individuals and trusts?

Section 9A(2) applies to personal returns under section 8 and trustee returns under section 8A. If the return was delivered on or before the filing date, HMRC can give notice up to the end of 12 months after the day the return was delivered. HMRC’s Enquiry Manual at EM1506 gives the example of a return received on 20 June 2018, where the window runs to 20 June 2019.

This means the clock starts when you file, not on 31 January. Before 1 April 2008 the window for on time returns ended on a fixed date, but since then it has run for a full 12 months from receipt (EM1506). Filing early therefore closes the window early.

The filing date depends on how and when the return is made. EM1505 explains that for paper returns it is 31 October after the end of the tax year, and for online returns 31 January, with a three month period instead where the notice to file is given late. A paper return delivered in December is late, even though an online return delivered then would not be.

What happens if the return is late or amended?

If the return is delivered after the filing date, section 9A(2)(b) extends the window to the quarter day next following the first anniversary of the day it was delivered. The quarter days are 31 January, 30 April, 31 July and 31 October.

A taxpayer can amend a return within 12 months after the filing date (section 9ZA). Each amendment starts its own window under section 9A(2)(c): up to the quarter day next following the first anniversary of the amendment. EM1506 notes that this does not change the window for the original return. If HMRC opens an enquiry because of an amendment after the original window has closed, section 9A(5) limits it to the points the amendment relates to or affects. Our guide to amending a tax return covers the taxpayer’s side.

HMRC’s Self Assessment Claims Manual at SACM8010 sets out separate time limits for claims made outside a return.

How does it work for partnerships?

Section 12AC(2) mirrors the individual rules: 12 months after delivery for an on time partnership return, and the quarter day after the first anniversary for late or amended returns. There is also a window following a referral of a dispute to a tribunal under section 12ABZB.

Section 12AC(6) is the point many partners miss. A notice of enquiry into the partnership return is treated as including a notice of enquiry into each partner’s own return. An enquiry into the firm can therefore draw in every partner’s personal tax affairs for that year.

What are the rules for companies?

Paragraph 24 of Schedule 18 to FA 1998 applies to company tax returns. For an on time return HMRC has 12 months from the day the return was delivered. Paragraph 24(6) changes this for a company in a group that is not a small group, as defined by the Companies Act 2006: the 12 months start from the filing date instead. Late and amended returns follow the same quarter day rule as individuals.

The filing date is generally 12 months from the end of the accounting period, with different rules for long periods of account and late notices (paragraph 14). EM1510 adds that a company can amend its return up to 12 months from the filing date, and that where an amended return is opened while the original window is still open, the enquiry covers the whole return. Amending a company return is covered in our guide to amending a CT600.

When is a notice treated as given?

EM1506 and EM1510 say the notice must be received by the taxpayer before the time limit, not merely sent. Unless the contrary is proved, HMRC treats a posted notice as delivered in the ordinary course of post. HMRC officers are told to keep evidence of posting and to use tracked delivery when a notice is issued close to the deadline. Enquiry notices must be in writing.

If a notice arrives after the window has closed, the enquiry is invalid. That is worth checking on the date of receipt before anything else is sent.

How does the window work in practice?

Illustration only.

Situation Delivered or amended Window closes
2025 to 2026 personal return filed online on time 10 September 2026 10 September 2027
Same return filed online late 15 March 2027 30 April 2028
On time return later amended 20 December 2027 31 January 2029, for the amendment
Standalone company, year to 31 March 2026, filed early 20 November 2026 20 November 2027
Same company in a group that is not small 20 November 2026 31 March 2028 (12 months from the 31 March 2027 filing date)

The amendment in the third row is possible because the 2025 to 2026 online filing date was 31 January 2027, leaving until 31 January 2028 to amend.

What happens after the window closes?

A closed window does not end HMRC’s interest. It can still make a discovery assessment. Under TMA 1970 the normal time limit is 4 years after the end of the tax year (section 34), extended to 6 years where a loss of tax was brought about carelessly and 20 years where it was deliberate or linked to certain failures, such as a failure to notify chargeability (section 36). Our guide to how far back HMRC can go explains these limits. An enquiry opened in time ends with a closure notice.

What mistakes come up most often?

  • Counting the window from 31 January rather than from the date the return was received.
  • Forgetting that each amendment opens a new window, so a late correction can extend HMRC’s time on that point.
  • Filing a paper return after 31 October and not realising it is late, which extends the window.
  • Overlooking that a partnership enquiry also reaches each partner’s return.
  • Assuming a large group company’s window runs from delivery, when it runs from the filing date.

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Sources

Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 19 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.