Amending a tax return gives HMRC a fresh window to open an enquiry, running to the quarter day after the first anniversary of the date you made the amendment. If HMRC opens that enquiry after the normal window for the original return has closed, or after an earlier enquiry has been completed, section 9A(5) of the Taxes Management Act 1970 limits it to what the amendment relates to and anything the amendment affects. The rest of the return stays closed unless HMRC can use its discovery powers.

How long do you have to amend a return?

For individuals and trustees, section 9ZA of the Taxes Management Act 1970 (TMA 1970) lets you amend a Self Assessment return by notice to HMRC, but not more than 12 months after the filing date. The filing date for this purpose is 31 January after the end of the tax year or, if HMRC issued the notice to file after 31 October, the last day of the three months beginning with the date of that notice. A company can amend its company tax return at any time up to 12 months from the statutory filing date, as HMRC’s Enquiry Manual at EM1510 explains.

HMRC also has its own correction power. Under section 9ZB an officer can correct obvious errors, or anything the officer has reason to believe is wrong in the light of information HMRC holds, within nine months of the return being delivered or, where the correction follows an amendment, within nine months of the amendment. You can reject a correction by notice within 30 days. A correction is not an amendment by you, so it does not create the new enquiry window described below.

The mechanics are covered in our guides to amending a tax return and amending a CT600.

What enquiry window does an amendment create?

Section 9A(2) TMA 1970 sets three time limits for a notice of enquiry into a personal or trustee return:

  • Return delivered on time: up to the end of 12 months after the day the return was delivered.
  • Return delivered late: up to and including the quarter day next following the first anniversary of the day it was delivered.
  • Return amended under section 9ZA: up to and including the quarter day next following the first anniversary of the day the amendment was made.

The quarter days are 31 January, 30 April, 31 July and 31 October. HMRC’s guidance at EM1506 confirms that the amendment does not change the enquiry period for the return itself; it creates a separate window for the amendment. The company rules in paragraph 24 of Schedule 18 to Finance Act 1998 are the same in substance: paragraph 24(4) gives HMRC until the quarter day after the first anniversary of the company’s amendment. For a company in a group that is not small, the ordinary 12 month window for an on time return runs from the filing date rather than the delivery date (paragraph 24(6)).

For a notice to be given by a deadline, HMRC’s guidance at EM1510 says the recipient must receive it by that date, so a notice posted on the last day may be too late.

When is the enquiry limited to the amendment?

A return can normally be the subject of only one notice of enquiry. Section 9A(3) makes an exception for a notice given in consequence of an amendment, or a further amendment, under section 9ZA. Section 9A(5) then sets the scope. Where the notice is given as a result of an amendment:

  1. at a time when HMRC can no longer open an enquiry into the original return under the normal time limits;
  2. after a final closure notice has been issued in an enquiry into the return; or
  3. after a partial closure notice has been issued covering the issue to which the amendment relates or which it affects,

the enquiry is limited to what the amendment relates to or affects. For companies, paragraph 24(5) of Schedule 18 allows a second notice only in consequence of an amendment, and paragraph 25(2) limits the enquiry in the same way where the notice is given after the ordinary window under paragraph 24(2) or (3) has closed.

The reverse also holds. If the ordinary window is still open when HMRC acts, the amendment does not shrink HMRC’s rights. EM1520 tells officers that where the enquiry window for the return is open they should normally enquire into the return as amended rather than the amendment alone, and EM1510 says an enquiry opened while the original deadline is still running covers the entire return.

What does “affected by the amendment” cover in practice?

The statute does not define the phrase, so the scope depends on what the amendment changed. HMRC’s guidance at EM1520 gives the main example: if a return contained an estimated or provisional figure and the correction changed only that one entry, any later enquiry would normally be limited to that entry. If the correction changed several entries or radically altered the overall self assessment, HMRC may consider the whole return.

Where HMRC’s look at the amendment suggests problems elsewhere in the return but no enquiry window is open for them, EM1520 directs officers to the discovery rules instead. Discovery has its own conditions, broadly careless or deliberate behaviour or a failure to disclose information HMRC needed (EM1530), and is explained in our page on HMRC discovery assessments. An amendment does not by itself reopen the parts of the return it did not touch.

It helps to describe the change clearly in the white space or a covering letter: which figures changed, why, and which other boxes moved as a result. Our page on aspect and full enquiries explains how HMRC frames the scope of an enquiry more generally.

What happens if you amend while an enquiry is already open?

Section 9B covers amendments made while an enquiry is in progress into anything the amendment relates to or affects. The amendment does not restrict the scope of the enquiry, but the officer may take it into account. So far as it changes the tax payable, it does not take effect while the enquiry into that issue is in progress. When the officer issues a partial closure notice or final closure notice, either the notice states that the amendment has been taken into account or found to be incorrect, in which case the amendment itself does not take effect, or the amendment takes effect when the notice is issued.

If HMRC thinks tax may be lost before the enquiry ends, it can make a jeopardy amendment under section 9C. Where the enquiry is limited to an amendment under section 9A(5), section 9C(3) restricts a jeopardy amendment to the deficiency attributable to that amendment.

A worked example

Illustration only. Priya, a self employed designer, files her 2024 to 2025 return on 20 January 2026, before the 31 January 2026 filing date. HMRC’s ordinary window to open an enquiry into the whole return therefore ends on 20 January 2027.

On 15 December 2026, still within her 12 month amendment period, she amends the return to claim business mileage she had missed. The first anniversary of the amendment is 15 December 2027, so the next quarter day, 31 January 2028, is the last day for a notice of enquiry into the amendment.

Date HMRC’s notice is received Scope of the enquiry
10 January 2027 The whole return, because the ordinary window is still open.
1 March 2027 Limited to the mileage claim and the figures it changes, such as her taxable profit and the tax calculation.
1 February 2028 Too late for any enquiry; only discovery is available, if its conditions are met.

HMRC’s company example at EM1521 works the same way: an amendment made on 29 December 2021, after the ordinary window closed, opens a window to 31 January 2023 covering only the amended repairs deduction.

Are claims outside a return treated the same way?

No. A claim made outside a return is enquired into under paragraph 5 of Schedule 1A to TMA 1970. HMRC has until whichever of these ends latest: the quarter day next following the first anniversary of the claim or its amendment; for a claim relating to a tax year, the first anniversary of 31 January following that year; or, for a claim relating to another period, the first anniversary of the end of that period. A claim that has been enquired into cannot be the subject of a further notice, even if it is later included in a return. HMRC’s guidance at EM3831A also notes that a partial closure notice cannot be issued in an enquiry into a claim made outside a return.

What mistakes do people make?

  • Assuming a late amendment is low risk. It can give HMRC well over a year more to examine that issue.
  • Thinking the amendment reopens everything. Once the ordinary window has closed, an enquiry into an amendment is limited to the amendment and its effects.
  • Forgetting that one enquiry does not use up the amendment window. Even after a final closure notice, an amendment can be enquired into separately under section 9A(3).
  • Expecting an immediate repayment from an amendment made during an enquiry. Section 9B delays its effect until the relevant closure notice.
  • Missing the amendment deadline. After 12 months from the filing date, corrections need other routes, covered in how far back HMRC can go.

Partnership returns follow the same pattern under section 12AC(5) TMA 1970, which HMRC lists alongside section 9A(5) at EM1520.

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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.