How do I amend a corporation tax return?
Answered 17 March 2026
Here is a comprehensive answer on how to amend a corporation tax return, based on the sources retrieved.
Amending a Corporation Tax Return
What the law says
Under the Corporation Tax Self Assessment (CTSA) regime (FA 1998, Schedule 18), the key legal rules are:
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Time limit: A company can amend its return, including the self assessment, at any time within 12 months of the filing date. The time limit is not extended if a return is delivered late — so if a return is delivered more than 12 months late, no amendment can be accepted.
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Corporate Interest Restriction (CIR) returns: The time limit is extended to within 24 months of the filing date.
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Form of amendment: An amendment must be made by notice to HMRC and must be in such form, contain such information, and be accompanied by such statements as HMRC may reasonably require.
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Amendments during an open enquiry: Amendments may be accepted while an enquiry is open, but only within the normal time limits. Under FA 1998, Sch 18, para 31(3), an amendment made during an enquiry does not take effect until the enquiry is completed — any changes only become effective at the closure of the enquiry.
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Late amendments: If a late amendment reveals additional liability, HMRC can bring the tax into charge by making a discovery assessment.
HMRC guidance / practice
- How to submit: HMRC has not prescribed a specific official form for amending returns. Companies and their agents can amend Company Tax Returns either:
- Informally in correspondence with HMRC, or
- By submitting an amendment through the Corporation Tax Online Service.
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Digital filing requirement: For most purposes (e.g. Creative Industries Tax Relief claims), amendments must be made digitally through the online Corporation Tax gateway. Amendments in other formats (e.g. via email) will not be accepted.
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Refused amendments: HMRC must tell a company when an amendment is refused as being too late.
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CTSA framework: Under CTSA, responsibility for assessing the company's tax liability lies with the company itself, and every company tax return must include a self assessment. Finality comes either by the passage of time or following the formal closure of an enquiry.
Summary
| Step | Detail |
|---|---|
| Deadline | Within 12 months of the filing date (24 months for CIR) |
| Method | Via the Corporation Tax Online Service, or informally in correspondence |
| During an enquiry | Permitted within normal time limits, but takes effect only on closure of the enquiry |
| Late amendment | Cannot be accepted; HMRC may raise a discovery assessment instead |
Citation sources
Claims for the Creative Industries Tax Reliefs can only be made through Corporation Tax Self-Assessment (CTSA). This means that claims must be made in reference to an accounting period, and a company must file a Company Tax Return with HMRC (form CT600). The return must be accompanied by the company’s accounts for the accounting period. Claims can be made either in an original return or via an amendment to a return. However, claims must be made digitally through the online Corporation Tax gatew
CT Self Assessment applies to Accounting Periods (APs) ending on or after 1 July 1999. The current process, briefly, is as follows: a notice to deliver a company tax return is issued the company makes a return that includes a self assessment of the tax due the return must be submitted through the CT Online Service for the vast majority of companies and include accounts and computations in iXBRL format HMRC has the right to correct obvious mistakes in the return, such as arithmetic errors, but in
The Creatives Expenditure Credits are Corporation Tax regimes. Production companies can only claim credits through Corporation Tax Self Assessment (CTSA). A valid claim consists of two parts: The additional information form, with the supporting evidence (including a British cultural certificate) as attachments (CREC081000) A Company Tax Return, including a completed CT600, accounts and computations (CREC082000) The claim is considered to be made at the time at which it is submitted in the compa
eturn. Para 25 Sch 18 FA 1998 Scope of enquiry. Para 25(1) Sch 18 FA 1998 Scope of enquiry into company return. Para 25(2) Sch 18 FA 1998 Scope of enquiry where amendment made to company return. Para 26 Sch 18 FA 1998 Enquiry into return for the wrong period. Para 26(2)/(4) Sch 18 FA 1998 Circumstances in which return is a 'return for the wrong period'. Para 26(5) Sch 18 FA 1998 Filing date and period within which notice of enquiry may be given where return is for the wrong period. Para 30 Sch 1
A company can amend its return, including the self assessment, at any time within 12 months of the filing date (see Glossary). The time limit is not extended if a return is delivered late. It follows that if a return is delivered more than 12 months late, no amendment to the return or self assessment can be accepted. For Corporate Interest Restriction (CIR) returns this time limit is extended to within 24 months of the filing date. You must tell a company when an amendment is refused as being to