The legislation provides strict statutory mechanisms for taxpayers seeking to amend their submitted tax returns. For individuals and trustees, section 9ZA of the Taxes Management Act 1970 (TMA 1970) establishes the primary statutory right to amend a self-assessment return. Taxpayers may amend their personal or trustee tax return simply by giving notice to an officer of the Board.
For corporate entities, Schedule 18, Paragraph 15 of the Finance Act 1998 (FA 1998) creates a parallel right. Companies can amend their company tax returns by providing notice to HMRC, provided it contains such information and statements as HMRC reasonably requires. Currently, HMRC does not prescribe an official form for corporate amendments; companies and their agents can amend company tax returns informally in correspondence, except where the amendment involves Research and Development (R&D) or Creative Industries reliefs.
Newly introduced regimes also follow this general framework. For the Multinational Top-up Tax (MTT), the filing member may amend a self-assessment return by submitting a notice to HMRC, as set out in Schedule 14 of the Finance (No.2) Act 2023.
Statutory Time Limits for Amendments
Taxpayers face strict statutory time limits when amending a tax return. If a taxpayer misses these deadlines, their statutory right to amend the return automatically falls away.
Under section 9ZA(2) of TMA 1970, an individual or trustee may not make an amendment more than twelve months after the statutory filing date. Similarly, Schedule 18, Paragraph 15 of FA 1998 dictates that a company can amend its return at any time up to twelve months from the statutory filing date. For MTT returns, the time limit is also 12 months beginning with the filing date.
Table: Key Amendment Time Limits
| Entity / Return Type | Statutory Time Limit | Relevant Legislation |
|---|---|---|
| Individuals & Trustees (ITSA) | 12 months after the statutory filing date. | s 9ZA(2) TMA 1970 |
| Companies (CTSA) | 12 months from the statutory filing date. | Sch 18 Para 15 FA 1998 |
| Multinational Top-up Tax (MTT) | 12 months beginning with the filing date. | Sch 14 F(No.2)A 2023 |
| Claims outside a return | 12 months after HMRC receives the claim. | Sch 1A Para 3(1)(b) TMA 1970 |
Capital Gains Tax on UK Property Restrictions
When handling amendments for Capital Gains Tax (CGT) on UK Property accounts, taxpayers must navigate specific legislative restrictions. Schedule 2 to the Finance Act 2019 limits the extent to which taxpayers can amend these 60-day returns. Amendments to a property return are only permitted so far as they could have been included when the return was originally delivered, based on events that had already occurred at that point.
Amendments cannot be made to the property return at any time on or after the date on which the person has submitted their Self Assessment return taking account of the disposal or the date by which the person has by notice been required to submit a Self Assessment return.
Once the taxpayer submits the Self Assessment return, it must include the final figures for the year. This annual return effectively subsumes the property return, capturing any late-arising losses or adjustments that would otherwise necessitate an amendment to the standalone CGT property return.
MTD ITSA Implications for the 2026/27 Tax Year
The 2026/27 tax year marks the introduction of Making Tax Digital for Income Tax Self Assessment (MTD ITSA). From 6 April 2026, sole traders and landlords with a qualifying income over £50,000 must use MTD for ITSA.
Under this regime, taxpayers must provide digital quarterly updates. However, the amendment window still anchors to the final end-of-year return. The final MTD return for the 2026/27 tax year remains due by 31 January 2028, meaning the standard 12-month amendment window runs until 31 January 2029.
Interplay with HMRC Enquiries
Amending a tax return while an HMRC enquiry is actively open triggers specific procedural rules. If a company amends its return during an enquiry into any matter affected by the amendment, the amendment does not restrict the scope of the officer’s enquiry. HMRC will take the amendment into account alongside any matters arising during the investigation.
Crucially, the legislation defers the tax effect of the amendment. Any resulting changes to the tax payable do not take effect while the enquiry remains in progress. The changes will only become legally effective once HMRC issues a closure notice to conclude the enquiry.
Remedies After the Statutory Deadline
When a taxpayer misses the 12-month deadline for amending a tax return, they must rely on alternative remedies. If the out-of-time error resulted in the taxpayer overpaying tax, they must submit a formal claim for Overpayment Relief (under Schedule 1AB TMA).
Conversely, if an error reveals underpaid tax and the amendment window has closed, the taxpayer cannot simply amend the return. Instead, HMRC will typically recover the underpayment by issuing a discovery assessment under section 36 of TMA 1970. The assessment windows depend strictly on the taxpayer’s behaviour:
- Innocent errors: 4 years after the end of the relevant tax year.
- Careless conduct: 6 years after the end of the relevant tax year.
- Deliberate conduct (or failure to notify): 20 years after the end of the relevant tax year.
Consider reviewing the procedural requirements for Overpayment Relief claims under Schedule 1AB TMA 1970 to assist clients who have discovered errors outside the standard 12-month amendment window.
Join 5,000+ businesses comparing today