The obligations of a taxpayer do not cease immediately after filing your tax return. For the 2026/27 tax year, under the Making Tax Digital (MTD) for Income Tax regime, taxpayers must submit their final Self Assessment return by 31 January 2028. MTD introduces a distinct pre-filing and post-filing validation process, requiring taxpayers to actively check information automatically populated by HMRC (such as PAYE and benefits) and manually add other sources like savings or dividends before formal submission.
Missing this critical submission deadline exposes the taxpayer to a late submission penalty point under the points-based penalty regime. Furthermore, the financial obligation materialises simultaneously; under section 59B of the Taxes Management Act 1970 (TMA 1970), the final balancing payment for the tax year is strictly due by 31 January following the end of the tax year. Taxpayers who fail to settle this balance face a 5% surcharge if the tax remains unpaid 28 days after the due date, with a further 5% surcharge applying at the six-month mark.
Amending the Tax Return
If a practitioner or taxpayer discovers an error after filing their tax return, statute provides a specific window for correction. Section 9ZA of TMA 1970 allows a person to amend their personal or trustee return by giving notice to an officer of the Board.
“(2) An amendment may not be made more than twelve months after the filing date.”
For a 2026/27 return due on 31 January 2028, the taxpayer retains the absolute right to amend the return until 31 January 2029. If a company amends its Corporation Tax return while an HMRC enquiry is already in progress, that amendment does not restrict the scope of the ongoing enquiry, but HMRC may take it into account alongside other matters.
HMRC Enquiry Windows
Following the submission, HMRC has a statutory window to open a formal enquiry into the return under section 9A of TMA 1970. The duration of this window depends entirely on when the taxpayer submitted the return:
- Returns filed on or before the deadline: The enquiry window ends exactly 12 months after the date on which HMRC received the return.
- Returns filed late: The enquiry window extends to the quarter day (31 January, 30 April, 31 July, or 31 October) following the first anniversary of the date HMRC actually received the late return.
If the taxpayer amends their return under section 9ZA, this triggers a separate, secondary enquiry window specifically restricted to the matters affected by the amendment. This secondary window runs until the quarter day following the first anniversary of the date the amendment was made.
Table: Statutory Time Limits After Filing
| Action | Statutory Authority | Time Limit (Standard) |
|---|---|---|
| Taxpayer Amendment | s 9ZA TMA 1970 | 12 months after the statutory filing date. |
| HMRC Enquiry (On-time return) | s 9A TMA 1970 | 12 months from the date of receipt. |
| HMRC Enquiry (Late return) | s 9A TMA 1970 | Quarter day following the first anniversary of receipt. |
Long-Term Compliance: Discovery Assessments
Even after the standard enquiry window closes, HMRC retains the power to assess additional tax if they discover an insufficiency. Section 29 of TMA 1970 grants HMRC the authority to issue “discovery assessments” outside the normal enquiry window, provided they meet specific statutory criteria.
The time limits for these assessments, governed by sections 34 and 36 of TMA 1970, escalate severely based on the taxpayer’s behaviour:
- Ordinary Time Limit: HMRC may make an assessment up to 4 years after the end of the relevant tax year.
- Careless Behaviour: If the loss of tax was brought about carelessly, the assessment window extends to 6 years after the end of the tax year.
- Deliberate Behaviour or Failure to Notify: If the loss was deliberate, or attributable to a failure to notify chargeability under section 7 of TMA 1970, HMRC can assess up to 20 years after the end of the tax year.
Corporation Tax: FA 2026 Penalty Updates
For practitioners handling corporate clients, the Finance Act 2026 (FA 2026) significantly alters the penalty landscape after filing your tax return. Section 265 of FA 2026 doubles the flat-rate penalties for late-filed company tax returns where the filing date falls on or after 1 April 2026. The initial penalty immediately increases from £100 to £200, escalating to £400 if the return remains outstanding for more than three months. Repeated failures over successive periods now trigger automatic penalties of £1,000 and £2,000.
Consider researching the specific interaction between the MTD penalty point system and the FA 2026 cancellation powers under Section 262 to effectively manage client appeals during the post-filing period.
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