Amending a tax return after an MTD final declaration

Q: How might this work in practice?

Illustration only. Tom is a landlord in MTD. He submits his 2026 to 2027 return through his software on 20 January 2028. In June 2028 he finds a £1,200 repair invoice that was never recorded. He adds the digital record, amends the return in the same software before 31 January 2029, and checks the revised calculation in his online account. If the result is a credit, he asks for the repayment through his online account. If he only found the invoice in March 2029, the section 9ZA window would have closed. He would then need a written overpayment relief claim, which for 2026 to 2027 must be made by 5 April 2031.

Once an MTD for Income Tax return (often still called the final declaration) has been submitted, you correct it through your MTD compatible software, and the change works as an ordinary amendment under section 9ZA of the Taxes Management Act 1970. The deadline is 12 months after the 31 January filing date, so a 2026 to 2027 return can be amended until 31 January 2029. After that, you write to HMRC, and an overpayment can only be recovered through an overpayment relief claim.

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Is the final declaration a different kind of return?

No. HMRC’s guidance simply calls the end of year step “submit your tax return”. In the Income Tax (Digital Obligations) Regulations 2026 (SI 2026/336), “return” means a personal return required by a notice under section 8(1)(a) of the Taxes Management Act 1970 (TMA), and regulation 8 requires it to be delivered using functional compatible software. Because it is the same section 8 return, the normal amendment, correction and enquiry rules in the TMA apply to it. The submission process itself is covered in the MTD final declaration.

How long do you have to amend?

Section 9ZA(2) TMA says an amendment may not be made more than twelve months after the filing date. The filing date for a tax year is 31 January after the end of that year or, if the notice to file was given after 31 October, three months from the date of the notice (section 9ZA(3)). HMRC’s MTD guidance puts it the same way: a change can be made within 12 months of the submission deadline.

Tax year	Normal filing date	Last day to amend
2026 to 2027	31 January 2028	31 January 2029
2027 to 2028	31 January 2029	31 January 2030

The window runs from the deadline, not from the date you filed. Filing early does not shorten it.

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How is the amendment made in software?

Regulation 16 of SI 2026/336 allows a person who delivered the return using functional compatible software to give notice of an amendment under section 9ZA using that software, and regulation 3 requires functional compatible software to be capable of giving notice of amendments. HMRC’s guidance tells you to make the change in your compatible software; your HMRC online services account, or the agent services account, then shows an updated tax calculation.

Two practical points follow. First, check that your product supports amendments before relying on it, and if you have changed software since filing, that the new product can amend an earlier year. Second, for agents, HMRC’s guidance says only the main agent can submit a client’s MTD return, so a supporting agent should expect the main agent to handle the amendment. If software problems stop you, HMRC’s guidance is to go to the provider first and then tell HMRC as soon as possible.

If the return has not yet been submitted, it is not an amendment at all. HMRC’s guidance says you should correct your digital records instead.

Do the digital records and quarterly updates also need correcting?

The records should support the amended figures. Regulation 17(1) requires anyone who finds an error or omission in a digital record to correct it using functional compatible software as soon as possible. Before the return goes in, HMRC offers two routes once the fourth update has been sent: correct the record and resend the fourth quarterly update, or adjust the category total in the software and reflect the change in the records. Regulation 17(3) sets the latest date for correcting fourth quarter information as the return deadline.

After submission, the amendment to the return is what changes the tax. The underlying record still needs fixing so the books, the return and any later enquiry all tell the same story. In year errors are covered in correcting errors in MTD quarterly updates.

What if the amendment means you are owed money?

HMRC’s guidance says you cannot claim a tax refund in your tax return using MTD for Income Tax. If you submitted with MTD software you request the refund through your HMRC online services account, and an agent requests it through the agent services account with the client’s bank details. Where the amendment increases the tax due, the updated calculation shows the extra liability.

What can you do once the 12 months have passed?

HMRC’s general Self Assessment guidance says that if you miss the deadline, you need to write to HMRC. The route depends on the direction of the error:

Tax underpaid. Tell HMRC in writing so the liability can be corrected.
Tax overpaid. Make an overpayment relief claim under Schedule 1AB TMA. Paragraph 3 sets a limit of 4 years after the end of the relevant tax year, and paragraph 3(4) says the claim cannot be made by including it in a return.

HMRC can also correct a return itself under section 9ZB TMA, for obvious errors or for anything an officer has reason to believe is incorrect based on information available. That must be done within nine months of the return (or of an amendment that prompted the correction), and you can reject the correction within 30 days of the notice. For the general rules, see how to amend a Self Assessment tax return.

Can an amendment reopen an enquiry or change your MTD status?

An amendment gives HMRC a fresh window. Under section 9A(2)(c) TMA, where a return is amended under section 9ZA, HMRC can open an enquiry up to and including the quarter day after the first anniversary of the amendment; the quarter days are 31 January, 30 April, 31 July and 31 October. Under section 9A(3) a return already enquired into can be enquired into again only as a result of an amendment. See enquiries into amended returns.

An amendment can also change whether you need MTD. HMRC considers amendments to work out qualifying income. A change made before the start of a tax year counts in either direction. After the start of a tax year, a reduction that takes income below the threshold removes the requirement for that year, but an increase is ignored, as regulation 26(c) of SI 2026/336 confirms. If an amendment to the previous year’s return takes you below the threshold, HMRC’s guidance says you can opt out or continue voluntarily.

How might this work in practice?

Illustration only. Tom is a landlord in MTD. He submits his 2026 to 2027 return through his software on 20 January 2028. In June 2028 he finds a £1,200 repair invoice that was never recorded. He adds the digital record, amends the return in the same software before 31 January 2029, and checks the revised calculation in his online account. If the result is a credit, he asks for the repayment through his online account.

If he only found the invoice in March 2029, the section 9ZA window would have closed. He would then need a written overpayment relief claim, which for 2026 to 2027 must be made by 5 April 2031.

What mistakes come up most often?

Counting the 12 months from the date of filing rather than from 31 January.
Amending the return but leaving the digital records showing the old figures.
Trying to claim a refund within the return instead of through the online account.
Assuming the supporting agent can file the amendment when only the main agent can submit.
Forgetting that an amendment restarts HMRC’s enquiry window for that return.

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Sources

Reviewed by Christine Anderson, Broker, Solar Protect. Last reviewed 24 October 2026. Part of Accountapedia, the Solar Protect guide to UK tax and accounting. This page is general information, not personalised advice.