Amending Your MTD Quarterly Update After the Deadline: What's Allowed

You Filed on Time - But the Numbers Are Wrong. Now What?

You hit the 7 August deadline. You submitted your Q1 quarterly update before the clock ran out. Then, a day or two later, you opened your records and spotted a mistake - a missed expense, an income figure you typed incorrectly, or an entire category that looks wrong. The first question most people ask is: can I even change it now? The short answer is yes, in most cases. But there are important things to understand before you rush back in to resubmit.

This guide covers exactly what happens when you amend an MTD quarterly update after the deadline has passed - whether HMRC allows it, how to do it, whether you risk a penalty, and when it is actually better to leave the mistake alone and fix it at a later stage.

Does HMRC Allow Post-Deadline Amendments?

Yes. HMRC's MTD for Income Tax system is designed to accept amended quarterly updates after the filing deadline. A quarterly update is not a locked, final document once submitted. You can go back in through your bridging software, correct the figures, and resubmit.

This is an important feature of MTD and one that is often misunderstood. Many people assume that once the deadline has passed, the figures are fixed. They are not. The quarterly update is an in-year estimate - it tells HMRC roughly where your income and expenses are trending. It is not your final tax bill. That comes later, through the Final Declaration.

Because the quarterly update is an estimate, HMRC built the system with flexibility. You can submit a corrected version of a quarterly update at any point during the tax year, even after the original deadline for that quarter has passed.

What Counts as an Amendment You Should Make?

Not every mistake needs a post-deadline amendment. Before you resubmit, it is worth asking whether the error is material - meaning, does it meaningfully affect the figures?

Errors that are generally worth correcting after the deadline include:

• Income figures that are significantly wrong - for example, you reported £8,000 when the correct figure is £12,000
• Expenses claimed in the wrong category, where the total amount claimed is also wrong (not just mislabelled)
• A large expense that was left out entirely
• Rental income figures for landlords where a property's rent has been double-counted or missed
• A gross income figure that is substantially understated and could raise questions at the Final Declaration stage

Errors that are often not worth fixing at the quarterly update stage include:

• Small rounding differences of a few pounds
• Expenses that were put in one allowable category instead of another - if the total claimed is still correct, the category label matters less at this stage
• Minor timing differences, such as an invoice you received in late April that you included in the April-to-June quarter rather than the next one

If you want a clearer framework for deciding which errors to act on, MTD Filing Errors: Which Ones to Fix Now and Which to Leave walks through the decision in more detail.

How to Amend Your Quarterly Update After the Deadline

The process is broadly the same as making a pre-deadline amendment - you go back into your MTD software, update the figures, and resubmit. But it helps to know exactly what happens at each step.

Step 1 - Identify what needs to change

Before you touch anything, write down what the error is and what the correct figure should be. This sounds obvious, but it prevents you from introducing new errors while fixing the old one. Check your records - bank statements, receipts, invoices - to confirm the correct numbers.

Step 2 - Open your software and locate the Q1 update

In AffordableMTD, you can access your submitted quarterly update and edit the figures directly. The software will show you what was previously submitted. Make the correction to the relevant income or expense field.

Step 3 - Resubmit the corrected update

Once you have updated the figures, resubmit the quarterly update through the software. The software sends the revised data to HMRC's API (the technical connection between your software and HMRC's systems). HMRC receives the new version and replaces the previous figures with the corrected ones.

There is no separate "amendment form" to complete. You simply resubmit the quarterly update with the correct numbers. HMRC does not require you to explain why you are amending at the quarterly update stage.

For a full walkthrough of the amendment process, see Amend Your MTD Quarterly Update After Submission: Step-by-Step.

Will You Face a Penalty for Amending After the Deadline?

This is the question that causes the most anxiety, and the honest answer is: amending a quarterly update does not itself trigger a penalty.

HMRC's penalty regime for MTD ITSA is focused on two main failures: missing a filing deadline entirely, and errors that result in understated tax on your Final Declaration. An amendment to a quarterly update - even one filed weeks after the original deadline - does not create a penalty in isolation.

Here is why. The quarterly update does not determine your tax liability. It is an estimate. Your tax is calculated and finalised at the point of the Final Declaration. If your quarterly updates have been corrected before the Final Declaration and the final figures are accurate, HMRC has what it needs.

Where penalties can arise is if:

• You deliberately understate income across your quarterly updates and do not correct it before or at the Final Declaration
• The error leads to an inaccurate Final Declaration and HMRC later identifies that you were careless or dishonest
• You miss the filing deadline for the quarterly update itself without a reasonable excuse - though this relates to the original filing, not the amendment

In short: correcting an honest mistake is not the same as making an error and doing nothing about it. Amending is the responsible thing to do, and HMRC's system is set up to allow it.

For a broader overview of how MTD penalties work, see MTD Penalties Explained: What Happens If You File Late or Miss a Deadline?

What If the Mistake Was in Your Income Figures?

Income errors carry slightly more weight than expense errors, because understating income is what tends to attract HMRC's attention. If you filed your Q1 update with income that was noticeably lower than your actual receipts - whether through a genuine mistake or because you forgot to include a source - it is worth correcting this.

Common income mistakes include:

• Sole traders who forgot to include cash payments or invoices paid late in the quarter
• Landlords who entered the wrong rent figure for one property
• People with mixed income - both self-employment and property - who accidentally left out one income stream entirely

If you have mixed income, Mixed Income and MTD: Landlords with Self-Employment (Q1 Setup Check) covers how the two income streams are treated differently and what to watch for.

What If the Mistake Was in Your Expenses?

Expense errors are common. The most frequent ones involve claiming a personal expense as a business expense, miscategorising an allowable expense, or forgetting to include an expense altogether.

If you overclaimed expenses - for example, you included a purchase that was not genuinely for your business - you should correct this. Overclaiming expenses means your taxable profit looks lower than it actually is. If that feeds through to your Final Declaration uncorrected, you will have paid less tax than you owe.

If you underclaimed expenses - you forgot to include something you were entitled to claim - you can add it in your amended update, or you can include it in a later quarterly update for the same tax year, or pick it up at the Final Declaration. All three approaches are technically valid, though amending the original update keeps your records tidy.

For guidance on what expenses are allowable in the first place, MTD Allowable Expenses: What You Can Actually Claim covers sole traders, and What Counts as a Landlord Expense in Your Q1 MTD Quarterly Update covers landlords specifically.

If the issue is that you put an expense in the wrong category, Common MTD Expense Miscategories: Spot and Fix Your Mistakes will help you check whether it matters and how to correct it.

Is There a Deadline for Submitting Amendments?

HMRC allows amendments to quarterly updates to be made at any point during the tax year - right up until the Final Declaration deadline. The Final Declaration for the 2026-27 tax year must be submitted by 31 January 2028.

In practice, it makes sense to correct errors as early as possible rather than leaving everything until the Final Declaration. If you wait, you may forget the detail of what the error was or lose the supporting records. Keeping your quarterly figures clean also makes the Final Declaration much simpler to complete.

You cannot amend a quarterly update after you have submitted the Final Declaration for that tax year. At that point, the process for correcting mistakes is different - you would need to amend the Final Declaration or, in some cases, contact HMRC directly.

When to Leave a Mistake Alone

There are situations where amending after the deadline is more effort than it is worth. These include:

• The error is very small and will have no meaningful impact on your final tax position
• You mislabelled an expense category but claimed the correct amount - the label is an administrative point, not a tax point, at the quarterly stage
• The mistake will naturally correct itself - for example, you included a refund as income in Q1 when it was not income, but you can offset it cleanly in Q2

If you are unsure whether a mistake matters, Just Filed Your Q1 MTD Update? What Mistakes Actually Matter gives a practical guide to separating the significant from the trivial.

Keeping Records of What You Changed

Whenever you amend a quarterly update - whether before or after the deadline - keep a note of what you changed and why. This does not need to be formal. A line in a spreadsheet or a note in a folder saying "amended Q1 income figure from £9,400 to £11,200 - missed invoice from client X" is enough.

If HMRC ever looks into your records, being able to show that you identified and corrected a mistake promptly is a positive thing. It demonstrates that your record-keeping is active rather than passive.

For more on what records to keep and for how long, How Long to Keep MTD Records After Filing: HMRC Retention Rules covers the requirements.

A Note on Anxiety and Perspective

It is worth saying plainly: most post-deadline amendments are routine. MTD is a new system and HMRC expects that people will occasionally file an update with imperfect numbers. The entire design of the system - with quarterly estimates feeding into an annual Final Declaration - accounts for the fact that your figures may shift between quarter and year-end.

Finding and correcting a mistake is not a sign that something has gone badly wrong. It is exactly what the amendment process exists for. What HMRC is concerned about is not the occasional corrected figure - it is patterns of significant understatement left unaddressed.

If you have already filed an amendment and want to understand what happens after it is received, After Your First MTD Quarterly Update: What Happens Next explains the broader picture.

Summary

You can amend an MTD quarterly update after the 7 August deadline without triggering a penalty. The process involves resubmitting corrected figures through your bridging software, which sends the updated data to HMRC and replaces the previous submission. Amending does not itself attract a penalty - penalties relate to late filing and inaccurate Final Declarations, not corrected quarterly estimates. For significant errors, amend promptly and keep a record of what you changed. For minor errors - small amounts, category labels, tiny rounding differences - consider whether it is worth the effort, or whether the Final Declaration is the right place to tidy things up.