Already filed your Q1 MTD update but spotted an error? Don't panic. HMRC allows you to amend quarterly updates, and with the 7 August 2026 deadline approaching, there's still time to correct mistakes before they become permanent. Whether it's missing expenses, incorrect income figures, or categorisation errors, fixing them now saves headaches later.

This guide walks you through exactly how to amend your Q1 update, what triggers the need for amendments, and how to avoid common re-filing mistakes that cause more problems than they solve.

Why Amending Your Q1 Update Matters

Your Q1 quarterly update isn't just a formality - it feeds directly into your final tax calculation. Errors left uncorrected now will either cost you money (if you've understated income or overstated expenses) or result in overpayment (if you've missed legitimate deductions).

More importantly, HMRC uses quarterly updates to build a picture of your tax liability throughout the year. Significant discrepancies between quarterly updates and your final tax return trigger compliance checks. Getting it right from the start reduces scrutiny later.

Note: You can amend quarterly updates until the 7 August deadline. After that, corrections must wait until your annual tax return, and you'll lose the opportunity to adjust your payments on account based on accurate quarterly figures.

Common Reasons to Amend Q1 Updates

Late Expenses Discovery

The most common amendment reason is finding expenses after filing. This happens frequently because:

If these expenses relate to your Q1 period (6 April to 5 July 2026), they belong in your Q1 update regardless of when you discovered or paid them.

Income Corrections

Income amendments typically involve:

Remember, if you're cash-based (most sole traders and landlords), income is recorded when received, not when invoiced. But if a Q1 client payment arrived in your bank after you filed, you still need to include it in Q1.

Categorisation Errors

These might seem minor but can trigger HMRC queries if they're significant:

Our guide to Q1 expense categorisation covers the most common classification mistakes to watch for.

The Amendment Process Step-by-Step

Step 1: Gather Your Correction Evidence

Before touching any software, compile the documentation for your amendments:

This preparation prevents multiple amendment cycles and ensures you capture everything in one go.

Step 2: Access Your MTD Software

Log into your MTD-compatible software. Most platforms show submitted updates with an "Amend" or "Correct" option. The exact process varies by software, but the principle is the same: you're creating a superseding update that replaces your original.

Step 3: Make Your Changes

Work through your amendments systematically:

  1. Update income figures - Add any missed income or correct errors in amounts
  2. Add or correct expenses - Ensure they're properly categorised and business-related only
  3. Review calculations - Let the software recalculate your profit/loss figures
  4. Check against bank statements - Ensure your amended figures reconcile with actual bank activity

Don't make wholesale changes unless necessary. HMRC can see the amendment history, and dramatic alterations invite questions.

Warning: Only amend figures that are genuinely incorrect. Adding expenses you forgot to include is legitimate; inflating expenses to reduce tax liability is not. Keep evidence for everything.

Step 4: Review Before Re-Submission

Use our pre-submission checklist to verify your amended update. Pay particular attention to:

Step 5: Submit the Amendment

Your software will submit the corrected update to HMRC, superseding the original. You should receive confirmation within a few minutes. Save this confirmation - it's proof your amendment was accepted before the deadline.

Deadline Implications and Timing

The 7 August 2026 deadline applies to amendments as well as original filings. This means you have until 11:59 PM on 7 August to submit corrected Q1 updates.

However, don't leave amendments until the last minute. HMRC's systems can become congested near deadlines, and if technical issues prevent submission, you'll face late filing penalties even though your error was attempting to correct mistakes.

Aim to complete amendments by 31 July. This gives you a week's buffer for any technical issues or additional corrections you might need.

What Happens After Amendment

Once HMRC accepts your amended update, the corrected figures replace your original submission entirely. Your payment on account calculations (if applicable) will adjust based on the new figures.

If your amendment increases your tax liability significantly, HMRC might adjust your payment dates or amounts. Conversely, if you've reduced your liability, you might receive credit against future payments.

Keep records of both your original and amended submissions. HMRC compliance checks sometimes compare amendment patterns across multiple periods, so having a clear audit trail helps if questions arise later.

Avoiding Re-Filing Errors

Common mistakes when amending quarterly updates often create bigger problems than the original errors:

Double-Counting Income

This happens when you add Q1 income that you've already recorded in a later period. Before amending, check your Q2 records to ensure you're not duplicating entries.

Period Allocation Errors

Expenses must be allocated to the correct quarterly period based on when they occurred, not when you discovered or paid them. A March 2026 expense belongs in your Q4 2025-26 figures, not Q1 2026-27, even if you found it while preparing Q1.

Incomplete Corrections

If you're correcting a bank reconciliation error, ensure all related transactions are updated. Fixing the income side but leaving expense errors creates new discrepancies.

Format and Categorisation Consistency

Maintain consistent expense categorisation across all quarters. If you're reclassifying expenses in Q1, ensure your Q2 and future periods use the same classification logic.

Note: If you've already started your Q2 update and made changes that affect Q1, you might need to amend both periods. Our Q2 planning guide covers how quarterly periods interact.

When Not to Amend

Not every discrepancy requires an amendment. Small errors (under £100) that don't affect your tax liability meaningfully can often wait until your annual tax return. HMRC expects minor variations between quarterly estimates and final figures.

Similarly, if you're unsure whether an expense qualifies as allowable, it's safer to exclude it from quarterly updates and seek professional advice before your annual return. Quarterly amendments are visible to HMRC immediately and can trigger inquiries.

Consider whether the amendment actually benefits you. If correcting an error increases your tax liability without providing any compliance benefit, you might prefer to address it through your annual return instead.

Technical Issues and Backup Plans

If you encounter software problems when trying to amend:

  1. Try different browsers or devices - Sometimes it's a simple compatibility issue
  2. Contact your software provider - They can often process amendments manually in urgent cases
  3. Use HMRC's online services directly - Some MTD software providers offer emergency backup submission methods
  4. Document your attempts - If system failures prevent submission, this evidence supports a reasonable excuse claim

Our guide to MTD technical issues covers backup options if HMRC's systems experience problems near the deadline.

Need Help Amending Your Q1 Update?

AffordableMTD makes quarterly amendments straightforward with clear error checking and automatic calculations. Fix your Q1 mistakes before the 7 August deadline.

Start Your Amendment

Amending your Q1 MTD update doesn't have to be stressful. With proper preparation, systematic corrections, and attention to deadlines, you can fix errors and ensure your quarterly reporting accurately reflects your business activity. The key is acting promptly while giving yourself enough time to handle any complications that arise during the amendment process.