Q1 MTD Submission Errors: 5 Common Mistakes and How to Fix Them

With just 8 weeks until the Q1 MTD deadline on 7 August 2026, thousands of sole traders and landlords are submitting their quarterly updates right now. And that means we're seeing the same five errors crop up repeatedly in MTD filings. The good news? Every single one is fixable, and most can be sorted out in minutes once you know what to look for.

Whether you're filing your first quarterly update or you're an accountant helping clients through the submission process, these are the mistakes causing the most headaches - and the step-by-step fixes that actually work.

1. Incorrect Quarter Type Election

This is the most common error we're seeing, and it's catching people off guard because it seems so basic. You've entered all your income and expenses correctly, but when you try to submit, HMRC's system rejects the filing with an error about "quarter type mismatch" or "incorrect period selection".

The problem is usually that you've selected the wrong accounting basis for your quarterly update. Most MTD software asks you to choose between "cash basis" and "accruals basis" when you set up your filing, but many people pick the wrong one or don't realise this choice affects how HMRC processes your submission.

How to Fix It

1. Check your original MTD registration details - log into your HMRC business tax account and look at your "accounting method" under your business details
2. In your MTD software, go to your Q1 submission settings (usually under "Filing Options" or "Submission Settings")
3. Make sure the accounting basis matches exactly what you registered with HMRC
4. If you need to change it, you'll probably need to create a new quarterly update rather than editing the existing one
5. Resubmit with the correct quarter type

2. Duplicate Expense Entries from Spreadsheet Imports

CSV imports are brilliant for getting your expenses into MTD software quickly, but they're also creating a specific type of error that's hard to spot until you submit. The problem happens when people import their expenses from a spreadsheet, then manually add some of the same expenses again because they're not sure the import worked properly.

HMRC's validation system is getting better at spotting these duplicates, especially identical amounts on the same date. Your quarterly update gets rejected with a "duplicate transaction" error, but the software doesn't always make it clear which expenses are causing the problem.

How to Fix It

1. Export your current expense list from your MTD software (most have an "Export to CSV" option)
2. Sort by date and amount to spot obvious duplicates
3. Look for expenses with identical amounts on the same date - these are usually the culprits
4. Delete the duplicate entries (keep the one with the most complete description)
5. For future imports, clear out all existing expenses before importing your CSV to avoid this happening again

Our guide to importing Q1 expenses with CSV format has more detail on setting up clean imports that won't create duplicates.

3. Mixed-Income Categorisation Errors

If you're both self-employed and a landlord, this error is probably familiar. You've got rental income, business income, maybe some freelance work on the side, and the MTD software is throwing errors about income being assigned to the wrong category or business stream.

The issue is that HMRC treats each income source as a separate "business" for MTD purposes, but many people lump everything together in one quarterly update. Your rental income needs to be filed under your property business, your self-employment income under your trade business, and so on.

How to Fix It

1. List out all your income sources for Q1 (rental, self-employment, freelance contracts, etc.)
2. Check your HMRC business tax account - each income source should be registered as a separate business with its own UTR or business ID
3. In your MTD software, make sure you're filing separate quarterly updates for each business
4. Move any income or expenses that are in the wrong business category to the correct one
5. If your software doesn't handle multiple businesses well, you might need different software or separate filings

We covered this in detail in our post about mixed income and MTD for landlords with self-employment, including which software handles multiple income sources best.

4. Timing Mismatches Between Bank Records and Submissions

This error is subtle but increasingly common. Your bank statement shows a payment going out on 30 June, but you've recorded it as 1 July in your MTD submission. Or you've got income that hit your account on 5 April but relates to work done in March, and you're not sure which quarter it belongs in.

HMRC's validation is getting stricter about dates, especially for cash basis filers where the date money actually changes hands matters more than invoice dates.

How to Fix It

1. Download your bank statements for April, May, and June 2026
2. Go through your Q1 quarterly update line by line
3. For each entry, check the date you've used matches when the money actually moved (not when you raised an invoice or received a bill)
4. Move any transactions that actually happened outside 6 April - 5 July 2026 to the correct quarter
5. For cash basis filers: use the date the money hit your bank account, not the invoice date
6. For accruals basis filers: use the date you earned the income or incurred the expense, even if money moved later

5. Missing Supporting Documents

HMRC isn't asking for receipts with your quarterly update, but they are checking that your submission includes basic supporting information where it's required. The most common missing elements are:

• Mileage logs for vehicle expense claims
• Home office percentage calculations for working from home expenses
• Description fields left blank on large expense items
• Missing postcode information for property income

These don't always cause immediate rejection, but they can trigger queries from HMRC later, and some MTD software won't let you submit without them.

How to Fix It

1. Go through your quarterly update and check every expense over £100 has a proper description
2. If you've claimed vehicle expenses, make sure you have mileage records that add up to your claim
3. For home office expenses, note down what percentage of your home you use for business
4. For landlords, check that property addresses and postcodes are complete
5. Add any missing information before you submit

Getting Help Before the Deadline

With 8 weeks still to go before 7 August, there's plenty of time to fix these errors and get your Q1 quarterly update submitted correctly. The key is not to panic if you hit one of these problems - they're all common, and they're all solvable.

If you're struggling with any of these issues, our pre-submission checklist covers the most important checks to run before you file. And if you need to make changes to a quarterly update you've already submitted, our guide to amending Q1 MTD updates explains exactly how the correction process works.

The most important thing to remember is that these errors are part of the learning process. Nearly everyone hits at least one of them in their first few quarterly updates. The difference is knowing how to spot and fix them quickly, so you can get your filing done and move on to preparing for Q2.