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

With the Q1 MTD deadline on 7 August 2026 approaching, many sole traders and landlords are preparing their first quarterly update. The same handful of mistakes come up again and again. The good news? Every one is fixable, and most can be sorted out in minutes once you know what to look for.

One point first: a quarterly update only sends HMRC category totals for each income source - not individual transactions, receipts or descriptions. So most of these errors won't be "rejected" by HMRC. They will simply make your figures wrong, which is why it pays to catch them before you submit.

1. Choosing the Wrong Quarter Type or Accounting Basis

Before your first quarterly update of the tax year, your software asks two set-up questions:

• Quarter type - standard quarters (6 April to 5 July, and so on) or calendar quarters (1 April to 30 June, and so on). The deadlines are the same either way.
• Accounting basis - cash basis (the default for most sole traders and landlords) or traditional accruals accounting.

Picking the wrong one means transactions near the quarter boundaries, or unpaid invoices and bills, end up in the wrong period.

How to Fix It

1. Decide which quarter type fits your records. If you have no preference, standard quarters are the default.
2. Use the same accounting basis you use for your tax return. Most sole traders use the cash basis.
3. Check both settings in your MTD software before you submit Q1. You can't change quarter type once you have submitted the first update of the tax year.

2. Duplicate Expenses from Spreadsheet Imports

CSV and Excel imports are a fast way to get expenses into MTD software, but they can create duplicates - for example, when people import a file and then also type in some of the same expenses because they weren't sure the import worked. HMRC only sees your totals, so it won't flag duplicates. Your expenses will simply be overstated.

How to Fix It

1. Sort your records by date and amount to spot obvious duplicates
2. Look for identical amounts on the same date from the same supplier
3. Remove the duplicate entries
4. Check your category totals against your bank statements before submitting

Our guide to importing Q1 expenses with CSV format has more detail on setting up clean imports.

3. Mixing Up Income Sources

If you're both self-employed and a landlord, HMRC treats each as a separate income source with its own quarterly update. Rental income and property costs go in the property update. Business income and expenses go in the self-employment update. A cost that lands in the wrong one gives you the wrong profit for both.

How to Fix It

1. List your income sources for Q1 (self-employment, UK property, foreign property)
2. Check that your software shows each one, matching what HMRC holds for you
3. Make sure each transaction is assigned to the right source
4. Split shared costs, such as a phone used for both, on a reasonable basis

We cover this in detail in our post about mixed income and MTD for landlords with self-employment.

4. Timing Mistakes

Your bank statement shows a payment going out on 3 July, but you recorded it as 7 July. Or you received income on 5 July for work done in June and you're not sure which quarter it belongs in. On the cash basis, the date the money moves decides the quarter.

How to Fix It

1. Download your bank statements covering 6 April to 5 July 2026 (or 1 April to 30 June on calendar quarters)
2. Check each entry's date against when the money actually moved
3. Move anything outside the quarter's dates to the right period
4. For cash basis filers: use the date the money hit or left your account, not the invoice date
5. For accruals basis filers: use the date you earned the income or incurred the expense

5. Missing Supporting Records

HMRC doesn't ask for receipts with your quarterly update, but you must be able to support every figure if it asks later. The gaps that cause most trouble are:

• Mileage logs for simplified mileage claims (55p per mile for the first 10,000 miles in 2026-27)
• Home office calculations for working from home expenses
• Receipts or invoices for large expense items
• Records of which property each landlord expense relates to

How to Fix It

1. Check every expense over £100 has a receipt, invoice or clear note
2. Make sure your mileage log adds up to your claim
3. Write down how you worked out any home office or mixed-use percentage
4. Keep digital copies of everything for at least five years after the 31 January filing deadline

Getting Help Before the Deadline

There's time to fix these errors and get your Q1 update right. Our pre-submission checklist covers the most important checks to run before you file. If you need to correct an update you've already submitted, our guide to amending Q1 MTD updates explains 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.