Quick answer: The most common MTD quarterly update mistakes are miscategorised expenses (especially repairs vs improvements for landlords), missing bank transactions from unconnected accounts, confusing quarterly updates with a full tax calculation, and leaving personal and business spending mixed on the same card.
Quarterly updates are simpler than a full tax return, but that simplicity hides a few traps that catch people repeatedly, particularly in the first couple of quarters before habits settle in.
Miscategorised expenses
This is the single most common issue, and the most consequential for landlords in particular. Repairs are deductible; improvements are capital and are not. Getting this wrong doesn’t just affect one quarter, since it flows into your final declaration too. See our guide to repairs vs improvements for concrete examples.
Missing transactions from unconnected accounts
If you have a second business account, a savings account you occasionally move business funds through, or a card that isn’t linked to your MTD software, transactions there simply won’t appear unless you add them manually. Before each quarterly submission, it’s worth checking every account income or expenses could plausibly have passed through.
Treating a quarterly update like a tax calculation
A quarterly update is a summary of income and expenses, not a tax bill. There’s no box for personal allowances, pension contributions, or Gift Aid; those adjustments happen later, in your final declaration. Submitting an update doesn’t mean you owe HMRC money immediately, and a running tax estimate in your software is an estimate, not a demand.
Mixing personal and business spending
If personal and business transactions run through the same card, every quarter becomes a manual sorting exercise, and it’s easy to miss or wrongly include something. A dedicated business account, even as a sole trader without a legal requirement to have one, makes categorisation dramatically more reliable. See do sole traders need a business bank account.
Forgetting the cumulative model
Each quarterly update reports year-to-date totals, not just that quarter in isolation. A common source of confusion is expecting each submission to be a standalone figure; it’s the running total that matters, and it’s also what makes correcting an earlier mistake straightforward. See how to correct or amend a quarterly update for how that works in practice.
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