Made a mistake in a filed quarter? How fixing works

Everyone eventually files a quarter and then spots something wrong: a missed expense, a rent payment in the wrong category, a duplicate line.

The key fact: updates are cumulative

Each quarterly update reports your whole tax year so far, not just the three months. Quarter 2’s update contains Quarter 1’s figures again, refreshed from your records. So when you correct a transaction, the very next update you file automatically sends HMRC the corrected year-to-date totals.

How do you correct a filed quarterly update?

• Correct the transaction in your records now: change the category, amount, or delete the duplicate.
• Carry on as normal. Your next quarterly update carries the correction to HMRC by itself.
• Nothing extra to file today, no amendment form, no phone call.

What if the mistake was in the last quarter of the year?

Quarter 4 has no later quarterly update to carry the correction, so the fix lands at the year end instead. Correct the transaction in your records, and the final declaration works from the corrected figures, because that is where the year is finalised. Nothing is lost by spotting it after 7 May.

When it’s more than a small mistake

The year-end final declaration is where your figures are finalised: that’s the number tax is actually calculated on, usually with an accountant’s eye. If you’ve found something big (a whole account you forgot, income in the wrong tax year), talk to your accountant or HMRC rather than trying to force it through a quarterly update.