Can I amend a return I have already filed?
Yes, up to 12 months after the 31 January filing deadline for that year. After that you have to write to HMRC.
Last checked 12/09/2026.
Yes. You have until 12 months after the 31 January deadline for that tax year to amend it yourself online, and the change recalculates the bill automatically.
Beyond that window you can still put things right, but it takes a letter to HMRC explaining what was wrong and why. Overpayment relief claims generally have a four-year limit.
Amending because you forgot a deduction is worth doing. A driver who did not realise they could claim mileage on their first year is usually owed a meaningful refund.
Amending because you under-declared income is also worth doing, and quickly. An unprompted correction is treated far more leniently than one HMRC finds.
Related
When do I have to pay my tax bill?
By 31 January following the end of the tax year, with a second instalment on 31 July if you are making payments on account.
HMRC & taxUpdated 19/08/2026
How long do I need to keep my records?
At least 5 years after the 31 January filing deadline for that tax year.
HMRC & taxUpdated 19/08/2026
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