A filed return is not final. HMRC expects figures to be corrected, and there is a defined window in which you can do it yourself without anyone's permission: 12 months after the 31 January online filing deadline for that tax year. Within it, amending is routine. Outside it, the route is different and narrower. This page covers both, and what an amendment does to the money.
The window, precisely
You can amend a return for 12 months after the 31 January online filing deadline for that tax year. Note that the clock runs from the deadline, not from the date you filed, so filing early does not shorten it and filing late does not extend it. In practice that gives most people the best part of two years from the end of the tax year to notice a mistake, which is generous, and is why the great majority of corrections never involve anything more than editing the return.
What an amendment changes
The tax for the year is recalculated, and the difference becomes an additional amount owed or a repayment. Interest runs from the original due date on anything additional, so an amendment that increases the bill costs a little more than the tax itself; there is no penalty for the correction as such, provided the original return was made in good faith. If the amendment reduces the bill, the overpayment comes back, and any interest you paid on it is adjusted.
The knock-on to payments on account
Payments on account for the following year are set from the bill for this one, so amending this year's figures moves next year's instalments too. This surprises people who amend after the first instalment has already been paid: the instalment is recalculated, and the difference appears as an adjustment rather than as a new demand. It is worth checking the following year's position after any material amendment rather than assuming the change is confined to one year.
After the window has closed
You can no longer amend the return itself. Where you have overpaid, a claim for overpayment relief is the route, with its own longer time limit and its own conditions, and it is refused in some circumstances that a simple amendment would not have been. Where you have underpaid, the right move is to disclose it rather than to wait, because coming forward materially changes how the matter is treated. Both of those are worth an adviser rather than a worksheet, and this site will not pretend otherwise.
Questions people ask about amend a tax return
Is there a penalty for amending a return?
Not for the correction itself where the original was made in good faith. Interest runs from the original due date on any additional tax, and a careless or deliberate error is treated differently from an honest one.
Does the 12 months run from when I filed?
No, from the 31 January filing deadline for that tax year. Filing early does not shorten the window and filing late does not extend it.
How do I amend a paper return?
By writing to HMRC with the corrections rather than by resubmitting the form. Keep a copy of what you sent and when.
Will amending trigger a compliance check?
Not by itself. Amendments are an ordinary part of the system. A large or unexplained change is more likely to attract a question, which is a reason to keep the evidence, not a reason to avoid correcting it.