Yes, within twelve months of the filing deadline — so for the year ended 5 April 2026, you have until 31 January 2028 to amend the return you filed by 31 January 2027. Amend online and the correction flows through automatically; if additional tax is due, interest runs from the original due date, not the date you noticed. Beyond that window you are into overpayment relief, which has a four-year limit and is a formal claim rather than a simple edit. If you have realised you underdeclared, disclose it rather than wait: unprompted disclosures attract dramatically lower penalties than the same error found by HMRC, and the difference is often the whole penalty. Amending a return does not extend HMRC’s enquiry window on the rest of it.