I tried posting this in the Self-Assessment forum but have had not a single reply, so trying here. I am employed and pay PAYE, but at times have been over the 40% tax threshold and also have rental income from a property, therefore I have submitted a Self-Assessment return every year since 2017-2018 inclusive. When my Self-Assessment has been processed I either get a refund for tax overpaid or if I have underpaid then my tax code for the following year is adjusted to recover the underpayment. This year (2022/2023) after my return was processed I received a refund for tax overpaid, and then shortly afterwards a new notice of coding which claims that I owe an amount of underpaid tax for a previous year (year unspecified). Given that I have submitted self-assessment every year and my tax code adjusted shortly afterwards how can I owe tax unpaid for a previous year, and why can HMRC not at least tell me which year and why? I have written to HMRC with the exact details and asked for an explanation but have received no reply.