Register by 5 October following the end of the tax year in which you started; file a paper return by 31 October; file online by 31 January; and pay by 31 January. So for the year ended 5 April 2026: register by 5 October 2026, file online and pay by 31 January 2027. If you make payments on account, the second is due 31 July. There is one deadline that quietly matters more than it looks: filing online by 30 December lets you have tax under £3,000 collected through your PAYE code over the following year instead of paying it in a lump, which is a genuine cash flow benefit for anyone with employment income. Registering late is its own failure, separate from filing late.