Key Self-Assessment Deadlines for 2026/27

Register for self-assessment by 5 October if this is your first year filing. Paper returns are due by 31 October, online returns by 31 January, and that same January deadline is when any tax owed for the year is due alongside your first payment on account for the year ahead. Missing the online deadline triggers an automatic £100 penalty even if you owe no tax, so file early if your figures are ready rather than waiting for the wire.