The UK Self Assessment online return is due by 31 January following the end of the tax year. The paper deadline (if you''re still filing on paper) is 31 October. Miss either by even one day and the late-filing penalty is £100, regardless of whether tax is owed.

This catches out thousands of people every year. The £100 isn''t the headline — it''s the daily and percentage penalties that pile up for longer delays.

The core dates

For the tax year that ends 5 April 2026 (a "2025/26 return"):

• 5 October 2026 — register for Self Assessment if you''re filing for the first time.
• 31 October 2026 — paper return deadline.
• 31 January 2027 — online return deadline AND the balancing payment deadline AND the first payment on account for 2026/27.
• 31 July 2027 — second payment on account for 2026/27.

Payments on account are a half-and-half advance against the following year''s tax bill. They''re payable in two instalments (January and July) and are due if your previous year''s Self Assessment liability exceeded £1,000.

The late-filing penalty cascade

It compounds. The 31 January deadline is hard. Here''s what happens after:

• 1 day late — £100 fixed penalty, even if you owe no tax or are due a refund.
• 3 months late — £10 per day, for up to 90 days (maximum £900).
• 6 months late — the greater of £300 or 5% of the tax owed, on top of the above.
• 12 months late — another £300 or 5%, on top of everything above.

A return filed 12 months late, where you genuinely owe nothing, costs £1,600 in penalties. With tax owed, the percentage charges scale with the bill.

The late-payment penalty regime

Filing the return is one obligation. Paying the tax is a separate one — even if you file on time but don''t pay, late-payment penalties apply.

• 30 days late — 5% of the unpaid tax.
• 6 months late — another 5%.
• 12 months late — another 5%.

Interest also runs from the original due date, at the HMRC rate (which tracks the Bank of England base rate plus 2.5% — currently meaningfully high).

Reasonable excuse appeals

Penalties can be appealed if you have a "reasonable excuse" — typically defined as something outside your control that prevented you from meeting the deadline, such as a serious illness, bereavement, or a fire / flood destroying your records. Forgetting, being busy, or not understanding the rules do not count.

You appeal online through the HMRC portal, or in writing. Submit the appeal as soon as you can after the deadline; don''t wait. Provide evidence (medical notes, death certificate, insurance claim) where it exists.

What to do this year

• Register by 5 October if it''s your first time. Registration takes a few weeks because HMRC posts your UTR by letter.
• File before Christmas if you possibly can. The HMRC portal gets noticeably slower in January. If you''re using an accountant, get them everything they need by late November — early December at the latest.
• Pay by bank transfer at least 3 working days before the deadline. HMRC card payment surcharges aside, faster payments can still take a working day to land.
• Set the dates as recurring calendar entries. The dates don''t change. They will catch you out next year if you don''t.