HMRC’s penalties are automatic. There is no warning letter before the first £100 lands. Here is the full ladder, because the top of it is worse than most people think.

The dates

For the 2025/26 tax year, which ended on 5 April 2026:

• Paper return: 31 October 2026
• Online return: 31 January 2027
• Tax owed: 31 January 2027
• If you want the bill collected through your tax code: file by 30 December 2026

If you need to register for the first time, the deadline to tell HMRC was 5 October 2026. Miss that and, on top of everything below, you risk a separate “failure to notify” penalty based on the tax left unpaid at 31 January.

Penalties for filing late

• Day one: £100
• After 3 months: £10 a day, up to £900
• After 6 months: 5% of the tax due or £300, whichever is more
• After 12 months: another 5% or £300, whichever is more

Add those up for someone who owes nothing and files a year late: £100 + £900 + £300 + £300 = £1,600. That is the floor. If you do owe tax, the six and twelve month charges grow with the bill.

Penalties for paying late

Separate from filing. HMRC charges 5% of the tax still unpaid at 30 days, again at 6 months, and again at 12 months. Interest runs on top from the due date.

A worked example

You owe £4,000 for 2025/26. You file online on 1 February 2028, a year and a day late, and pay at the same time.

• Filing: £100 + £900 + £300 + £300 = £1,600. (The 5% options do not bite here: 5% of £4,000 is £200, less than the £300 minimum.)
• Late payment: £200 at 30 days + £200 at 6 months + £200 at 12 months = £600
• Interest on £4,000 for a year at HMRC’s late payment rate on top

Total penalties: £2,200, before interest. On a £4,000 bill.

If you cannot pay

HMRC would rather you file on time and talk about the money. If you cannot pay in full, you may be able to set up a payment plan with HMRC to pay in instalments. The filing penalties still apply if the return itself is late, so file even if you cannot pay.

If you have a reasonable excuse, you can appeal a penalty. “I forgot” is not one, and neither is “HMRC did not remind me”. What can count: a serious or life-threatening illness, a close relative dying shortly before the deadline, your computer failing while you were preparing the online return.

Sources

All pages checked 20 October 2026.

• Self Assessment deadlines: https://www.gov.uk/self-assessment-tax-returns/deadlines
• Self Assessment penalties: https://www.gov.uk/self-assessment-tax-returns/penalties
• If you cannot pay your tax bill on time: https://www.gov.uk/difficulties-paying-hmrc
• Reasonable excuses: https://www.gov.uk/tax-appeals/reasonable-excuses

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