Self Assessment: the 31 January deadline and penalties

Self Assessment is how HM Revenue and Customs (HMRC), the UK tax authority, collects income tax from people whose tax is not taken automatically from wages: the self-employed, landlords, higher earners and those with side income. For the 2025/26 tax year (6 April 2025 to 5 April 2026) the deadlines are strict.

Which deadlines matter?

• Tell HMRC you need to file — by 5 October 2026.
• Paper return — must reach HMRC by 11:59pm on 31 October 2026.
• Online return — by 11:59pm on 31 January 2027.
• Pay your tax — by 11:59pm on 31 January 2027.
• To pay through your tax code, file online by 30 December 2026.

Depending on the amount, a second "payment on account" may be due by 31 July.

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What if the return is late?

• straight away — £100, even if you owe no tax;
• after 3 months — an extra £10 a day, up to £900;
• after 6 months — a further 5% of the tax due or £300, whichever is greater;
• after 12 months — again 5% or £300, whichever is greater.

And if you pay the tax late?

Late payment is penalised separately: 5% of the unpaid tax at 30 days, 6 months and 12 months. Interest is also charged on top of the amount owed.

Who needs to file?

Self Assessment applies to people whose tax is not taken automatically by an employer. Most often these are the self-employed and sole traders, landlords with rental income, business partners, and people with high incomes or extra income not taxed at source. If you are unsure whether you must file, GOV.UK has a checker that decides from a few questions.

How do you pay?

You can pay by online banking, by card or using HMRC's details; the money must arrive by 31 January. If the bill is large, HMRC may require a "payment on account": part of next year's tax is paid in advance — the first instalment by 31 January, the second by 31 July. So January can cost more than one return suggests.

What to do now

If you are filing for the first time, you must register with HMRC and get sign-in details first, which takes time — so leaving it to late January is risky. You can file before the deadline and still pay by 31 January; filing early tells you the bill in advance.