Self Assessment 2025/26: key deadlines, who must file and what late penalties are

The 2025/26 Self Assessment paper deadline is 31 October 2026 and the online and payment deadline is 31 January 2027. This guide explains who must file, how late penalties and interest build up, and where to get help.

What Self Assessment is and the 2025/26 deadlines

Self Assessment is the system HMRC uses to collect Income Tax from people whose tax is not fully taken from pay or pensions at source. If you have other income, you report it on a Self Assessment tax return, a form that sets out your income, reliefs and the tax you owe for one tax year. The return for 2025/26 covers 6 April 2025 to 5 April 2026. GOV.UK sets out the dates for this return. A paper return must reach HMRC by 11:59pm on 31 October 2026, an online return must be submitted by 11:59pm on 31 January 2027, and the tax itself is due by 11:59pm on 31 January 2027. If you file online and want any tax owed to be collected through your tax code instead, the online deadline is earlier, 30 December 2026. The 2026/27 tax year, which runs from 6 April 2026 to 5 April 2027, is the current one, and its return will follow a year later.

Who has to send a return, and who may not

HMRC will tell some people to file, and if you receive a notice you must send a return even if you would not otherwise need to. Beyond that, the rules depend on the type of income. GOV.UK says that for self-employment (trading) income, a gross income of £1,000 or less can generally be left alone because of the £1,000 trading allowance, but over £1,000 means registering for Self Assessment. For property income, £1,000 or less generally needs no action, over £1,000 up to £2,500 means contacting HMRC, and over £2,500 means registering. Other income over £1,000 up to £2,500 also means contacting HMRC, and over £2,500 means registering. The allowances cannot be used for income from a company you own or control, a partnership you are in, or your employer. If you are new to Self Assessment, you had to register by 5 October 2026, and GOV.UK warns that telling HMRC after that date could lead to a penalty. You may also choose to file when your income is £1,000 or less, for example to claim a loss or to pay voluntary Class 2 National Insurance. These rules are for general guidance only, and the GOV.UK checker covers other cases.

How late filing and late payment penalties build up

The filing penalties and the payment penalties are separate. According to GOV.UK, for a late return there is an initial penalty of £100. After three months, daily penalties of £10 per day apply, up to a maximum of £900. After six months there is a further penalty of 5% of the tax due or £300, whichever is greater, and after twelve months another of 5% of the tax due or £300, whichever is greater. For tax paid late, GOV.UK lists a penalty of 5% of the unpaid tax at 30 days, a further 5% at six months and a further 5% at twelve months. Interest is also charged on the amount owed, and HMRC lists the late payment rate for Self Assessment as 7.75%, effective from 9 January 2026. That rate is variable and can change. Penalties must be paid within 30 days of the date on the penalty notice, and payments due on a weekend or bank holiday should reach HMRC by the last working day before, except for Faster Payments or card payments.

A worked example

These figures are illustrative only and do not describe any real person. Suppose someone owes £4,000 for 2025/26 and neither files nor pays for just over six months after 31 January 2027. Filing penalties would be £100, plus £10 a day for the maximum of 90 days, which is £900, plus the six-month penalty. Five per cent of £4,000 is £200, which is less than £300, so £300 applies. That is 100 + 900 + 300 = £1,300. Late payment penalties would be 5% of £4,000 at 30 days, which is £200, plus another 5% at six months, which is £200, a total of £400. Together the penalties come to 1,300 + 400 = £1,700. Interest is separate. At 7.75% a year, £4,000 for half a year is 4,000 × 0.0775 × 0.5, which is about £155, though the real figure depends on the rate and the exact days. So a £4,000 bill would grow by roughly £1,855 in this example, before any twelve-month penalties.

Paying, getting help and appealing

GOV.UK lists several ways to pay a Self Assessment bill. Faster Payments, CHAPS and debit or corporate credit card online usually arrive the same or next day, while Bacs and an existing Direct Debit take about three working days, and a new Direct Debit takes five working days. You can no longer pay at the Post Office. If you cannot pay in full, you can ask HMRC to set up a payment plan to pay in instalments, and HMRC checks whether the plan is affordable. If you have a reasonable excuse for missing a deadline, you can appeal a penalty. GOV.UK describes it as something that stopped you meeting a tax obligation for a valid reason, such as a close relative dying shortly before the deadline, an unexpected hospital stay, or problems with HMRC's online services. Not having enough money to pay, not receiving a reminder, and mistakes on the return do not count. You must file or pay as soon as you can after the excuse ends.

Common mistakes to look out for

• Assuming that paying the tax by 31 January 2027 also files the return. Filing and paying are separate tasks.
• Forgetting that the paper deadline, 31 October 2026, is three months earlier than the online deadline.

Check before you act

Dates, rates and penalties are set by HMRC and the Government and can change. This guide is general information, not personal advice, so confirm everything on the official GOV.UK pages linked below, and consider free help from HMRC, Citizens Advice or MoneyHelper if you are unsure about your own position.

Sources

• GOV.UK, Self Assessment tax returns: deadlines
• GOV.UK, Self Assessment tax returns: penalties
• GOV.UK, Tax-free allowances on property and trading income
• GOV.UK, Register for Self Assessment
• GOV.UK, Pay your Self Assessment tax bill
• GOV.UK, Reasonable excuses
• GOV.UK, HMRC interest rates for late and early payments
• GOV.UK, If you cannot pay your tax bill