Late Self Assessment penalties and deadlines

The online Self Assessment return for the 2025/26 tax year, which ended on 5 April 2026, is due by 11:59pm on 31 January 2027, and so is any tax you owe. Miss it and HMRC charges a £100 penalty straight away, with more added over the following year. Paying late carries separate penalties plus interest. If you have a reasonable excuse, you can appeal.

The deadlines

For the tax year that ran from 6 April 2025 to 5 April 2026, GOV.UK gives these dates:

What	Deadline
Tell HMRC you need to file, if you have not before	5 October 2026
Paper return reaches HMRC	11:59pm on 31 October 2026
Online return and payment of tax	11:59pm on 31 January 2027
Online return if you want the bill collected through your tax code	11:59pm on 30 December 2026
Second payment on account, if you make advance payments	31 July

If you register after 5 October 2026, HMRC sends a letter or email with a new filing deadline, three months from the date on it. The tax itself must still be paid by 31 January 2027. Whether you need a return at all is covered in Do employees need to do Self Assessment?, and the usual public sector trigger for casual income is in side income and the trading allowance.

Late filing penalties

If your return is late, you get:

• an initial £100 penalty
• after 3 months, extra daily penalties of £10 a day, up to a maximum of £900
• after 6 months, a further penalty of 5% of the tax due or £300, whichever is greater
• after 12 months, another 5% or £300, whichever is greater

Those stack, so a return a year late can attract at least £1,600 in filing penalties (£100, £900, then £300 twice) before any tax is counted. All partners in a partnership are charged if a partnership return is late. HMRC’s advice is to send the return as soon as possible, because the penalties keep growing until you do.

Late payment penalties

If you pay your tax late, you get penalties of 5% of the unpaid tax at 30 days, 6 months and 12 months, and interest is charged on the amount owed. Filing on time does not stop a late payment penalty, and paying on time does not stop a late filing one. They are separate.

If you register late, after 5 October, and do not pay all of your tax by 31 January, you may also get a failure to notify penalty. It is based on the amount still unpaid and arrives within 12 months after HMRC receives your return.

Paying a penalty

You must pay within 30 days of the date on the penalty notice. HMRC suggests considering paying even if you appeal, because if the appeal is rejected you owe interest on the penalty from the date it was due. If the appeal succeeds, HMRC repays what you paid with interest.

How to appeal

If you have a reasonable excuse, you can appeal against a penalty. You need the date the penalty was issued, the date you filed (if you have), the date you paid (if you have), and the details of your reasonable excuse. You can appeal online or download form SA370 or SA371 to appeal by post. HMRC then tells you when to expect a reply, and you can ask for a review if you disagree with the decision.

If you did not need to submit a return, ask HMRC to cancel the penalty. Write down what happened and when, as the notice will have dates you can quote back.

If you cannot pay the bill

Contact HMRC as early as you can, before the penalties start to build. Free help for people on low incomes is available from the tax charities TaxAid and Tax Help for Older People, and a tax adviser can file for you.

You can see what HMRC holds about your tax through your personal tax account. Tax year dates are in UK tax year dates.