What is the Self Assessment tax return deadline?

Self Assessment covers the UK tax year, which runs from 6 April to 5 April the following year. For the 2025/26 tax year (6 April 2025 to 5 April 2026) there are two filing deadlines depending on how you submit.

Paper returns must reach HMRC by 31 October 2026. Online returns must be submitted by 31 January 2027. Any tax owed for 2025/26 must also be paid by 31 January 2027, regardless of whether you file on paper or online. The 2026/27 return follows the same pattern one year later: online filing and payment by 31 January 2028.

If you have a payment on account obligation, the first payment on account for 2026/27 is due by 31 January 2027 and the second by 31 July 2027. Payments on account are advance payments towards your next year's tax bill. They are required when your Self Assessment tax liability exceeds £1,000 and less than 80% of your tax is collected at source. Each payment is 50% of the previous year's liability.

Penalties for missing the online deadline escalate automatically. One day late triggers an immediate £100 fixed penalty, even if no tax is owed and even if the tax has already been paid. After three months late (1 May 2027 for 2025/26 online returns), HMRC adds £10 per day for up to 90 days, which is a further £900 maximum. After six months late (1 August 2027), a further penalty of the higher of £300 or 5% of the tax due is charged. After twelve months late (1 February 2028), another penalty at the same level is added. In cases of deliberate withholding, HMRC can charge up to 100% of the tax due.

For late payment, HMRC charges interest at the Bank of England base rate plus 4 percentage points (the rate is currently 7.75% per annum) on the overdue amount. There are also 5% surcharges on unpaid tax at 30 days, 6 months, and 12 months after the payment deadline.

You must register for Self Assessment before you can file. The registration deadline for the 2025/26 tax year is 5 October 2026. You must register if you are self-employed with income over £1,000, a company director, a higher or additional rate taxpayer, have untaxed income such as rent or investment returns, or earn over £100,000 in the year.

If you receive a notice to file from HMRC, you must submit a return even if you have no tax to pay. Ignoring the notice does not remove the obligation and the £100 penalty will apply.

If you have a reasonable excuse for filing late, such as a serious illness, bereavement of a close relative, or a genuine system failure outside your control, you can appeal penalties through HMRC's online service. Not having your paperwork ready, forgetting the deadline, or being too busy are not accepted as reasonable excuses.

For the 2025/26 tax year the key dates in full are: 5 October 2026 to register for Self Assessment; 31 October 2026 for paper returns; 31 January 2027 for online returns, all tax payments and the first payment on account for 2026/27; 31 July 2027 for the second payment on account for 2026/27.

AccountsOS tracks your Self Assessment deadline automatically and sends reminders well in advance. Finn can help you pull together salary, dividend, and expense figures so you have everything ready before January.