Self Assessment deadlines: returns, payments on account and penalties

For most practices, Self Assessment is the busiest season of the year. The rules are well known, but with hundreds of individual clients, the challenge is knowing at any moment who has sent their records, whose return is filed and who still owes tax.

The key dates

For a tax year ending on 5 April:

So for the 2025/26 tax year, the online return and balancing payment are due by 31 January 2027, together with the first payment on account for 2026/27.

Payments on account

Taxpayers whose Self Assessment liability is £1,000 or more usually make two payments on account towards the next year’s tax, each half of the previous year’s liability. They are not needed where more than 80% of the tax was collected at source, for example through PAYE.

Clients whose income has fallen can apply to reduce their payments on account, but should be warned that interest is charged if the reduction turns out to be too large.

Late filing penalties

• 1 day late: £100, even if there is no tax to pay
• 3 months late: £10 a day for up to 90 days (up to £900)
• 6 months late: the greater of 5% of the tax due or £300
• 12 months late: a further 5% or £300, and more where information was deliberately withheld

Late payment penalties and interest

Penalties of 5% of the unpaid tax apply at 30 days, 6 months and 12 months after the due date, on top of late payment interest. Interest runs at the Bank of England base rate plus 4%. Taxpayers who are within Making Tax Digital for Income Tax move to HMRC’s newer points-based penalty regime, described in our MTD quarterly updates guide.

Making Tax Digital for Income Tax

From 6 April 2026, sole traders and landlords with qualifying income over £50,000 must keep digital records and send quarterly updates through MTD-compatible software. The thresholds fall to £30,000 from April 2027 and £20,000 from April 2028. For these clients the Self Assessment year now includes four MTD quarterly updates before the final return.

Practical tips for practices

1. Chase records in the summer, not December. Set an internal deadline well before 31 January.
2. Record the last tax year filed for each client. In Deadlio, Last SA tax year filed (for example 2025 for 2024/25) moves the next deadline on automatically, and the Mark done button does it in one click.
3. Send clients their tax figures early. A return filed in October with tax due in January gives them time to budget.
4. Do not forget partnerships. Partnership returns follow the same dates, and each partner has their own personal return.

Related guides

• MTD for Income Tax quarterly updates
• VAT return deadlines
• AML client review cycle
• See it in practice: A landlord’s first MTD quarter
• All deadline guides

This guide is general information for accountancy practices and is not a substitute for checking the current rules on GOV.UK. Last reviewed October 2026.