Self Assessment Deadlines: What You Need to Know for 2025/26

Self Assessment can catch people out — not because the process is complicated, but because the deadlines creep up quickly. Missing a deadline results in an automatic £100 penalty from HMRC, even if you don't owe any tax. Here's a complete guide to the key dates for the 2025/26 tax year.

The key Self Assessment deadlines

What happens if you miss the deadline?

HMRC applies automatic penalties for late filing and late payment:

• 1 day late: £100 automatic penalty (even if no tax is owed)
• 3 months late: £10 per day for up to 90 days (maximum £900)
• 6 months late: Additional 5% of tax due or £300, whichever is greater
• 12 months late: Further 5% of tax due or £300, whichever is greater

Late payment of tax also incurs interest charges at the HMRC late payment rate, plus surcharges if the tax remains unpaid after 30 days.

Payments on account

If your Self Assessment tax bill exceeds £1,000, HMRC may require you to make payments on account — advance payments towards next year's tax bill. These are due on 31 January and 31 July each year, each equal to 50% of your previous year's tax bill. This often catches people out, as your January payment includes both the balance for the previous year and the first payment on account for the current year.

Tips for staying ahead of the deadline

• Don't wait until January — start gathering your records as soon as the tax year ends in April
• Set aside tax as you earn to avoid a large bill in January
• If you can't pay, contact HMRC early — they may agree a Time to Pay arrangement
• Consider appointing an accountant to manage your return and keep track of deadlines on your behalf

Need help with your Self Assessment?

At White & Co Accountants in Sutton Coldfield, we handle Self Assessment tax returns for individuals, sole traders, landlords, and company directors across Birmingham and the West Midlands. We manage all deadlines on your behalf and provide a clear summary of your tax liability and payment instructions once your return is filed.