Self Assessment Tax Deadlines 2025/26: Key HMRC Dates You Must Not Miss

Missing an HMRC Self Assessment deadline results in automatic penalties — even if you don't owe any tax. This guide covers every key tax deadline for 2025/26, including the 31st January filing deadline, payment on account dates, and how Accounted4Tax ensures you never pay an unnecessary fine.

Why Self Assessment Deadlines Matter

Missing an HMRC Self Assessment deadline results in automatic financial penalties — even if you have no tax to pay, or you are owed a refund. Knowing the key dates and filing on time is essential for every self-employed person, landlord, CIS worker and anyone else required to complete a tax return.

5th April 2026 — End of the 2025/26 Tax Year

The UK tax year runs from 6th April to 5th April. The 2025/26 tax year ends on 5th April 2026. After this date, you can begin filing your Self Assessment tax return for that year. The sooner you file after 5th April, the sooner any refund you are owed lands in your bank account.

31st July 2025 — Second Payment on Account

If HMRC has determined that you must make payments on account — which happens when your tax bill exceeds £1,000 — your second payment on account for the 2024/25 tax year is due on 31st July 2025. This is an advance payment towards your current year's tax liability and must be paid on time to avoid interest charges.

Many self-employed people are caught off guard by payments on account, particularly in their first or second year of Self Assessment. If you are unsure whether this applies to you, contact Accounted4Tax and we will explain your position clearly.

5th October 2025 — Register for Self Assessment Deadline

If you became self-employed, started receiving rental income, or had any other untaxed income during the 2024/25 tax year, you must register with HMRC for Self Assessment by 5th October 2025. Missing this registration deadline can result in penalties on top of any tax owed.

31st October 2025 — Paper Tax Return Deadline

If you choose to file your Self Assessment tax return on paper rather than online, the deadline is 31st October 2025 for the 2024/25 tax year. Most people file online, which gives you until 31st January 2026.

31st January 2026 — The Most Important Deadline of the Year

31st January is the critical Self Assessment deadline. By midnight on 31st January 2026 you must have completed all three of the following:

• Filed your 2024/25 Self Assessment tax return online with HMRC

• Paid any tax owed for the 2024/25 tax year in full

• Made your first payment on account for the 2025/26 tax year (if applicable)

Missing any of these triggers automatic HMRC penalties. Even if you owe no tax at all, a late filing still results in a £100 penalty.

What Are the Penalties for Missing the January Deadline?

HMRC operates a tiered penalty system for late Self Assessment returns:

• 1 day late — an automatic £100 penalty, regardless of whether you owe tax

• 3 months late — an additional £10 per day, up to a maximum of £900

• 6 months late — a further 5% of the tax due or £300, whichever is greater

• 12 months late — an additional 5% of the tax due or £300, plus potential HMRC investigation

On top of filing penalties, HMRC charges interest on unpaid tax from the 31st January deadline onwards. Late payment surcharges of 5% of the outstanding tax can also apply at 30 days, 6 months, and 12 months after the deadline.

Making Tax Digital Deadlines From 2026

From April 2026, sole traders and landlords earning over £50,000 must comply with Making Tax Digital for Income Tax — quarterly digital submissions to HMRC throughout the year rather than a single annual return. Accounted4Tax helps clients transition to MTD smoothly and handles all quarterly submissions on their behalf.

Never Miss a Deadline With Accounted4Tax

Our Self Assessment service includes proactive deadline tracking, reminders, and prompt filing — so you never pay an unnecessary HMRC penalty. We file returns for our clients well before the January deadline to ensure everything is in order and any tax owed is known well in advance. Start your Self Assessment tax return from £99 or book a free consultation today. Call us on 07563 811938 or WhatsApp us.