Self Assessment deadlines, penalties and compliance for the tax year 2025/26 tax year

In this guide, we outline who needs to file a Self Assessment tax return, the key deadlines for the 2025/26 tax year, and practical steps you can take to avoid penalties and stay compliant.

Who needs to file a Self Assessment tax return in the UK?

You may need to complete a Self Assessment tax return if any of the following apply to you:

• You are self-employed as a sole trader and earned more than £1,000 in gross income during the tax year.
• You received untaxed income, such as rental income, foreign income, or investment income.
• You are liable to pay the High Income Child Benefit Charge.
• You need to report and pay Capital Gains Tax (CGT) following the sale or disposal of an asset.
• HMRC has specifically requested that you complete a tax return.

Making Tax Digital (MTD)

Some taxpayers may also need to comply with HMRC’s Making Tax Digital (MTD) requirements. From April 2026, self-employed individuals and landlords with qualifying income over £50,000 will generally be required to:

• keep digital accounting records
• use compatible software
• submit quarterly updates to HMRC

The income threshold is expected to reduce further in future years, meaning more taxpayers are likely to fall within the Making Tax Digital (MTD) regime over time.

If you are unsure whether you need to file a Self Assessment tax return or comply with Making Tax Digital requirements, please contact us for advice and assistance.

2025/26 Self Assessment tax return deadlines

Here are some important dates to note for the 2025/26 tax year.

5 October 2026 - Deadline to register for Self Assessment

If you need to file a Self Assessment tax return for the first time for the 2025/26 tax year, you must register with HMRC by 5 October 2026.

First-time submissions - If you are newly self-employed as a sole trader and have not submitted a Self Assessment tax return before, you will need to register with HMRC. Once registered, HMRC will issue you with a 10-digit Unique Taxpayer Reference (UTR) number, which is required to file your tax return online.

Existing self-employed taxpayers - If you have previously registered for Self Assessment and already file tax returns, you do not normally need to register again or complete another CWF1 form unless your circumstances have changed.

31 October 2026 – Deadline for paper tax return submissions

Although HMRC continues to encourage digital filing, some taxpayers still choose to submit paper tax returns by post. The deadline for paper submissions for the 2025/26 tax year is 31 October 2026. If you miss this deadline, you can still file your return online before the online filing deadline.

31 January 2027 - Deadline for online tax return filing and payments

This is the deadline for submitting online Self Assessment tax returns for the 2025/26 tax year and paying any tax owed. Taxpayers who are required to make payments on account will usually need to make their first payment on account by this date. Payments on account are typically calculated as 50% of the previous year’s tax liability.

31 July 2027 - Deadline for second payment

Taxpayers who are required to make payments on account must usually make their second payment on account by 31 July 2027.

HMRC penalties for late filing and tax payments

If you miss HMRC’s deadlines for filing your Self Assessment tax return or paying tax owed, penalties and interest charges may apply.

Penalties for late filing

HMRC currently applies the following penalties for late Self Assessment tax returns:

• An automatic fixed penalty of £100 if your return is filed after the deadline, even if no tax is due.
• Daily £10 fine for filing after 90 days and up to £900 for the next three months.
• £300 or 5% of tax due (whichever is higher) for filing after six months.
• Additional £300 or 5% (whichever is higher) after 12 months

Penalties for late payment

If tax remains unpaid after the payment deadline, HMRC may charge:

• 5% of unpaid tax after 30 days
• Additional 5% after six months
• Extra 5% after 12 months

How to be compliant and avoid penalties

Meeting HMRC deadlines and keeping accurate records can help reduce stress, avoid unnecessary penalties and ensure your tax affairs remain compliant.

Some practical steps taxpayers can take include:

• Estimating annual income and tax liabilities early in the tax year.
• Using accounting software to maintain accurate digital records and reduce errors.
• Setting calendar reminders for key filing and payment deadlines.
• Budgeting regularly and setting aside funds to cover future tax liabilities.
• Seeking professional advice where tax affairs are more complex.

At Abbott & Brown, our team of ICAEW and ACCA-qualified chartered accountants can assist landlords, self-employed individuals and business owners with all aspects of accounting, tax compliance and HMRC reporting requirements.

FAQs

Q1. What is the Self Assessment deadline in the UK for 2025/2026 tax year?

For the 2025/26 tax year:

• the deadline for paper tax returns is 31 October 2026
• the deadline for online tax returns and payment of tax owed is 31 January 2027

Q2. What will happen if I miss the deadline?

If you miss the filing deadline, HMRC will usually issue an automatic £100 late filing penalty. Additional daily penalties and percentage-based penalties may apply for longer delays, together with interest charges on unpaid tax.

Q3. Do I need an accountant to file my Self Assessment return?

You are not legally required to use an accountant to complete your Self Assessment tax return. However, the services of a professional accountant, such as Abbott & Brown can help ensure compliance, reduce the risk of errors and identify legitimate tax-saving opportunities.