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Self Assessment 2025/26: Key Deadlines, Common Mistakes and How to File on Time

Who needs to file a Self Assessment?

You must file a Self Assessment tax return if you are:

• A self-employed sole trader earning more than £1,000
• A director of a limited company
• A landlord receiving rental income
• Someone earning over £100,000
• Receiving income from abroad
• Claiming certain tax reliefs or Child Benefit (if income exceeds £50,000)

Key deadlines for 2025/26

• 5 October 2026: Register for Self Assessment if you are new to it
• 31 October 2026: Paper return deadline
• 31 January 2027: Online return deadline and payment of tax owed
• 31 July 2026: Second payment on account (if applicable)

When are Self Assessment tax returns due?

The Self Assessment tax year runs from 6 April to 5 April the following year, which is what people usually mean when they ask about the tax year for Self Assessment. For the 2025/26 tax year, online returns and any tax owed are due by 31 January 2027, with the earlier paper return deadline on 31 October 2026. These are the HMRC tax return deadlines that apply whether you are a sole trader, a landlord or a director filing personally.

If you are asking when do Self Assessment tax returns need to be submitted for the first time, registration comes before filing: you must register by 5 October following the end of the tax year you need to declare. Miss the 31 January deadline and the automatic £100 penalty applies even if no tax is owed.

Common mistakes that cost UK taxpayers money

1. Missing the 31 January deadline. HMRC issues an automatic £100 fine even if you owe no tax. After 3 months, daily £10 penalties begin. After 6 months, a further 5% of tax owed is added.

2. Not claiming all allowable expenses. Sole traders and landlords often under-claim on expenses. Home office costs, mileage, equipment, professional subscriptions and training costs are commonly missed.

3. Forgetting bank interest and investment income. All taxable income must be declared, including savings interest, dividends and cryptocurrency gains.

4. Getting payments on account wrong. If your tax bill is over £1,000, HMRC requires advance payments. Many people do not budget for these and get caught out.

How to make it easier

Use accounting software like Xero or QuickBooks to track income and expenses throughout the year so there is no scramble in January. Better still, work with a qualified accountant who can file your return accurately, identify every deduction you are entitled to, and ensure you are not overpaying.