Not everyone in the UK needs to complete a Self Assessment tax return, but if you have untaxed income, run a business, or earn over a certain threshold, you might need to. This guide will help you understand whether you need to file one for the 2025/26 tax year, and what happens if you don’t.

What Is a Self Assessment Tax Return?

Self Assessment is the process HMRC uses to collect tax from individuals who don’t have it automatically deducted through PAYE. You complete a tax return once a year to declare your income and pay any tax due.

Who Needs to File a Tax Return?

You usually need to file a Self Assessment return if, during the 2025/26 tax year (6 April 2025 to 5 April 2026), any of the following apply:

• You were self-employed as a sole trader and earned more than £1,000 in gross income.
• You were a partner in a business partnership.
• You had untaxed income, such as:
  • Rental income
  • Dividends or savings interest above your tax-free allowances
  • Foreign income
  • Cryptocurrency profits
  • eBay, Etsy, or other side hustle income above £1,000
• You received Child Benefit and your income (or your partner’s) was over £50,000.
• You had income from a trust or settlement.
• You want to claim tax relief on things like work expenses, donations, or pension contributions.

You Don’t Need to File If...

You don’t need to file a return if you’re fully employed, have no other sources of income, and your tax is handled through PAYE — unless you fall into one of the above categories.

Important Deadlines

• Register for Self Assessment: by 5 October 2026
• Paper tax return deadline: 31 October 2026
• Online tax return deadline: 31 January 2027
• Pay any tax owed: also by 31 January 2027

HMRC may charge penalties and interest if you miss these deadlines, even if you have no tax to pay.

Example Scenarios

Example 1: Side Hustle Income

• Alice works full time but made £2,200 selling on Etsy in her spare time.
• She exceeds the £1,000 trading allowance and must register and file a return.

Example 2: Landlord

• Sophie rents out a house and earns £2,000 in rental income.
• She must report this through Self Assessment.

How to Register

If you haven’t filed before, you need to register with HMRC first. This gives you a Unique Taxpayer Reference (UTR) and allows you to file online.

Filing and Paying

You can file your return online using HMRC’s website or compatible software. After submitting, you’ll get a final tax calculation and must pay anything due by 31 January.