Not everyone in the UK needs to complete a Self Assessment tax return. However, if you have untaxed income, run a business, or receive money from certain sources, HMRC may require you to file. This guide explains exactly who needs to file for the 2025/26 tax year, how the process works, and how to avoid common mistakes that can lead to fines.

What Is a Self Assessment Tax Return?

Self Assessment is the system HMRC uses to collect tax from individuals whose income is not fully taxed at source through PAYE. Rather than your employer automatically deducting all the tax you owe, you submit a return each year to declare your income and expenses. HMRC then calculates what you owe (or what you are owed back if you have overpaid).

Who Must File a Tax Return?

For the 2025/26 tax year (6 April 2025 to 5 April 2026), you will usually need to file a Self Assessment return if 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 received untaxed income such as:
  • Rental income from property or land
  • Dividends or savings interest above your personal allowances
  • Foreign income
  • Profits from cryptocurrency
  • Online sales from eBay, Etsy, Amazon, or other platforms over £1,000 in a year
• You or your partner had income over £50,000 and claimed Child Benefit.
• You had income from a trust or settlement.
• You need to claim tax relief on expenses, charitable donations, or pension contributions.
• You owe additional tax on income that was taxed at source but not enough was deducted.

When You Do Not Need to File

You generally do not need to file a Self Assessment return if all your income is from employment, your tax is handled entirely through PAYE, and you have no other income. There are exceptions, so check HMRC’s guidance if you are unsure.

Key Deadlines for the 2025/26 Tax Year

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

Missing these dates can result in an automatic £100 fine, with additional daily penalties and interest if you delay further.

Example Scenarios

Example 1: Side Hustle Income

Alice works full-time but makes £2,200 selling on Etsy. She exceeds the £1,000 trading allowance, so she must register and file a return.

Example 2: Landlord

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

Example 3: Dividends and Savings

David earns £2,000 in dividends and £1,000 in bank interest. This is above his allowances, so he needs to file a return to pay the extra tax.

Example 4: Cryptocurrency Profits

Ravi sells cryptocurrency and makes a £3,000 profit. Even if paid in crypto rather than cash, this counts as taxable income and must be declared.

Common Mistakes to Avoid

• Not registering in time – you must register by 5 October following the tax year.
• Thinking you don’t need to file if you made a loss – HMRC still requires a return if you meet other criteria.
• Forgetting about “payment on account” – many first-time filers are surprised by having to pay part of next year’s bill early.
• Leaving it until the last minute – filing late increases the risk of mistakes and penalties.

How to Register

If you have never filed before, you must register with HMRC to get your Unique Taxpayer Reference (UTR). This number is used every year when you file.

Filing Your Return

You can file online through HMRC’s system or using commercial tax software. You will need records of all income, expenses, and relevant reliefs. Once you submit, HMRC will confirm the amount you owe and how to pay it.

Tips for a Smooth Self Assessment

• Keep records all year rather than scrambling in January.
• Consider using bookkeeping software or a spreadsheet to track income and expenses.
• Set aside money regularly for your tax bill so you are not caught short.
• Double-check all figures before submitting to avoid penalties for careless errors.

Frequently Asked Questions