Do I need to complete a self assessment tax return? It’s one of the questions I get asked most often, and the honest answer is: it depends on your situation, but it’s worth checking rather than assuming someone else handles it for you.

Who Actually Needs to File a Self Assessment Tax Return?

The most common reason people need to file is self-employment. If you’re a sole trader and your income from self-employment was more than £1,000 in the tax year, HMRC expects you to complete a self assessment tax return. That £1,000 figure is called the trading income allowance, and it’s confirmed in HMRC’s official guidance for the 2025-26 tax year.

But self-employment isn’t the only trigger. You’ll also need to file if you earned more than £10,000 from savings or investments, received rental income, had untaxed income from abroad, or earned over £100,000 in the year. If you’re a company director, a partner in a business partnership, or you need to claim certain tax reliefs, those all count too.

What Are the Deadlines and What Happens If You Miss Them?

The two dates that matter most are 31 October (if you’re filing a paper return) and 31 January (if you’re filing online). The January deadline also covers payment of any tax you owe for the previous tax year, so it’s doing double duty. Most people file online, which gives you a few extra months.

Miss the 31 January deadline and the penalties start immediately. HMRC’s penalty structure works like this: £100 the moment you’re late, then £10 per day after three months (up to £900), then a further charge of 5% of the tax due or £300 (whichever is greater) at six months, and again at twelve months. If you’ve already missed the deadline, that doesn’t mean you’re stuck, it just means you should act now rather than wait any longer.

What Information Do You Need to Pull Together?

Before you can complete a return, you’ll need a few things in order. Your Unique Taxpayer Reference (UTR) is the ten-digit number HMRC assigned when you registered for self assessment. You’ll also need your National Insurance number, your Government Gateway login, and records of all your income and expenses for the tax year (6 April to 5 April).

For self-employed income, that means your total turnover and any allowable business expenses. If you have employment income on top, you’ll need your P60 or P45. Bank interest, dividend income, rental figures, and anything else HMRC doesn’t already know about all need to go in. The cleaner your records, the faster the whole thing goes.

Something Important Is Changing From April 2026

If your income from self-employment or property is over £50,000, you’re now caught by Making Tax Digital for Income Tax (MTD for IT). From 6 April 2026, HMRC requires those people to keep digital records and send quarterly updates rather than one annual return. Over 860,000 sole traders and landlords are affected by this first wave of changes.

The threshold drops further in coming years. From April 2027 it applies to income over £30,000, and from April 2028 it extends to income over £20,000. If you’re not sure whether this affects you yet, it’s worth getting ahead of it now rather than finding out at the last minute. I work with QuickBooks, FreeAgent, Xero and Sage, so getting the right software in place is something I can help with directly.