Self Assessment is one of those phrases everyone hears and few have explained to them. If you've started a side hustle, gone self-employed, or HMRC has written asking you to file, this is the plain-English version: what Self Assessment actually is, whether it applies to you, and the handful of dates you can't afford to miss.
What Self Assessment is
If you're employed, your employer takes Income Tax and National Insurance off your pay before you see it — that's PAYE, and HMRC gets the money without you doing anything. Self Assessment is the system for everything PAYE doesn't catch: profit from self-employment, rental income, investment income, foreign income, and so on.
The name says it: you assess what you earned, work out the tax, report it on a return, and pay it. HMRC then checks and can ask questions. It's run against your UTR — your unique 10-digit tax reference.
Who has to file
You generally need to send a return for a tax year if any of these applied:
- You were self-employed (a sole trader) and your gross income was more than £1,000
- You had £1,000+ of other untaxed income — renting out property, savings or investment income above your allowances, a side hustle, or content/creator income
- You were a partner in a business partnership
- You (or your partner) had to pay the High Income Child Benefit Charge
- You had Capital Gains to report — for example selling a second property or shares above the allowance
- You had untaxed income from abroad, or you're a higher earner HMRC has asked to file
Not sure? HMRC has a free "check if you need to send a tax return" tool on gov.uk — answer a few questions and it gives you a definitive yes or no for that year.
The deadlines that matter
The UK tax year runs 6 April to 5 April. For each year you need to file:
So for the 2025/26 tax year (ended 5 April 2026): register by 5 October 2026, file online and pay by 31 January 2027. If your bill is large enough you may also make payments on account — advance instalments towards next year's tax, due 31 January and 31 July.
How it works, step by step
- Register. Tell HMRC you need to file (by 5 October). You'll get a UTR and set up online access.
- Keep records. Through the year, keep your income and expense records — invoices, bank statements, receipts, mileage.
- Work out your figures. Total your income, subtract your allowable expenses, and that profit is what's taxed.
- File the return. Submit online (or on paper by the earlier deadline). The system calculates the tax and National Insurance due.
- Pay. Settle what you owe by 31 January, plus any payment on account.
What happens if you're late
The penalties stack up the longer you leave it:
- £100 the moment you miss the filing deadline — even if you owe no tax, even if it's a day late
- After 3 months, daily penalties can start adding up
- Further penalties at 6 and 12 months
- Separate interest and penalties apply to tax paid late, on top of the filing penalties
The practical lesson: register early and file on time even if money is tight. Filing and paying are separate — if you can't pay, file anyway and arrange a payment plan with HMRC rather than missing both.
How Making Tax Digital changes it
For higher earners, the annual return is being replaced. Making Tax Digital for Income Tax (MTD ITSA) starts on 6 April 2026 for sole traders and landlords with qualifying income over £50,000, dropping to £30,000 in 2027 and £20,000 in 2028. Instead of one return, you keep digital records, send four quarterly updates, and submit a final declaration after year end.
Below those thresholds, nothing changes yet — you keep filing the normal Self Assessment return. The detail is in our MTD ITSA quarterly updates guide.