Freelancer guides
Self-Assessment Tax in the UK — A Beginner's Guide
Who must file, the key deadlines, payments on account, and how self-employed National Insurance works.
8 min read · Reviewed March 2026
Who needs to file
Most employees never touch Self Assessment because PAYE handles their tax automatically. You generally need to file a return if you're self-employed earning above the trading allowance, a company director, a high earner with extra income, or you have significant untaxed income from property, dividends or abroad.
If in doubt, HMRC's online checker tells you whether you must register. Registering late or missing the return triggers automatic penalties, so it's worth confirming early.
The deadlines that matter
The UK tax year runs 6 April to 5 April. For online returns, the filing and payment deadline is 31 January following the end of the tax year. Paper returns are due earlier, on 31 October.
Miss the 31 January deadline and there's an immediate £100 penalty, with more piling on after three, six and twelve months — plus interest on unpaid tax. Set a reminder well in advance.
Payments on account
A surprise for first-time filers: HMRC often asks for 'payments on account' — advance instalments toward next year's bill, due 31 January and 31 July. In your first profitable year this can mean paying roughly 150% of your tax bill at once.
It's not an extra tax, just prepayment, but it catches people out. Budget for it so the January bill doesn't blindside you.
Self-employed National Insurance
The self-employed pay Class 4 National Insurance on profits (at lower rates than employees' Class 1), and historically Class 2 as well. You also pay income tax on profit after allowable business expenses.
Keeping clean records of income and expenses throughout the year makes the return far easier — and ensures you don't overpay. Our UK freelancer calculator gives a rough estimate.
Related
Frequently Asked Questions
+Who has to do a Self Assessment tax return?
Mainly the self-employed above the trading allowance, company directors, high earners with extra income, and people with significant untaxed income from property, dividends or overseas. Most PAYE-only employees don't need to.
+What are payments on account?
Advance instalments toward next year's tax bill, due 31 January and 31 July. In your first profitable year you may pay your current bill plus the first instalment together — around 150% — so budget ahead.
Estimate only — not tax advice. Figures are estimates based on publicly available tax rules and may not reflect your full circumstances. See our methodology & sources (last reviewed June 2026). Always confirm with an official tax authority or a licensed adviser before making decisions.