This guide covers self Assessment for first-time filers for UK freelancers and small businesses in 2025/26.

Self Assessment is HMRC's system for people whose tax isn't collected automatically through PAYE — which includes almost every freelancer.

It runs on an April-to-April tax year, with the online filing deadline the following 31 January. Getting comfortable with the cycle is the single biggest thing you can do to keep tax stress manageable.

Key facts

• Register for Self Assessment by 5 October following the tax year in which you first became self-employed. Missing this deadline can trigger a Failure to Notify penalty.
• The online filing deadline is 31 January following the end of the tax year. Paper returns are 31 October.
• The tax year runs 6 April to 5 April. Income earned in that window is reported on the return due the following January.
• If your tax bill exceeds £1,000, HMRC typically requires payments on account — two prepayments toward the following year's bill.
• You need your UTR (Unique Taxpayer Reference), Government Gateway login, and records of income and expenses to file. Missing UTR = you cannot submit.
• The £1,000 trading allowance means if your gross self-employed income is under £1,000 in the tax year, you generally don't need to register or file at all.

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Run your own numbers with our Self-Employed Tax Calculator. It's free, mobile-friendly, and updated for the 2025/26 tax year — no signup required.

Getting registered

Register for Self Assessment via GOV.UK using your Government Gateway ID. You'll receive a UTR (Unique Taxpayer Reference) by post — this is the number you'll need forever.

The registration deadline is 5 October following the tax year in which you first became self-employed. Missing this triggers a Failure to Notify penalty on top of any tax owed.

Once registered, HMRC will expect a return every year, even if income is nil. If you stop trading, you have to formally deregister to avoid ongoing filing obligations.

Filing the return

The return is filed online by 31 January following the tax year end. You'll need your UTR, Government Gateway login, records of self-employment income and expenses, and any other income (employment P60, dividends, interest).

The self-employment section reports gross income, allowable expenses, capital allowances, and net profit. HMRC calculates tax and Class 4 NI from this.

Payments on account become payable if your tax bill exceeds £1,000 — the first is due 31 January, the second 31 July. These are advance payments toward next year's bill.

Common complications

Freelancers with both employment and self-employment income file both sections of the same return. Tax owed is calculated on total income after allowances.

The £1,000 trading allowance can be claimed instead of actual expenses if it's higher than your actual expenses — useful for very small side hustles.

Amendments to filed returns can be made within 12 months of the original filing deadline. After that, overpayment relief claims are possible but more restrictive.

Free Expense Tracker

Download our free Expense Tracker — HMRC-compliant, editable, and ready to use with UK clients.

Related: PayslipCheck

If you also have PAYE income alongside your freelance work, our sister site PayslipCheck helps you decode the tax and NI on your payslip.

Try our free Day Rate Calculator

Run your own numbers with our Day Rate Calculator. It's free, mobile-friendly, and updated for the 2025/26 tax year — no signup required.

Free Invoice Generator

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