What you're actually being asked
Strip the jargon and Self Assessment asks three questions: what did you earn (from everything — self-employment, any PAYE job, interest, dividends), what did it cost you to earn it (allowable expenses), and what tax adjustments apply to you (pension contributions, student loan, Gift Aid). HMRC's software does the arithmetic; your job is complete, honest inputs.
The tax year runs 6 April to 5 April. Your return for 2026/27 covers that window and is due online by 31 January 2028, with the tax due the same day — and registration due by 5 October 2027 if you're new. Those dates feel far away; the whole trick of a calm first return is not treating them that way.
The full guide: the gather-list, a worked calculation you can adapt, the penalty table, and the classic first-timer mistakes.
Gather these seven things
- Your UTR (from registering) and Government Gateway login
- Income records — invoices and bank statements for the year (or a FreeAgent account where they already live)
- Expense records — receipts, mileage log, home-working workings
- P60/P45 from any employment in the year
- Bank interest figures (banks report these to HMRC — match them)
- Pension contributions made personally (they extend your basic-rate band — free money often forgotten)
- Student loan plan type, if you have one
The calculation, worked
Freelance income £38,000, expenses £6,000 → profit £32,000. No other income:
- Income tax: (£32,000 − £12,570 allowance) × 20% = £3,886
- Class 4 NI: (£32,000 − £12,570) × 6% = £1,166
- Total ≈ £5,052 — about 16% of the profit
Because that's over £1,000, HMRC also wants payments on account: 50% of it (£2,526) added on 31 January towards next year, and another £2,526 on 31 July. First-January total: ~£7,578. This is the number that ambushes first-timers — if you've been setting aside 20–25% of receipts, it's already sitting in your pot.
Deadlines and what missing them costs
- 5 October — register (first-timers)
- 31 October — paper return deadline (file online; ignore this one)
- 31 January — online return + payment. Miss the return: £100 immediately, even with no tax owed; then £10/day after 3 months (up to £900), with 6- and 12-month escalations. Miss the payment: interest immediately, 5% surcharges at 30 days, 6 and 12 months.
File early, pay late (legally) Filing in April–June doesn't bring the payment date forward — you still pay by 31 January. Early filing just means you know the exact number seven months ahead, refunds arrive quickly, and January contains no drama. There is no downside. It's the single best habit in self-employed tax.
The five first-timer mistakes
- Registering late — the 5 October deadline is missable precisely because the return feels distant.
- Reporting turnover as profit — you're taxed on income minus expenses; under-claiming expenses is donating money (our expenses guide covers what counts).
- Forgetting employment income — a mixed year needs the P45/P60 numbers in, or PAYE tax you already paid isn't credited.
- Ignoring payments on account — see above; budget 150%, not 100%.
- Starting the Gateway process on 30 January — activation codes historically arrive by post. Set up access weeks ahead, minimum.
Or hand the whole thing over: every one of our packages includes the return, from £19 + VAT a month — and we'll usually recover more than that in expenses you didn't know were claimable.
