The first time you do a Self Assessment, the worst part isn’t the maths. It’s the dread. You hear horror stories about HMRC, you assume you’ll miss something, and you put it off until the second week of January. The reality: for most sole traders and small directors, the actual return takes 60–90 minutes once your figures are in front of you. This guide walks you through what to gather, when to file, and how to do it without the dread.
Who needs to file
You need to file Self Assessment if you’re self-employed (any income above the £1,000 trading allowance), a partner in a partnership, a director of a limited company in most cases, anyone with untaxed income over £1,000 (rental, foreign, dividends outside ISAs), a high earner over £100,000, anyone claiming Marriage Allowance or other reliefs, or anyone HMRC has issued a return to. From 2025/26 onwards, HMRC has signalled that directors with only salary and dividends within their allowances may not need to register – but if you have any other untaxed income, you still file.
Registering for Self Assessment
You must register by 5 October following the tax year you became liable . The form is SA1 (general) or CWF1 (self-employed). HMRC issues you a Unique Tax Reference (UTR) – a 10-digit number – within about 10 working days. You can’t file without it. Register early; if you leave it until late October, the UTR may not arrive in time to use the online service before paper deadlines.
Key deadlines
Three dates anchor the year. 31 October is the paper return deadline – rarely used. 31 January is the online deadline AND the deadline to pay your tax for the year just ended. 31 July is the second payment-on-account date if your bill triggers payments on account. Penalties for late filing start at £100 immediately, then £10/day after three months, plus £300 (or 5% of tax, whichever is higher) at six and twelve months. The £100 minimum applies even if you owe nothing.
What to gather before you start
Spend an hour collecting before you log in. Income records (invoices issued, payments received). Expense records (receipts, mileage logs). Bank statements. P60 / P45 if you also had employment income. Pension contributions (relief-at-source vs net-pay matters). Charitable Gift Aid donations. Interest from savings accounts. Dividend income. Rental income. Foreign income. Your UTR and Government Gateway login.
Once you have everything in front of you, the actual return is mostly copying numbers into the right boxes. Filing without preparation is what makes Self Assessment feel like a black hole.
Step-by-step: filing online
- Sign in to Government Gateway. At gov.uk/log-in-file-self-assessment-tax-return, using the credentials you got when you registered.
- Select the relevant tax year. The 2025/26 tax year ends 5 April 2026 and is filed by 31 January 2027.
- Tell HMRC about your income sources. Employment, self-employment, property, dividends, interest, capital gains, foreign – tick whichever apply.
- Enter your self-employment income and expenses. Either summary figures or HMRC’s category breakdown, depending on your turnover.
- Enter any reliefs. Pension contributions, Gift Aid, Marriage Allowance.
- Review the calculation. HMRC’s online service calculates Income Tax, Class 4 NIC, and any voluntary Class 2 NIC for you.
- Decide on payments on account. Reduce them if you reasonably expect next year’s bill to be lower – but keep evidence.
- Submit the return. Save the on-screen acknowledgement.
- Pay any balance due. By 31 January, plus the first payment on account.
Class 4 NIC and Class 2 — what changed
Class 4 NIC (the main NIC for self-employed people) is 6% on profits between £12,570 and £50,270, and 2% above for 2026/27. Class 2 NIC was made voluntary in April 2024. Most self-employed people earning above the Lower Profits Limit get a State Pension qualifying year automatically without paying Class 2. Below the Small Profits Threshold (£6,725), you can pay Class 2 voluntarily to fill State Pension gaps – usually a good investment.
Common mistakes
Forgetting to register by 5 October. Filing without a UTR (waiting can take 10+ working days). Mixing personal and business bank accounts so income/expense tracing is a nightmare. Not declaring side income (HMRC’s data-matching is genuinely sophisticated). Forgetting payments on account. Submitting and then realising you’ve missed a relief – which you can amend within 12 months, but it’s easier to get it right first time.
When to use the Time to Pay arrangement
HMRC offers Time to Pay arrangements for tax owed up to £30,000 online, or by phone for higher amounts. Arrange before the deadline if possible – interest still accrues, but late-payment penalties may be avoided. HMRC are more accommodating than reputation suggests, provided you engage early and stick to the agreed instalments. Don’t ignore an HMRC bill; the worst outcome is silence.
Making Tax Digital changes from April 2026
From 6 April 2026, sole traders and landlords with qualifying income over £50,000 are in scope of Making Tax Digital for Income Tax (MTD for ITSA). That means quarterly digital updates plus an annual final declaration that replaces the Self Assessment return. The threshold drops to £30,000 in April 2027 and £20,000 in April 2028. For full detail see our MTD for sole traders guide.
Frequently asked questions
The questions sole traders and small directors ask most often when filing their first Self Assessment.
I earned £600 from a side hustle. Do I need to file Self Assessment?
No. There's a £1,000 trading allowance, so income up to £1,000/year doesn't need to be declared and Self Assessment isn't required. If you go over £1,000, you must register and file. The trading allowance is a tax-free amount, not an expense — you can use it instead of claiming actual expenses if your costs are low.
I'm a director of a limited company. Do I always need to file Self Assessment?
Until recently, yes — HMRC required all directors to file. From 2025/26 onwards, HMRC has signalled that directors with only salary and dividends within their allowances may not need to. If in doubt, check with your accountant or HMRC. If you have any other untaxed income (rental, savings, foreign), you'll still need to file.
When do I pay my Self Assessment tax bill?
31 January following the end of the tax year. So for the 2025/26 tax year (ending 5 April 2026), the deadline is 31 January 2027. If your bill is over £1,000, you'll also pay 'payments on account' — 50% on 31 January and 50% on 31 July, towards the next year's bill.
What if I've overpaid via PAYE — can Self Assessment claim it back?
Yes. You enter your P60 / P45 figures showing tax already deducted via PAYE, and HMRC's calculation reconciles the total. Any overpayment is refunded (or used against the next bill).
Can I file my Self Assessment by paper?
Yes, but the deadline is 31 October (3 months earlier than the online deadline). Most people file online. HMRC stopped automatically sending paper returns to most taxpayers in 2020.
I forgot to register by 5 October. What now?
Register as soon as possible. There's a 'failure to notify' penalty if you owe tax and didn't register on time, but it's mitigable if you register voluntarily and pay promptly. The longer you delay, the higher the penalty.
What expenses can I claim?
Anything wholly and exclusively for the business. Key categories: office costs, travel, vehicle (mileage rates), training that maintains existing skills, professional subscriptions, marketing, accountant fees. See our sole trader allowable expenses guide for the full list.
Can I file my Self Assessment using accounting software?
Yes. From April 2026, MTD-compatible software is mandatory if your qualifying income exceeds £50,000. Below that threshold you can still file directly through HMRC's online service, but software (FasCash, FreeAgent, Xero, QuickBooks) makes the process faster and more accurate.
