Self Assessment is HMRC's annual reconciliation: you tell them everything you earned that wasn't already taxed at source, claim the expenses and allowances you're entitled to, and pay the difference. The form looks long because it covers everyone (landlords, partners, high earners, foreign income) but most sole traders only need 4–5 sections.
Direct answer
The first one is intimidating; the second is half an hour. This guide takes you screen-by-screen through HMRC's online form, what to gather first, and the figures most people miscalculate. Use the key facts, step list and official source links on this page to confirm the decision before you spend money or register anything.
- Online deadline
- 31 January
- Paper deadline
- 31 October
- Late filing fine
- £100 + daily
- Records to keep
- 5 years
Checklist
Quick checklist
- Gather UTR, Government Gateway login, NI number
- Total all self-employed income for 6 April–5 April
- List allowable expenses with receipts to back them up
- Collect P60, P45, interest summaries, dividend statements
- Add pension contributions and Gift Aid
- Submit by 31 January
- Diarise the 31 July payment on account
Section 01
Before you log in — gather these
- Your UTR (10-digit Unique Taxpayer Reference) — on any HMRC letter.
- Your Government Gateway login.
- All self-employed income — total invoiced or paid in the tax year (6 April–5 April).
- All allowable expenses — broken down by category (see below).
- P60 / P45 if you had any employed income.
- Interest from UK banks — most banks post a summary in your online statements.
- Pension contributions and Gift Aid donations — both reduce tax.
- Student Loan plan type if applicable.
Section 02
The form — section by section
- 01
Tell us about you
Confirm address, marital status, residency. Tick 'self-employed' to unlock the SA103 self-employment pages.
- 02
Self-employment
Trading name, description, accounting dates. Then turnover (gross income) and either: claim the £1,000 trading allowance, OR list expenses (rent, phone, software, travel, materials, use of home, capital allowances on equipment).
- 03
Other income
Bank interest, dividends (use the £500 dividend allowance), rental income, foreign income.
- 04
Tax reliefs
Pension contributions, Gift Aid, marriage allowance transfer if applicable.
- 05
Calculate
HMRC's system computes income tax + Class 2/4 NI. Check the figure against your own estimate — a 10%+ gap usually means you missed a section.
- 06
Submit and pay
Pay by 31 January via bank transfer, debit card, or direct debit. If your bill is over £1,000 you'll also pay 50% on account towards next year.
Section 03
Expenses sole traders forget to claim
- Use of home as office — £6/week flat or a proportion of bills.
- Mileage — 45p/mile for the first 10,000 business miles, 25p after.
- Mobile phone — proportion of contract used for business.
- Software subscriptions, domain names, hosting.
- Accountant or bookkeeper fees.
- Bank charges on a business account.
- Training that maintains existing skills (not new qualifications).
- Equipment under £1,000 — usually fully deductible via the Annual Investment Allowance.
Section 04
Payments on account — the bit no one warns you about
If your tax bill exceeds £1,000 and less than 80% of your tax is collected at source, HMRC asks for two 'payments on account' towards the following year — 50% on 31 January and 50% on 31 July. So your first big bill is effectively 150% in one go. Set aside cash from day one.
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Common questions