How to Fill In Your Self-Assessment Tax Return (Self-Employed) — UK 2025/26

Filing your first Self-Assessment tax return as a self-employed person feels daunting. The forms look complicated, the terminology is confusing, and the stakes feel high. But here's the truth: for most sole traders, it's surprisingly straightforward once you know what goes where.

This guide walks you through every section of the Self-Assessment tax return, box by box, with plain English explanations. No accounting jargon. No assumptions that you've done this before.

Before You Start: What You'll Need

Gather these before you sit down to file:

• Your UTR (Unique Taxpayer Reference) — 10-digit number from HMRC. If you don't have one, you need to register as self-employed first.
• Government Gateway login — username and password for HMRC online services
• Records of all income — every invoice, payment, and bank deposit from your self-employment
• Records of all business expenses — receipts, bank statements, mileage logs (see our complete expenses guide)
• P60 or P45 — if you also had employed income during the year
• Bank interest statements — your bank should provide these
• Student loan details — if applicable
• Pension contribution records — if you paid into a personal pension

Step 1: Log Into HMRC Online

Go to gov.uk/self-assessment-tax-returns and sign in with your Government Gateway credentials. Select "Complete your tax return" and choose the 2025/26 tax year.

HMRC will ask you a series of questions about what types of income you had. For most sole traders, you'll tick:

• ✅ Self-employment
• ✅ Bank or building society interest (if any)
• ✅ Employment income (if you also had a job)

Only tick what applies. More ticked boxes = more sections to fill in.

Step 2: Personal Details (SA100)

The main return form is the SA100. Most of this will be pre-filled with your name, address, and UTR. Check everything is correct, especially:

• Your name and address
• Date of birth
• National Insurance number
• Marital status (affects Marriage Allowance eligibility)

Step 3: Self-Employment Section (SA103S or SA103F)

This is the core section for your business income. Most sole traders use the short version (SA103S) — you qualify if your annual turnover is under £85,000.

Business Details

• Business name: Your trading name (or just your own name)
• Business description: Brief description like "freelance graphic designer" or "web development services"
• Business start date: When you first started trading
• Accounting period: Usually 6 April 2025 to 5 April 2026 (the standard tax year)

Income

• Turnover: Your total business income for the year — everything you invoiced or were paid, before any deductions. This is your gross income, not profit.
• Other business income: Grants, COVID support payments, interest from business accounts, etc.

Expenses

You can either claim actual expenses or use the simplified expenses method (flat rates for vehicle costs, working from home, living on business premises).

Common expense categories:

• Cost of goods: Materials, stock, direct costs of delivering your service
• Car, van, and travel expenses: Business mileage at HMRC approved rates, or actual running costs
• Wages, salaries, and other staff costs: If you hired anyone
• Rent, rates, power, and insurance: Office costs, or a proportion for working from home
• Repairs and maintenance: Equipment repairs, not improvements
• Phone, fax, stationery, and other office costs: Business portion of phone bills, software subscriptions, postage
• Advertising and business entertainment: Website costs, Google Ads, networking (not client meals — these aren't deductible)
• Interest on bank and other loans: Business loan interest only
• Bank, credit card, and other financial charges: PayPal fees, Stripe fees, bank charges
• Irrecoverable debts written off: Invoices you'll never be paid for (see bad debt relief guide)
• Accountancy, legal, and other professional fees: Accountant fees, legal advice
• Depreciation and loss/profit on sale: Capital allowances (equipment over £1,000)
• Other expenses: Anything else legitimate that doesn't fit above

Net Profit

The form calculates this for you: Turnover minus Expenses = Net Profit. This is the figure your tax is calculated on.

Step 4: Other Income Sections

Employment Income

If you had a job alongside your self-employment, enter details from your P60. HMRC often pre-populates this from your employer's records — check it matches your P60.

Bank Interest

Enter total interest received from all bank and building society accounts. Remember: the first £1,000 of savings interest is tax-free for basic rate taxpayers (Personal Savings Allowance).

Dividends

If you received dividends from shares, enter the total. The first £500 (2025/26) is tax-free.

Step 5: Tax Reliefs and Allowances

Pension Contributions

If you contributed to a personal pension (SIPP or stakeholder), enter the gross amount. You'll get tax relief — basic rate relief is usually applied automatically by your pension provider, but higher rate relief needs to be claimed here.

Gift Aid

Charitable donations made via Gift Aid can reduce your tax bill, especially if you're a higher rate taxpayer.

Marriage Allowance

If your spouse/civil partner earns less than £12,570, they can transfer £1,260 of their personal allowance to you, saving up to £252 in tax.

Step 6: Review the Tax Calculation

Before submitting, HMRC shows you a tax calculation. Check these numbers:

Component	2025/26 Rate
Personal Allowance	£12,570 (tax-free)
Basic rate (£12,571–£50,270)	20%
Higher rate (£50,271–£125,140)	40%
Additional rate (over £125,140)	45%
Class 2 NI	£3.45/week
Class 4 NI (£12,570–£50,270)	6%
Class 4 NI (above £50,270)	2%

See our National Insurance guide for full details on how NI is calculated.

Step 7: Payments on Account

If your tax bill is over £1,000, HMRC will require payments on account — advance payments towards next year's bill. Each payment is 50% of the current year's tax.

• First payment on account: Due 31 January 2027 (same time as current year's bill)
• Second payment on account: Due 31 July 2027

This means your first January payment can be up to 150% of one year's tax (this year's bill + 50% advance). Budget for it.

Step 8: Submit and Pay

Once you've reviewed everything:

1. Click "Submit" — you'll get a confirmation with a reference number
2. Save or print your submission receipt
3. Pay by 31 January 2027 (options: bank transfer, debit card, Direct Debit, or budget payment plan)

Common Mistakes to Avoid

• Forgetting payments on account: Budget for 150% of your tax bill in January
• Missing the £1,000 trading allowance: If your total self-employment income is under £1,000, you might not need to file at all
• Not claiming all expenses: Most new freelancers under-claim. Read our complete expenses guide
• Using wrong dates: The tax year runs 6 April to 5 April, not January to December
• Forgetting to include all income: Including small cash jobs, foreign income, and platform payments
• Filing late: Even one day late = automatic £100 penalty

Making Tax Digital (From April 2026)

If your self-employment income exceeds £50,000, you'll need to use Making Tax Digital for Income Tax from April 2026. This means quarterly digital updates instead of one annual return. The threshold drops to £30,000 from April 2027.

Should You Use an Accountant?

For a straightforward sole trader return (one income source, simple expenses), you probably don't need an accountant. The online form guides you through it.

Consider an accountant if:

• You have multiple income sources
• You're approaching the VAT threshold (£85,000)
• You have complex expenses (vehicle, home office, overseas income)
• You're considering switching to a limited company

Your End-of-Year Checklist

Use our end-of-year tax checklist to make sure you haven't missed anything before filing. And keep records for at least 5 years — HMRC can enquire into any return within that window.