You’ve read the guidance, watched the videos, and you still aren’t sure what a completed Self Assessment actually looks like. The official HMRC guidance on completing your tax return tells you what each box means, but it doesn’t show you a real return filled in with real figures from start to finish.
This article does. We take a freelance graphic designer with £45,000 turnover and £8,500 expenses, and walk through every box they need to fill in on the SA100 main return and the SA103S self-employment pages. You’ll see the figure that goes in each box, why it goes there, and how HMRC turns those figures into the final tax bill. Follow along with your own numbers — the structure is the same regardless of your trade or income level.
If you haven’t registered yet or aren’t sure what records to gather, start with our first-year self-employed checklist — it covers registration deadlines and record-keeping from day one.
1. The Scenario: A Freelance Graphic Designer Earning £45,000
Meet Sarah. She’s been a freelance graphic designer for three years, working as a sole trader. For the 2025/26 tax year (6 April 2025 to 5 April 2026), her figures are:
| Item | Amount |
|---|---|
| Turnover (total invoiced income) | £45,000 |
| Allowable expenses | £8,500 |
| Trading profit | £36,500 |
| Bank interest (gross, from a savings account) | £250 |
| Pension contributions (net, to a relief-at-source scheme) | £0 |
She has no PAYE income, no property income, no dividends, no capital gains, and no benefits in kind. Her turnover is under £90,000, so she uses the SA103S (self-employment short) pages rather than the full SA103F. She uses the cash basis — the default since April 2024 — so she records income when she’s paid and expenses when she pays them.
This is a deliberately simple return. One trade, one other small income source, no complex reliefs. If your situation is more complicated — multiple income streams, a limited company, capital gains — the structure below still applies, but you’ll fill in more supplementary pages. The official HMRC guidance lists every supplementary page and when each one applies.
2. Before You Start: What You Need to Gather
Before you log into HMRC’s online Self Assessment service, have these ready:
- Your 10-digit UTR number (Unique Taxpayer Reference) — if you can’t find it, see our guide on what to do if you’ve lost your UTR number
- Your National Insurance number
- Your Government Gateway user ID and password
- A summary of your income and expenses for the tax year — a spreadsheet or accounting software export works
- Bank statements showing any interest received (even if it’s tax-free under the Personal Savings Allowance)
- Records of any pension contributions or Gift Aid donations (if applicable)
The biggest time-waster in filing a return is not having your figures organised before you start. If you’re filing for the first time, our first-year self-employed checklist walks through what to record from day one so January is a 30-minute job, not a weekend-long panic.
3. SA100 Page TR1: Personal Details (Pre-Populated by HMRC)
Page TR1 is the simplest page. HMRC pre-populates it with the details they hold for you:
| Box | Field | What appears |
|---|---|---|
| — | Name | Sarah’s full name (as registered with HMRC) |
| — | Address | Her registered address |
| — | Date of birth | Pre-filled |
| — | National Insurance number | Pre-filled |
| — | UTR | Pre-filled |
Your only job here is to check everything is correct. A surprising number of returns get held up because the address is outdated or a name is misspelt. If anything is wrong, update it in your HMRC online account before you file — don’t try to correct it on the return itself.
That’s it for TR1. No figures, no calculations. Move on.
4. SA100 Page TR2: Ticking the Right Supplementary Pages
Page TR2 is a set of tick boxes that tell HMRC which supplementary pages apply to you. Each tick triggers a corresponding section in the online form. For Sarah:
| Box | Supplementary page | Tick? | Why |
|---|---|---|---|
| Employment | SA102 | No | She has no PAYE job |
| Self-employment | SA103S or SA103F | Yes — SA103S | She’s self-employed, turnover under £90,000 |
| Partnership | SA104 | No | Not in a partnership |
| UK property | SA105 | No | No rental income |
| Foreign | SA106 | No | No foreign income |
| Trusts | SA107 | No | No trust income |
| Capital gains | SA108 | No | No asset sales |
| Residence | SA109 | No | UK resident |
The online filing service unfolds only the sections you tick — so if you tick “self-employment” and nothing else, you’ll see the SA103S pages but not the employment or property sections. This is why most filers only see 8–12 boxes even though the full return has over 80: the tick boxes filter everything down to what’s relevant.
If you’re not sure which pages apply, HMRC’s guidance on supplementary pages lists every form and when each one is needed. The short version: if you have income from a source, you tick the box for that source.
5. SA103S: Entering Your Self-Employment Income and Expenses
This is where the real work happens. The SA103S (self-employment short) pages capture your trade income and expenses. Sarah fills in the following boxes:
Income
| Box | Field | Sarah’s figure | Notes |
|---|---|---|---|
| Box 1 | Turnover | £45,000 | Total invoiced income before expenses |
| Box 10.1 | Trading income allowance | Blank | She’s claiming actual expenses, not the £1,000 allowance |
She leaves box 10.1 blank because her actual expenses (£8,500) are well above the £1,000 trading allowance. If your expenses are under £1,000, claiming the allowance is simpler and gives a bigger deduction — see our trading allowance vs expenses guide for the full either/or decision.
Expenses
| Box | Field | Sarah’s figure | Notes |
|---|---|---|---|
| Box 17 | Office, phone, stationery | £1,200 | Laptop already bought (capital allowance, not here), software subscriptions, phone bill |
| Box 18 | Travel and subsistence | £800 | Train fares to client meetings, parking |
| Box 19 | Clothing and tools | £0 | No uniforms or specialised equipment |
| Box 20 | Staff costs | £0 | No employees |
| Box 21 | Goods bought for resale | £0 | She sells services, not products |
| Box 22 | Financial costs | £600 | Business bank account fees, professional indemnity insurance |
| Box 23 | Premises costs | £3,200 | Home office proportion (heating, lighting, internet — business use only) |
| Box 24 | Advertising and marketing | £1,500 | Website hosting, Google Ads, portfolio site |
| Box 25 | Training | £400 | Online design course |
| Box 26 | Other allowable expenses | £800 | Professional subscriptions, accountant’s fee for tax return |
| Box 32 | Total expenses | £8,500 | Sum of boxes 17–26 |
| Box 33 | Net profit (box 1 − box 32) | £36,500 | This figure flows to the SA100 tax calculation |
Every expense here must pass the “wholly and exclusively” test — it was incurred entirely for business purposes. If an expense has a personal element (like a phone used for both business and personal calls), you can only claim the business proportion. Our allowable expenses guide covers every category in detail, including the common mistakes that trigger HMRC questions. HMRC’s own self-employed expenses overview and the HS222 taxable profits helpsheet are the official references.
One thing Sarah does not put here: the cost of her laptop. Equipment like computers is a capital asset, claimed through capital allowances (or the £1,000 annual investment allowance for small purchases), not as a day-to-day expense. Mixing up capital and revenue items is one of the most common filing errors.
Capital allowances (if applicable)
Sarah bought a new laptop for £1,200 during the year. Because the cost is under £1,000, she claims it as a full expense in box 17 under the cash basis simplified rules (no separate capital allowance calculation needed). If the laptop had cost over £1,000, or if she used traditional accounting, it would go in the capital allowances section instead.
6. SA100 Page TR3: Other Income (Bank Interest, Dividends)
Back on the main SA100, page TR3 captures income that isn’t from self-employment or employment. For Sarah, this is just bank interest:
| Box | Field | Sarah’s figure | Notes |
|---|---|---|---|
| Box 1 | UK interest (gross) | £250 | Total interest from all UK bank/building society accounts |
Even though £250 is well below the £1,000 Personal Savings Allowance (so no tax is due on it), Sarah still declares it. HMRC uses the gross figure to calculate her adjusted net income, which affects things like the High Income Child Benefit Charge threshold and the Personal Allowance taper above £100,000. For most basic-rate taxpayers, savings interest is tax-free — but it still goes on the return.
If Sarah had dividends, they’d go in box 2 (UK dividends) or box 3 (foreign dividends). State pension goes in box 4, other pensions in boxes 5–7. She has none of these, so those boxes stay blank.
7. SA100 Page TR4: Tax Reliefs and Pension Contributions
Page TR4 captures tax reliefs — pension contributions, Gift Aid donations, and other deductions that reduce your taxable income. Sarah made no pension contributions this year and no Gift Aid donations, so this page is blank for her.
If she had contributed to a personal pension (a relief-at-source scheme like a SIPP), the net contribution amount would go in the relevant box here. HMRC adds 20% basic-rate relief automatically, and if she’s a higher-rate taxpayer, the gross contribution reduces her adjusted net income — pulling her below thresholds and potentially reclaiming tapered Personal Allowance. Our pension contributions on Self Assessment guide covers the mechanics, including the common mistake of entering gross instead of net contributions.
For this example, we skip TR4 entirely. Move to the calculation.
8. The Tax Calculation: How HMRC Turns £36,500 Profit Into £6,271.80
This is the part most people find opaque. Here’s exactly how HMRC gets from Sarah’s figures to her final tax bill, step by step.
Step 1: Total income
| Source | Amount |
|---|---|
| Self-employment profit (from SA103S box 33) | £36,500 |
| Bank interest (from SA100 TR3 box 1) | £250 |
| Total income | £36,750 |
Step 2: Apply the Personal Allowance
The Personal Allowance for 2025/26 is £12,570. This is the amount Sarah can earn before paying any Income Tax. It’s applied against her total income, not just her self-employment profit.
| Amount | |
|---|---|
| Total income | £36,750 |
| Minus Personal Allowance | £12,570 |
| Taxable income | £24,180 |
Step 3: Income Tax
The taxable income of £24,180 falls entirely within the basic rate band (£12,571 to £50,270 for 2025/26), so it’s all taxed at 20%:
| Band | Amount in band | Rate | Tax |
|---|---|---|---|
| £0 – £12,570 (Personal Allowance) | £12,570 | 0% | £0 |
| £12,571 – £50,270 (basic rate) | £24,180 | 20% | £4,836.00 |
| Above £50,270 (higher rate) | £0 | 40% | £0 |
| Total Income Tax | £4,836.00 |
Note: the £250 bank interest is technically taxed separately (savings income), but because it’s within the £1,000 Personal Savings Allowance, the tax due is £0. It still counts toward total income for the Personal Allowance calculation above.
Step 4: Class 4 National Insurance
Class 4 NIC is charged on self-employment profits (not total income — just the trading profit from SA103S):
| Band | Amount in band | Rate | NIC |
|---|---|---|---|
| £0 – £12,570 (Small Profits Threshold) | £12,570 | 0% | £0 |
| £12,571 – £50,270 | £23,930 | 6% | £1,435.80 |
| Above £50,270 | £0 | 2% | £0 |
| Total Class 4 NIC | £1,435.80 |
Step 5: Total tax due
| Component | Amount |
|---|---|
| Income Tax | £4,836.00 |
| Class 4 National Insurance | £1,435.80 |
| Total tax due | £6,271.80 |
That’s it. HMRC does this calculation automatically when you submit online — you’ll see a summary screen with these figures before you confirm. But understanding the steps helps you spot errors and plan ahead. If Sarah’s profit had been £55,000 instead of £36,500, she’d have crossed into the higher rate band and the calculation would look very different. Our Personal Tax Allowance guide explains the bands, the £100,000 taper trap, and how to stay below thresholds.
9. Payments on Account: The January Surprise
Here’s where most first-time filers get caught. Because Sarah’s tax bill is over £1,000 and less than 80% of her tax was collected at source (she has no PAYE), HMRC requires payments on account — advance payments toward next year’s tax.
Each payment is 50% of this year’s bill:
| Payment | Amount | Due date |
|---|---|---|
| Balancing payment (this year’s tax) | £6,271.80 | 31 January 2027 |
| First payment on account (toward 2026/27) | £3,135.90 | 31 January 2027 |
| Second payment on account (toward 2026/27) | £3,135.90 | 31 July 2027 |
| Total due 31 January 2027 | £9,407.70 | |
| Total due 31 July 2027 | £3,135.90 |
So Sarah doesn’t owe £6,271.80 on 31 January — she owes £9,407.70. The extra £3,135.90 is an advance toward next year’s tax, not an additional charge. When she files her 2026/27 return, these two payments are deducted from whatever she actually owes that year. If her income drops, she can reduce her payments on account — but if she reduces too far and her income doesn’t actually drop, HMRC charges interest on the underpayment.
This system catches almost every new self-employed person off guard. The first January you file, you pay 100% of your tax. The second January, you pay 150% — your full current-year bill plus the first instalment toward next year. Plan for it from the start by setting aside 25–30% of every invoice if you’re a basic-rate taxpayer.
10. What Happens After You Click Submit
Once you’ve reviewed the calculation and clicked submit, you’ll see a confirmation screen with:
- Your submission reference number — keep this in case of disputes
- The total tax due and the payment deadlines (31 January for the balancing payment and first POA, 31 July for the second POA)
- A link to pay your Self Assessment tax bill — you can pay by bank transfer, debit card, or corporate credit card (not personal credit card)
If you realise you’ve made a mistake after submitting, you have 12 months from the filing deadline to amend your return. Log back into your HMRC online account, make the correction, and resubmit. Our guide on what to do if you’ve made a mistake on your Self Assessment walks through the amendment process, including what happens if the mistake means you owe more (or less) tax.
If you miss the 31 January deadline, Self Assessment penalties start at £100 immediately — even if you owe no tax — and escalate the longer you delay. File on time even if you can’t pay; the penalty for late filing is harsher than the penalty for late payment.
11. Variation: If You Also Have a PAYE Job
Many sole traders have a part-time or full-time PAYE job alongside their self-employment. If Sarah also earned £25,000 through PAYE, the return changes in two key ways:
- She’d tick “Employment” on page TR2 and fill in an SA102 page with her gross salary (£25,000) and tax already deducted through PAYE. Her P60 provides these figures.
- The Personal Allowance is applied against total income — PAYE salary plus self-employment profit plus bank interest. Her PAYE salary of £25,000 uses up most of the £12,570 Personal Allowance, so her self-employment profit is taxed on top, potentially pushing her into the higher rate band sooner.
The structure of the return stays the same — SA100 plus SA103S plus SA102 — but the tax calculation changes because the income stacks. Our first-year self-employed checklist covers the PAYE-plus-self-employment scenario in detail, including how to avoid the common mistake of assuming your PAYE tax code covers everything.
12. The Bottom Line
A Self Assessment tax return is less intimidating than it looks once you see one filled in. For a sole trader with one trade and straightforward expenses, you’re filling in roughly a dozen boxes across two forms — the SA100 main return and the SA103S self-employment pages. The online service calculates the tax for you; your job is to put the right figures in the right boxes.
The three things that trip people up aren’t the boxes themselves — they’re the decisions around them: whether to claim the trading allowance or actual expenses, how to handle capital items versus day-to-day costs, and the payments on account that inflate your first January bill by 50%. Get those three right and the rest is data entry.
If your return is more complex than this example — multiple income streams, a limited company, capital gains, or foreign income — consider getting help rather than filing alone. Our Self Assessment help guide compares free HMRC support, accounting software, online filing services, and traditional accountants so you can match the help to your situation. And if you’re approaching the £50,000 MTD threshold, see our MTD vs Self Assessment guide to understand how this process changes from April 2026 — the return structure shifts from one annual filing to four quarterly updates plus a final declaration.