UK Self Assessment Tax Return: A Step-by-Step Guide for the Self-Employed

If you’re self-employed in the UK, filing a Self Assessment tax return is one of those annual obligations that can feel overwhelming – especially the first time. But the process is more straightforward than it appears, and once you’ve done it once, each subsequent year gets easier. This guide walks you through every step, from registration to payment, so you know exactly what to expect.

Disclaimer: This article provides general information only and does not constitute tax advice. Consult a qualified tax adviser for advice specific to your circumstances.

Who Needs to File a Self Assessment Tax Return?

If you’re employed and your tax is handled entirely through PAYE, you generally don’t need to file. But you do need to file if any of the following apply:

• You’re self-employed and your gross trading income exceeds £1,000 in the tax year (6 April to 5 April).
• You receive rental income from property.
• You have other untaxed income – tips, commission not in PAYE, income from abroad, or savings income above your personal savings allowance.
• You’re a partner in a business partnership.
• Your total taxable income exceeds £150,000, even if most is taxed through PAYE.
• You need to claim tax relief on expenses or pension contributions.
• You owe Capital Gains Tax on the sale of assets.

If you’re unsure, HMRC’s Self Assessment page has a tool to check. When in doubt, register and file – the penalties for not filing when you should are considerably worse than filing unnecessarily.

Key Deadlines You Cannot Miss

Self Assessment runs on a strict calendar, and missing deadlines triggers automatic penalties:

• 5 October – Register for Self Assessment by this date following the tax year you became self-employed.
• 31 October – Paper tax return deadline (rarely used today).
• 31 January – Online filing deadline and payment deadline. For the 2025/26 tax year, this means 31 January 2027. Your first payment on account for the following year is also due on this date.
• 31 July – Second payment on account is due.

Late Filing Penalties

Miss the 31 January deadline and penalties escalate quickly: a £100 fixed penalty from day one, £10 per day after three months (up to £900), and further percentage-based penalties at six and twelve months. Late payment also attracts interest charges. The message is clear: file and pay on time.

Step-by-Step: Filing Your Self Assessment Return

Step 1: Register for Self Assessment

If you haven’t already registered, do so through the HMRC website. You’ll need your National Insurance number and basic personal details. HMRC will send you a Unique Taxpayer Reference (UTR) – a 10-digit number that identifies you in the Self Assessment system. This arrives by post and can take 7-10 working days, so don’t leave it until the last minute.

Step 2: Set Up Your Government Gateway Account

To file online, you need a Government Gateway account. Create one at gov.uk and link your UTR to it. HMRC may send an activation code by post, so get this sorted well ahead of the filing deadline.

Step 3: Gather Your Records

Before completing the return, gather everything you need:

• Income records – Invoices, bank statements, and any P60 or P45 forms if you also have employment income.
• Expense records – Receipts, invoices, and bank statements for all business expenses you intend to claim, organised by category.
• Bank interest statements and pension contribution records.
• Student loan details – Your repayment plan type and any repayments already made.
• Other relevant documents – Gift Aid donations, property income records, Capital Gains records, and so on.

Step 4: Complete the SA100 and SA103S Forms

The main Self Assessment form is the SA100. If you’re self-employed, you also complete the SA103S (short, for turnover below £85,000 and no stock) or SA103F (full). The key sections cover your total turnover, allowable expenses by category, and net profit – income minus expenses, which is the figure that gets taxed.

The online filing system calculates your tax automatically as you enter figures, which is one of the strongest reasons to file online rather than on paper.

Step 5: Submit Online and Pay What You Owe

Once you’ve completed all sections, review everything carefully, then submit. HMRC will confirm receipt and show your tax calculation. You can pay by Direct Debit, online banking, Faster Payments (using your UTR as reference), or debit card. Credit cards are no longer accepted. Pay by 31 January to avoid interest and penalties.

Allowable Expenses: What You Can Claim

HMRC allows a wide range of expenses, provided they are incurred “wholly and exclusively” for business purposes:

• Office costs – Stationery, printing, postage, phone bills (business portion), software subscriptions, and computer equipment. For home workers, claim a proportion of household costs or use HMRC’s simplified flat rate (£6 per week for 25+ hours per month from home).
• Travel – Train tickets, bus fares, flights, taxis, parking, and accommodation for business trips. For your own car, claim 45p per mile for the first 10,000 miles and 25p thereafter. Commuting to a regular workplace is not allowable.
• Clothing – Only items exclusively for work and not suitable for everyday wear: uniforms, protective clothing, branded workwear.
• Staff costs – Salaries, subcontractor fees, employer’s NI contributions, pension contributions, and agency fees.
• Things you buy to sell on – Stock, raw materials, and direct costs of goods sold.
• Financial costs – Business bank charges, loan interest, hire purchase interest, accountancy fees, and tax software costs.
• Advertising and marketing – Website costs, business cards, online advertising, networking event fees, and directory listings.
• Training courses – Training that updates existing skills related to your current business. Training for an entirely new profession is generally not allowable.

Payments on Account: What They Are and Why They Matter

If your Self Assessment bill is £1,000 or more, and less than 80% of your tax is collected at source (through PAYE), HMRC will require payments on account for the following year. This catches many self-employed people off guard the first time.

Payments on account are advance payments towards next year’s tax bill. Each payment is 50% of the previous year’s bill: the first due 31 January (same date as your balancing payment), the second due 31 July.

For example, if your 2025/26 tax bill is £6,000, on 31 January 2027 you’d pay the £6,000 owed plus £3,000 as the first payment on account for 2026/27. On 31 July 2027, you’d pay the second £3,000. This means your first Self Assessment payment can feel shockingly large – you’re effectively paying 1.5 years of tax at once.

If your income drops significantly, you can apply to reduce payments on account, but underpaying attracts interest charges.

Tips for a Smoother Self Assessment

• Don’t wait until January. File in the spring or summer while everything is fresh, rather than working from faded memories nine months later.
• Keep records throughout the year. Scan receipts as you get them and categorise expenses monthly. This turns Self Assessment from a weekend ordeal into a one-hour task.
• Use the trading allowance if it makes sense. If your expenses are low, the £1,000 trading allowance lets you skip receipt-keeping – but you also can’t claim more than £1,000 in deductions.
• Consider Making Tax Digital. From April 2026, self-employed individuals earning over £50,000 must comply with MTD requirements. For details, see our guide to Making Tax Digital for the self-employed.

File Confidently, Not Anxiously

Self Assessment doesn’t have to be stressful. The people who find it overwhelming are the ones who leave it until the last week of January with a carrier bag full of unsorted receipts. Track your income and expenses throughout the year, and the annual return becomes a formality rather than an ordeal. Download Taxr to keep your receipts organised and your expense records ready for Self Assessment all year round.