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How to Do Your Self Assessment Tax Return as a Freelancer in the UK

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Introduction

Self Assessment is the thing most freelancers dread and most accountants make their money on. It doesn't have to be either.

If you've been freelancing for a full tax year, you need to file a Self Assessment tax return with HMRC. The deadline is January 31st for online returns. Miss it, and HMRC starts the penalty clock immediately: £100 on day one, more if you keep missing it.

This guide covers the full process — registering with HMRC if you haven't already, what income and expenses to include, how to calculate your tax, and how to actually submit the return. No jargon where it can be avoided. Plain steps in the right order.

Who Needs to File a Self Assessment Return

You need to file if, in the tax year (April 6 to April 5 the following year):

If you're newly self-employed: register with HMRC as soon as you start trading. You have until October 5th after the end of your first tax year to register. Miss this and the penalties apply retroactively.

The UK Tax Year: Getting Your Dates Straight

This confuses everyone at least once.

The UK tax year runs April 6 to April 5. Yes, April 6. This is a historical quirk going back to 1752 — you don't need to know why, just remember it.

So:

| Date | What Happens | |------|-------------| | April 6 | New tax year begins | | July 31 | Second payment on account (if applicable) | | October 5 | Deadline to register for Self Assessment if new | | October 31 | Paper return deadline | | January 31 | Online return deadline + tax payment due |

Step 1: Register for Self Assessment

If you've never filed before, you need to register with HMRC before you can submit a return.

1. Go to gov.uk and search "register for Self Assessment" 2. Create a Government Gateway account if you don't have one (you'll get a User ID) 3. Select "I am self-employed" or the relevant category 4. HMRC will post a UTR (Unique Taxpayer Reference) to your address — this takes up to 10 working days

Your UTR is a 10-digit number. Keep it. You'll need it every year.

You already have a UTR. Log in to your HMRC online account (Government Gateway) and proceed to Step 2.

Step 2: Gather Your Records

Before you start filling in the return, collect everything. Trying to find records mid-form is where mistakes happen.

invoices/income records, expense records, and potentially a P60 if they also had employment income during the year.

Step 3: Calculate Your Freelance Income

Your taxable freelance income is your turnover minus allowable expenses.

All money received from clients during the tax year. If you use the cash basis (most freelancers do), this is when you received the payment, not when you raised the invoice.

Business costs you can deduct. These reduce your taxable profit.

Common allowable expenses for freelancers:

If you work from home, you can claim a proportion of your household costs (broadband, electricity, heating). The simplest method: the HMRC simplified expenses flat rate.

| Hours worked from home per month | Monthly flat rate | |----------------------------------|------------------| | 25 to 50 hours | £10 | | 51 to 100 hours | £18 | | 101 or more hours | £26 |

Or you can claim the actual proportion (square footage of office ÷ total square footage × household costs). The flat rate is simpler; the actual method may give a larger deduction if you have a dedicated room.

Step 4: Log In to HMRC and Start the Return

Go to gov.uk/log-in-file-your-self-assessment-tax-return.

Sign in with your Government Gateway user ID and password. If you've forgotten these, there's a recovery option — allow 10 minutes for this if it's your first time back in a while.

Once in, select "Complete your tax return" for the relevant year.

Most freelancers starting out use SA103S.

Step 5: Understand What Tax You'll Pay

HMRC calculates your tax bill automatically once you enter your figures. But understanding what goes into it helps you avoid surprises.

For 2024/25:

Freelance profit: £35,000 Personal Allowance: £12,570 Taxable income: £22,430

Income Tax: £22,430 × 20% = £4,486 Class 4 NI: (£35,000 - £12,570) × 6% = £1,346

Total tax bill (approx): £5,832

This is a rough estimate — Gift Aid donations, pension contributions, and other deductions affect the final figure.

Step 6: Payments on Account

This catches many freelancers out in their second year.

If your tax bill is over £1,000 and less than 80% of it was collected at source (e.g., through PAYE), HMRC requires you to make payments on account — advance payments towards next year's tax bill.

Each payment on account is 50% of your current year's bill. They're due on:

So in year two, you might face:

This can be a significant cash flow shock if you're not expecting it. The solution: set aside roughly 25-30% of your income throughout the year in a separate account. When tax time comes, you'll have the money.

Step 7: Submit and Pay

Once you've filled in all sections, the HMRC form shows your tax calculation. Review it carefully — check that the figures match your records.

Click "Submit return" or equivalent. You'll get a confirmation with a reference number. Screenshot it or take note — this is your proof of submission.

Options include:

personal cheques (slow and risky for deadlines), credit cards (HMRC doesn't accept them).

Pay by January 31. The bank transfer takes a day or two — don't leave it to the last minute.

Common Mistakes to Avoid

If you start freelancing in 2025/26, register by October 5, 2026.

UK tax year is April 6 to April 5. Not January to December.

Entertainment, personal expenses, and the non-business portion of mixed costs cannot be deducted.

If you donate to charity with Gift Aid, declare it. It extends your basic rate band and can reduce your higher-rate tax.

If you earned more than £500 in savings interest (basic rate taxpayer), it's taxable and must be declared.

If you drove to client meetings, locations, or events, 45p/mile is real money that most freelancers leave on the table.

Understand that your second-year bill will include advance payments. Plan cash flow accordingly.

What If You Need More Time

If you're struggling to file by January 31, you can request more time from HMRC — but only in specific circumstances (serious illness, bereavement, technical problems with HMRC's systems). Normal busy-ness doesn't qualify.

If you're going to miss the deadline: file as soon as you can and pay as much as you can. Penalties are based on how late you file and how much you owe. Paying even part of your bill reduces the interest that accrues.

Using Software or an Accountant

• FreeAgent, Xero, QuickBooks, Sage — all have Self Assessment filing features
• HMRC's own Basic PAYE Tools (free, limited)
• Finny — AI-powered expense tracking that keeps your records organised year-round, so when January comes you're not untangling a year's worth of transactions
• Your income is above £50,000 (higher rate tax bracket — more planning to do)
• You have multiple income streams (employment + freelance + rental + investments)
• You're a limited company director (different forms, corporation tax)
• You want someone to take full responsibility for accuracy

For a simple freelance setup — one income stream, straightforward expenses — Self Assessment is genuinely manageable yourself with organised records.

Frequently Asked Questions

Filing is free through HMRC's online system. If you use an accountant, costs vary from around £150 for a simple return to £500+ for more complex situations.

HMRC charges a £100 penalty immediately. After 3 months, daily penalties of £10 (up to 90 days). After 6 months, an additional penalty of 5% of tax owed or £300 (whichever is greater). Interest also accrues on unpaid tax from the deadline date.

HMRC expects you to keep records for at least 5 years after the filing deadline for a given tax year. No receipt doesn't automatically mean no deduction, but you need to be able to demonstrate the expense was genuinely for business if HMRC asks. Bank statements showing business payments are acceptable evidence for many expenses.

If your total freelance income was below £1,000 in the tax year, the trading allowance applies — no tax is due and you don't need to register for Self Assessment. If you earned over £1,000, you must register and file.

Legally: claim all allowable expenses, make pension contributions (these reduce your adjusted net income and can extend basic rate band access), use ISA allowances for savings and investments, and if your income is variable, consider whether a limited company structure would be more tax-efficient above a certain income level. An accountant is useful once income is high enough that tax planning pays for itself.

Cash basis: you record income when received and expenses when paid. This is simpler and the default for most small freelancers. Accruals basis: you record income when earned (invoiced) and expenses when incurred (regardless of when paid). Required once turnover exceeds certain thresholds or if you choose it. Most freelancers use cash basis — it matches your actual bank movements.

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