Self-Assessment Tax Returns: A Step-by-Step Guide for UK Freelancers and Side Hustlers

The first time I filed a self-assessment tax return, I did it on 30 January at 11pm — one hour before the deadline, with a cold sweat and a browser full of panicked Google searches. The payment was wrong, I'd missed a deductible expense worth £900, and HMRC's website crashed twice. I know I'm not alone in this. Around 12 million people in the UK file self-assessment returns each year, and a significant portion leave it until the absolute last minute, making mistakes that cost them real money.

Whether you're a full-time freelancer, a contractor, or someone who earns a bit on the side selling crafts on Etsy or tutoring on weekends, here's exactly how self-assessment works — from registering with HMRC to pressing submit.

Who Actually Needs to File a Self-Assessment Return?

Not everyone who earns money needs to file. If you're employed and paid through PAYE with no other income, your employer handles your tax and you never touch a tax return. Self-assessment is for people whose tax affairs are too complicated for automatic deduction.

You need to file if any of the following apply:

• You earned more than £1,000 from self-employment (that's the trading allowance threshold — below £1,000, you don't need to report it)
• You're a partner in a business partnership
• You had untaxed income — rental income, foreign income, savings interest above your allowance, or investment gains above the annual exempt amount
• You earned over £150,000 in a tax year (even if all through PAYE)
• You need to claim tax relief on pension contributions or Gift Aid donations
• You received the High Income Child Benefit Charge (income over £60,000 while receiving Child Benefit)
• HMRC has sent you a notice to file (in which case you must, regardless of circumstances)

For most freelancers and side hustlers, it's the £1,000 self-employment threshold that triggers the requirement. If your side income from freelancing, selling goods, or providing services exceeds £1,000 gross (that's before expenses), you need to file.

Step 1: Register with HMRC

Before you can file a return, you need to register for self-assessment. If you're newly self-employed, you must register by 5 October following the end of the tax year in which you started trading. So if you began freelancing in June 2025, you'd need to register by 5 October 2026.

Registration is done online through the HMRC website. You'll need your National Insurance number and some basic personal details. HMRC will then send you a Unique Taxpayer Reference (UTR) — a 10-digit number that identifies you in their system. This arrives by post and can take up to 10 working days, sometimes longer. You'll also need to set up a Government Gateway account if you don't already have one, and you'll receive an activation code for that separately.

The whole process can take two to three weeks from start to finish, which is why registering well before the filing deadline matters. People who leave this until January discover they literally cannot file on time because they don't have their UTR yet.

Step 2: Understand the Tax Year and Deadlines

The UK tax year runs from 6 April to 5 April. So the 2025/26 tax year covers income earned between 6 April 2025 and 5 April 2026. Your self-assessment return for that year must be filed by one of two deadlines:

• 31 October 2026: If you're filing a paper return (yes, some people still do this)
• 31 January 2027: If you're filing online

The 31 January deadline is also when any tax you owe must be paid. Miss it, and you'll face an automatic £100 penalty — even if you don't owe any tax. After three months, daily penalties of £10 start accruing (up to a maximum of £900). After six months, there's an additional penalty of 5% of the tax due or £300, whichever is greater. After twelve months, things get properly serious.

The message is simple: don't miss the deadline. And the easiest way to not miss it is to file months early. HMRC opens online filing for the 2025/26 tax year from 6 April 2026. You can file in April and not pay until January — there's no advantage to filing late.

Step 3: Gather Your Records

Before you start filling in the return, you need all your financial records for the tax year. Good record-keeping throughout the year makes this painless. Last-minute scrambling through bank statements and email receipts makes it miserable.

What you'll need:

• Income records: Invoices sent, payments received, bank statements showing all income. If you're employed alongside self-employment, your P60 from your employer.
• Expense records: Receipts, invoices, and bank statements showing business expenses (more on what qualifies below).
• Bank interest statements: Your bank should provide a summary of interest earned, or you can find it in your annual statements.
• Pension contributions: If you pay into a personal pension, you'll need the amounts for tax relief claims.
• Student loan details: If you're repaying a student loan through self-assessment.
• Any P11D from your employer: This shows benefits in kind, like company cars or private medical insurance.

HMRC requires you to keep records for at least five years after the 31 January submission deadline. So records for the 2025/26 tax year must be kept until at least 31 January 2032. Digital copies are fine — you don't need paper filing cabinets anymore.

Step 4: Know What Expenses You Can Claim

This is where freelancers leave the most money on the table. You can deduct any expense that is "wholly and exclusively" for business purposes. That phrase is doing a lot of heavy lifting, and HMRC applies it strictly, but the list of legitimate deductions is longer than most people realise.

Commonly Claimed Expenses

• Office and premises costs: Rent for an office or co-working space, business rates, utilities for business premises
• Working from home: If you work from home, you can claim a proportion of your household costs (heating, electricity, broadband, rent or mortgage interest). The simplified method lets you claim a flat rate based on hours worked: £10/month for 25-50 hours, £18/month for 51-100 hours, £26/month for 101+ hours. The actual cost method requires more calculation but often yields a higher deduction.
• Travel: Business travel costs including train fares, fuel for business journeys (at HMRC's approved mileage rate of 45p per mile for the first 10,000 miles, 25p thereafter), parking, and hotel accommodation. Commuting between home and a regular place of work does not count.
• Equipment and tools: Computers, phones, software subscriptions, cameras (if you're a photographer), tools (if you're a tradesperson). Items used partly for personal use must be apportioned — if your laptop is 70% business, claim 70%.
• Professional subscriptions: Membership fees for professional bodies relevant to your work.
• Training: Courses that maintain or update existing skills (but not courses to acquire entirely new skills outside your current trade — that's a grey area worth understanding).
• Insurance: Professional indemnity insurance, public liability insurance, business equipment insurance.
• Marketing and advertising: Website hosting, domain names, business cards, advertising costs, portfolio hosting.
• Accountancy fees: The cost of an accountant to prepare your return is itself tax-deductible. Satisfyingly circular.
• Bank charges: Fees on a business bank account, PayPal or Stripe transaction fees.

What You Cannot Claim

Personal expenses, obviously. But also fines and penalties (including parking tickets and late filing penalties), entertainment costs for clients (HMRC disallows these for sole traders), clothing that could also be worn as normal attire (a suit doesn't count, even if you only wear it for meetings — but branded uniforms or specialist protective clothing does), and the personal portion of any mixed-use expense.

Step 5: Fill In Your Return

The online self-assessment form is divided into sections. You'll start with personal details, then move through employment income (if applicable), self-employment income, other income (property, investments, foreign), and deductions.

The self-employment section asks for your total business income and total allowable expenses. You can either enter a single figure for all expenses (simpler) or break them down by category (more detailed but useful if HMRC ever queries your return). You'll also need to declare your accounting basis — most small freelancers use the cash basis, which means you record income when received and expenses when paid, rather than when invoiced.

HMRC's system will calculate your tax bill based on what you enter. The calculation considers your personal allowance (£12,570 for 2025/26), basic rate band, higher rate band, National Insurance contributions, and any tax already paid through PAYE if you're also employed.

National Insurance: Class 2 and Class 4

Self-employed people pay two types of National Insurance Contributions in addition to income tax.

Class 2 NICs are a flat weekly rate — £3.45 per week for 2025/26 (around £179 per year). You only pay these if your profits exceed the Small Profits Threshold of £6,725. Class 2 contributions count towards your state pension entitlement, so paying them is actually beneficial even though they're a cost.

Class 4 NICs are profit-based: 6% on profits between £12,570 and £50,270, and 2% on profits above £50,270 for 2025/26. These are calculated automatically as part of your self-assessment and collected alongside your income tax payment.

Together, income tax and NICs mean that a freelancer earning £40,000 profit will pay roughly £5,486 in income tax and £1,646 in NICs — a total tax bill of around £7,132. That's an effective rate of about 17.8%, which is lower than many people expect, largely thanks to the personal allowance shielding the first £12,570.

Payments on Account: The Nasty Surprise

Here's the thing that blindsides almost every first-time self-assessment filer. If your tax bill exceeds £1,000 and less than 80% of your total tax is collected at source (through PAYE), HMRC will require payments on account. These are advance payments towards next year's tax bill, each equal to half of the current year's liability.

So if your 2025/26 tax bill is £6,000, you'll pay:

1. £6,000 for the 2025/26 tax year (due 31 January 2027)
2. £3,000 as the first payment on account for 2026/27 (also due 31 January 2027)
3. £3,000 as the second payment on account for 2026/27 (due 31 July 2027)

That means your first self-assessment payment is effectively 150% of one year's tax. For a £6,000 bill, you'd owe £9,000 on 31 January. This catches people off guard every single year, and it's the number one reason freelancers end up in payment difficulties with HMRC. Start setting aside 25-30% of every payment you receive throughout the year, and this won't be a shock.

Making Tax Digital

HMRC's Making Tax Digital (MTD) programme is rolling out to income tax self-assessment. From April 2026, self-employed individuals and landlords with income over £50,000 must keep digital records and submit quarterly updates to HMRC using MTD-compatible software. The threshold drops to £30,000 from April 2027.

This means that if your self-employment income is above these thresholds, you'll need to use accounting software like FreeAgent, Xero, QuickBooks, or another HMRC-recognised product — spreadsheets won't be compliant. The quarterly updates aren't full tax returns; they're summaries of income and expenses. You'll still file an annual return, but the quarterly data feeds into it.

For freelancers under the threshold, MTD isn't mandatory yet, but using proper accounting software is still a good idea. It automates categorisation, tracks expenses in real time, calculates your estimated tax bill, and makes the annual filing process dramatically less painful.

Common Mistakes That Cost Money

Based on what HMRC sees most frequently — and what accountants tell their clients — these are the errors worth avoiding:

• Not claiming legitimate expenses: The single most expensive mistake. Track everything, claim everything you're entitled to.
• Mixing personal and business finances: Get a separate bank account for business income and expenses. It doesn't have to be a "business account" — a second personal current account works fine for sole traders.
• Forgetting to include all income: HMRC receives data from banks, employers, and platforms. If your figures don't match theirs, it triggers an enquiry. Report everything.
• Wrong accounting basis: If you switch between cash basis and accrual basis, make sure you understand the implications and are consistent.
• Missing the payment on account: Budget for 150% of your annual tax bill in the first year. After that, the payments smooth out.
• Not registering for self-assessment: Earning over £1,000 from self-employment and not registering is not a grey area — it's a legal requirement you're failing to meet.

Should You Hire an Accountant?

If your affairs are genuinely simple — one source of self-employment income, straightforward expenses, no property or investment income — you can absolutely file yourself using HMRC's online system. It's clunky but functional.

If your situation is more complex — multiple income streams, a limited company, property income, overseas earnings, or you're approaching the higher rate threshold — an accountant will almost certainly save you more than they cost. A good sole trader accountant charges £200–£500 per year for self-assessment preparation and filing, and they'll typically identify deductions and reliefs that offset their fee several times over. They also deal with HMRC correspondence and reduce the odds of an enquiry.

The third option is to use accounting software to keep your records throughout the year and then file yourself at year-end. FreeAgent, which is free with certain NatWest and Mettle business accounts, is particularly well-suited to UK freelancers and generates the self-assessment return directly.

The Bottom Line

Self-assessment isn't inherently difficult — it's just badly explained by the people who designed it. Register early, keep records as you go, claim every expense you're entitled to, set money aside for tax throughout the year, and file well before the deadline. Do those five things and you'll never be the person sweating at 11pm on 30 January, wondering why HMRC's website is showing an error page. That person is having a terrible evening. Don't be that person.