Self Assessment Tax Return: A Beginner's Guide

The first time I filed a self-assessment tax return, I left it until January 28th. Three days before the deadline. I had a shoebox of receipts, no idea what I was doing, and a growing sense of dread. I got it done, but I overpaid by roughly £1,400 because I didn’t claim expenses I was entitled to. That stung more than the process itself.

Self-assessment isn’t complicated once you understand what’s expected. But HMRC doesn’t exactly make it intuitive, and the penalties for getting it wrong (or getting it late) are unforgiving. If you’re filing for the first time, here’s everything you need to know.

Who needs to file a self-assessment?

You need to file a self-assessment tax return if any of the following apply:

You’re self-employed (sole trader or partner in a partnership) and earned more than £1,000
You earned over £150,000 in the tax year (though this threshold has shifted; check the current rules)
You received rental income from property
You earned over £100,000 from any source (because you lose your personal allowance gradually above this)
You have untaxed income such as tips, commission, or foreign income
You need to pay capital gains tax on the sale of assets
You’re a company director (unless it was for a non-profit)
You receive child benefit and you or your partner earns over £60,000 (the High Income Child Benefit Charge)
HMRC has sent you a notice to file (even if you don’t think you need to, if they’ve asked, you must)

If you’re a PAYE employee with no other income and you earn under £100,000, you probably don’t need to file. But if any of the situations above apply, you do.

Registering for self-assessment

Before you can file, you need to register with HMRC. This is the bit that catches people out because it takes time.

If you’re newly self-employed, register by 5 October following the end of the tax year in which you started trading. So if you started freelancing in July 2025, you need to register by 5 October 2026.

You can register online through the HMRC website. You’ll get a Unique Taxpayer Reference (UTR) sent by post (yes, actual post) within 10 working days. You’ll also need to set up a Government Gateway account if you don’t already have one.

Don’t leave registration to the last minute. If you haven’t registered, you can’t file. And if you can’t file, you’ll get fined regardless of whether you owe any tax.

The timeline

Here are the key dates for the 2025/26 tax year:

6 April 2025 to 5 April 2026: The tax year
5 October 2026: Deadline to register for self-assessment (if newly self-employed)
31 October 2026: Deadline for paper returns (hardly anyone does this now)
31 January 2027: Deadline for online returns AND payment of any tax owed
31 July 2027: Second payment on account due (if applicable)

The 31 January deadline is the one that matters most. Miss it and you’ll get an automatic £100 penalty, even if you don’t owe any tax. Leave it three months late and daily penalties kick in. Leave it six months and it gets worse. Twelve months late and HMRC can charge up to 100% of the tax owed as a penalty.

File early. There is no advantage to waiting. You can file your return from 6 April onwards for the previous tax year. Filing early doesn’t mean you pay earlier; payment is still due on 31 January. But it does mean you know exactly what you owe and you have months to plan for it.

What records you need

HMRC requires you to keep records of all income and expenses for at least five years after the 31 January submission deadline. That means records for the 2025/26 tax year must be kept until at least 31 January 2032.

You’ll need:

Income records: Invoices, bank statements, payment confirmations
Expense records: Receipts, bank statements, mileage logs
Bank statements: For all business-related accounts
P60 or P45: If you also have employment income
Dividend vouchers: If you receive dividends from a limited company
Rental income records: Tenancy agreements, mortgage statements, maintenance receipts
Pension contribution statements: For claiming tax relief

Digital records are fine. You don’t need to keep physical receipts. Take photos, use accounting software, or scan everything. Just make sure you can produce the records if HMRC asks.

The most common mistakes

Having watched friends and fellow business owners file their returns over the years, the same mistakes come up repeatedly.

Missing the deadline. The £100 fine is the minimum. It escalates quickly. Set a reminder for December to start pulling your records together. Better yet, file in the summer when you have time and the numbers are fresh.

Not claiming legitimate expenses. This was my mistake the first time round. If you’re self-employed, you can claim for office costs, travel, phone bills, software subscriptions, professional development, and much more. Every expense you miss is money you’ve given to HMRC unnecessarily.

Forgetting about payments on account. This one blindsides almost everyone the first time. If your tax bill exceeds £1,000, HMRC will ask you to make “payments on account” for the following year. More on this below.

Not separating personal and business finances. If everything runs through one bank account, you’ll spend hours trying to identify which transactions were business-related. Open a separate account. It doesn’t have to be a business account; a second personal current account works fine for sole traders.

Underestimating the tax bill. When you’re employed, tax is deducted at source. You never see it. When you’re self-employed, you see your gross income and it feels like your money. Then January comes and HMRC wants 20% to 40% of it. Put aside 25% to 30% of your profit each month into a separate savings account. You’ll thank yourself later.

Payments on account explained

This is the one that properly shocked me. If your self-assessment tax bill is more than £1,000 (and more than 80% of it wasn’t collected at source through PAYE), HMRC assumes you’ll owe a similar amount next year and demands advance payments.

Here’s how it works. Say your tax bill for 2025/26 is £6,000.

31 January 2027: You pay the £6,000 owed, plus a first payment on account of £3,000 (50% of the bill) for 2026/27. Total due: £9,000.
31 July 2027: Second payment on account of £3,000.
31 January 2028: Your 2026/27 bill is calculated. If it’s £6,000 again, you’ve already paid £6,000 in advance, so nothing more is due. If it’s higher, you pay the difference. If it’s lower, you get a refund.

That first January payment is brutal. You’re paying your current year’s bill plus half of next year’s estimated bill. Nobody warns you. Well, I just did.

You can apply to reduce your payments on account if you know your income will be lower the following year. Do this through your HMRC online account. But be careful: if you reduce them too much and your actual bill ends up higher, you’ll be charged interest on the underpayment.

Accountant vs doing it yourself

For a straightforward self-assessment (one source of self-employment income, simple expenses), you can absolutely do it yourself using HMRC’s online system. It’s not beautiful, but it works.

If your tax affairs are more complex (multiple income sources, rental property, capital gains, or you’re a company director), get an accountant. A good one will save you more than they cost, because they’ll claim every deduction you’re entitled to and structure things efficiently.

Software like FreeAgent, Xero, and QuickBooks can help you track income and expenses throughout the year and generate the figures you need for your return. FreeAgent in particular integrates well with HMRC’s Making Tax Digital requirements and can file your return directly.

If you’re going to do it yourself, use software. Don’t try to manage everything in a spreadsheet. You’ll miss things and the time cost isn’t worth it.

Start early, stay calm

Self-assessment is not the monster HMRC’s reputation makes it seem. The process is logical: earn money, track it, report it, pay what you owe. The people who struggle are the ones who leave it until January, panic, and either overpay or underpay.

Register on time. Keep your records up to date. Set money aside each month. File early. That’s it. Your first return is the hardest. Every one after that is just updating the numbers.