Simple Guide to Completing a Self-Assessment Tax Return

If you have income that isn't taxed automatically through PAYE, there's a good chance you need to complete a self-assessment tax return. It can feel like a daunting chore, but most people find that once they've gathered everything together, the actual filing takes well under an hour. The trick is preparation — and keeping decent records all year round so that nothing is a scramble in January.

Do You Actually Need to File?

HMRC expects a return from anyone whose untaxed income is significant enough to matter. The common triggers include:

• Self-employed income above the £1,000 trading allowance.
• Rental income from property, again above £1,000.
• Dividends, savings interest or other investment income that pushes you over your allowances.
• Income from a side hustle — selling, tutoring, driving, content creation, anything that generates profit.
• Higher or additional rate income, or the High Income Child Benefit Charge.
• Foreign income, crypto gains or a large one-off capital gain.

If you're filing for the first time, you must register with HMRC by 5 October following the end of the tax year. The tax year runs from 6 April to 5 April, so that's an important date to set a reminder for. After that, you can file on paper by 31 October, or online by 31 January — and the tax itself is due on that same 31 January date.

Gather Everything Before You Start

Filing is much easier when you're not hunting for documents halfway through. Before you log in, collect:

• Your Unique Taxpayer Reference (UTR) — a ten-digit number on any letter from HMRC.
• Your National Insurance number.
• P60s from employers, plus P45s if you changed jobs.
• Bank statements showing savings or investment interest.
• Dividend vouchers or investment summaries.
• Records of any rental income and the expenses you paid out.
• Invoices and receipts for self-employed work.
• Details of pension contributions, Gift Aid donations and any other reliefs.
• Capital gains information if you sold assets such as shares or a second property.

If you've kept a simple spreadsheet throughout the year, this stage takes minutes. If not, set aside an evening — and make this the year you promise yourself you'll do better next time.

Knowing What You Can Claim

You only pay tax on profit, not on every pound that comes in, and plenty of people overpay simply because they don't claim what they're entitled to.

If you're self-employed, allowable expenses include office supplies, business insurance, software subscriptions, professional fees, travel to clients, and a proportion of your home costs if you work from home — typically calculated using HMRC's simplified flat rates or a reasonable business percentage of your bills.

If you're employed, you may be able to claim tax relief on:

• Professional subscriptions and union fees.
• Business mileage in your own car, above what your employer reimburses.
• Uniform that you wash and maintain yourself. for example.
• Pension contributions paid from your own pocket, which extend your basic rate band.
• Gift Aid donations, which also stretch your basic rate band.

Keep receipts for everything. If HMRC ever queries a figure, a bank statement showing a payment isn't proof of what it was for — the invoice is.

Working Through the Online Return

The online form is divided into sections, and you only complete the ones that apply to you. Start with the personal details, then work through employment, self-employment, property, and other income in turn. The system saves as you go, so you can pause and come back if you need a document you haven't got.

Two things worth double-checking before you submit. First, that your name, address and bank details are current — especially if you've moved. Second, that you haven't missed an income stream. It's easy to overlook a small savings account or a second platform holding a handful of shares.

When you're ready, submit the return and then check your calculation carefully. It will show your tax due, any Class 2 and Class 4 National Insurance if you're self-employed, and what you've already paid.

Payments on Account and Paying Your Bill

Once your bill exceeds £1,000 and you've paid less than 80% of it through PAYE, HMRC will usually ask for payments on account. These are advance instalments towards next year's bill, each worth half of what you owe, due on 31 January and 31 July.

They often catch people out in their first self-employed year, because you pay this year's tax plus half of next year's in one go. If your income has dropped and you expect a lower bill, you can apply to reduce your payments on account — but be careful, because if you reduce them too far, interest is charged on the shortfall.

Set the money aside as you earn it. Transferring a percentage of every invoice into a separate savings account makes January far less painful.

Keep Records and Avoid Penalties

Filing late costs you £100 immediately, even if you owe nothing, with further daily charges and percentage penalties stacking up over time. Paying late adds interest on top.

Keep your records — invoices, receipts, bank statements, contracts — for at least 22 months after the end of the tax year, or five years and ten months if you have trading or partnership income. Digital records make this easy: scan receipts, label them clearly, and file them by year.

Finally, file early. Submitting in April or May, rather than waiting until the last week of January, gives you time to correct mistakes calmly, and it takes the pressure off completely. Your future self will thank you.