Filing your first self assessment tax return can feel like stepping into a maze. The forms, the deadlines, the strange tax codes — it is enough to make even the most confident person pause. But here is the truth: it is not as bad as it looks. You do not need a degree in finance, and you do not have to panic every time you see the words “HMRC” in your inbox.

Whether you are newly self-employed, earning extra cash on the side, or working as a freelancer, this guide will walk you through what self assessment means, who needs to file, and how to get it done without losing your head.

What Is a Self Assessment Tax Return?

In simple terms, self assessment is how HMRC works out how much income tax you owe. If your income is not taxed automatically through PAYE (like it is for most employees), you are expected to tell HMRC about your earnings and pay what you owe.

You file a self assessment tax return service, HMRC calculates your bill, and you pay up. Easy? Sort of. But there are a few steps you need to get right — especially the first time around.

Do I Need to File One?

Not everyone needs to file a self assessment. If you are on PAYE and have no income from other sources, you are likely in the clear. But you will probably need to file if any of the following apply to you:

You are self-employed as a sole trader and earned more than £1,000 in the tax year

You are a partner in a business partnership

You received income from renting out property

You made money from investments, dividends, or crypto

You had foreign income

You earned more than £100,000 from any source

You claimed Child Benefit and earned over £50,000

If in doubt, HMRC’s online tool can help you check. But as a rule of thumb: if any of your income is not taxed automatically, you probably need to tell them about it.

When Are the Key Deadlines?

Let us say you are filing your first tax return for the 2024/25 tax year (which runs from 6 April 2024 to 5 April 2025). These are the dates you need to know:

5 October 2025 – Deadline to register for self assessment if you have never done it before

31 January 2026 – Final deadline to file your return online and pay your bill

31 July 2026 – Deadline to make your second “payment on account” (more on that later)

Missing these deadlines can lead to fines, so pop them in your calendar now.

How Do I Register?

If this is your first time, the first thing you need to do is register with HMRC. You can do this online through the HMRC website. You will need:

Your National Insurance number

Your personal details (name, address, date of birth)

Information about your self-employment or income sources

Once you register, HMRC will send you a Unique Taxpayer Reference (UTR) number by post. This can take a week or two, so do not leave it too late.

After you get your UTR, you can create a Government Gateway account and file your return online.

What Information Do I Need to File?

Before you sit down to fill out your return, gather all your paperwork. Here is what you will likely need:

Records of your self-employed income

Invoices and receipts for expenses

P60s or P45s if you also had a job

Details of bank interest, dividends, or rental income

Pension contributions

Charity donations (for Gift Aid relief)

If you are self-employed, you can usually deduct allowable expenses from your income before tax is calculated. These might include:

Office costs (software, stationery)

Business travel

Phone bills

Marketing

Professional services

Keep those receipts. HMRC might not ask for them straight away, but you need to hold onto them for at least five years after the 31 January deadline.

What Is “Payment on Account”?

This one trips up a lot of first-timers.

If your tax bill is over £1,000 and you do not pay tax through PAYE, you may be asked to make payments on account — basically, advance payments for next year’s tax bill.

Here is how it works:

You pay 50% of your tax bill on 31 January

You pay the second 50% on 31 July

If you owe more than expected, you top it up with a “balancing payment” next January

So if you owe £2,000 for 2024/25, HMRC will ask for £3,000 by January 2026: £2,000 for this year, and £1,000 toward next year. That catches a lot of people off guard, so do not get caught short.

What If I Make a Mistake?

Mistakes happen. The good news is HMRC gives you 12 months after the deadline to amend your return. So if you realize you forgot to claim a deduction or entered something wrong, you can go back and fix it.

But honesty is key. If HMRC thinks you have tried to hide income, they can issue fines or even open an investigation. Always be upfront, even if it means paying more than you expected.

Should I Hire an Accountant?

If your finances are simple, you can file your self assessment yourself using HMRC’s online portal. Many people do.

But if you have multiple income sources, claim lots of expenses, or just do not want the stress, an accountant is worth their fee. They can help you reduce your bill, avoid penalties, and make sure you are playing by the rules.

Prices vary, but expect to pay around £200 to £500 for a basic return — more if your affairs are complex.

Tips to Make It Easier Next Time

Keep digital records as you go. Use apps like QuickBooks, Free Agent, or even Google Sheets.

Track expenses in real time — not six months later.

Set money aside for tax every month. A good rule is 20-30% of your income.

File early. You do not have to wait until January. Filing in April gives you more time to save and plan.

It Is About Control

Doing your first self assessment tax return might not be fun, but it is a rite of passage for anyone running their own business or side hustle. It means you are earning, you are growing — and you are in charge.

By understanding the process, getting organized, and asking for help when you need it, you will not only avoid nasty surprises — you will build a solid foundation for your financial future.

And once you have done it once, you will wonder what all the fuss was about.