Self-Assessment Tax Return: A Complete Guide for First-Timers

Who needs to file a self-assessment return?

You must register for self-assessment and file a tax return if any of the following applied in the last tax year:

• You were self-employed as a sole trader and earned more than £1,000
• You were a partner in a business partnership
• You are a company director (unless your only income is PAYE salary with no untaxed income)
• Your total taxable income was over £100,000
• You had untaxed income over £2,500 from savings, investments or rental property
• You received income from abroad
• You need to claim certain tax reliefs
• HMRC sent you a letter asking you to file

If you are employed and pay tax through PAYE, you do not normally need to file a return — unless one of the above applies.

When to register — and why timing matters

If this is your first time filing, you need to register with HMRC before you can submit a return. Registration is separate from filing and takes time — HMRC will post your Unique Taxpayer Reference (UTR) to your home address, which can take up to 10 working days.

Register as soon as possible, even if the filing deadline feels far away. If you leave it too late, you risk missing the deadline before you have even started.

Register online at gov.uk/register-for-self-assessment. You will need a Government Gateway account, which you can create at the same time.

Key dates for the 2025/26 tax year (6 April 2025 to 5 April 2026):

• 5 October 2026 — deadline to register if newly self-employed
• 31 October 2026 — deadline to file a paper return
• 31 January 2027 — deadline to file online and pay tax owed
• 31 January 2027 — first payment on account due
• 31 July 2027 — second payment on account due

What information do you need to gather?

Before you sit down to file, gather the following:

Personal details

• Your UTR — 10 digits, on any letter from HMRC
• Your National Insurance number
• Government Gateway user ID and password

Income records

• P60 or P45 if employed at any point in the year
• Self-employment income records — invoices, bank statements or accounting software exports
• Interest statements from banks
• Dividend vouchers if applicable
• Any other income received

Expense records

For self-employed income, you can deduct allowable business expenses before calculating taxable profit. Keep records for:

• Office costs — stationery, software, phone
• Travel costs for business journeys
• Equipment and tools
• Professional subscriptions and fees
• Marketing and advertising
• Business insurance
• Use of home as office (if applicable)

Pension and Gift Aid

• Contributions to a personal pension
• Gift Aid donations — these extend your basic rate band

How to calculate your tax bill

Your tax bill is based on your taxable income — total income minus the personal allowance (£12,570 in 2026/27) and any allowable deductions. Income tax rates for 2026/27:

• 20% on income between £12,570 and £50,270
• 40% on income between £50,270 and £125,140
• 45% on income above £125,140

If you are self-employed, you also pay Class 4 National Insurance:

• 9% on profits between £12,570 and £50,270
• 2% on profits above £50,270

Payments on account — what first-timers always miss

This is the most common shock for first-time filers. When you pay your first self-assessment bill in January, HMRC does not just ask for the tax you owe for the year just gone. They also ask for an advance payment towards next year's bill.

This advance payment is called a payment on account. HMRC requires two per year — January and July — each equal to half your previous year's tax bill.

So if your first tax bill is £3,000, your January payment will be:

• £3,000 — tax owed for the year
• £1,500 — first payment on account
• Total: £4,500 due in January

The second payment on account of £1,500 is due the following July. This catches most first-timers off guard. Budget for it in advance.

How to actually file your return

1. Log in at gov.uk/log-in-file-self-assessment-tax-return
2. Select "Complete your tax return"
3. Work through each income source — the online form guides you
4. The system calculates your bill automatically as you go
5. Review the summary and submit
6. Pay any tax owed by 31 January

You can save your progress and return to it — you do not have to complete it in one sitting.

Common mistakes to avoid

Missing the registration deadline

You cannot file without a UTR. Register early — allow at least 2 weeks for your UTR to arrive by post.

Forgetting to include all income

HMRC receives data from banks, employers and other sources. Income you omit will likely be flagged.

Not claiming allowable expenses

Many first-timers under-claim expenses and overpay tax. Keep records throughout the year.

Ignoring payments on account

Assuming your bill is just the current year's tax. Budget for the full January payment.

Filing late

Even a day late triggers a £100 penalty. Set a reminder in late December.

Should you use an accountant?

For straightforward self-employment without complex income sources, many freelancers file successfully themselves. An accountant is worth considering if your income is complex, you are approaching the £100,000 threshold, or you want peace of mind. Fixed-fee filing services typically charge £100-£300.

Frequently asked questions

What is a UTR number?

A Unique Taxpayer Reference (UTR) is a 10-digit number HMRC assigns when you register for self-assessment. You need it to file your return and it never changes.

Can I file my self-assessment return early?

Yes. The online filing window opens on 6 April each year. Filing early means you know your bill sooner. The payment deadline remains 31 January regardless of when you file.

What happens if I cannot pay my tax bill?

Contact HMRC before the deadline. You may be able to set up a Time to Pay arrangement to spread the cost. Interest applies but penalties are lower if you contact HMRC proactively.

Do I need to file a return if I made a loss?

Yes, if registered for self-assessment. Filing when you have made a loss lets you carry it forward to offset against future profits, reducing future tax bills.

What is the difference between self-assessment and PAYE?

PAYE is used by employers to deduct tax automatically from salary. Self-assessment is used by self-employed people to calculate and declare their own tax, as there is no employer to deduct it.