Filing a Self-Assessment tax return can seem complicated, especially if you are completing one for the first time. Understanding what income needs to be reported, which expenses you can claim, and when your tax return and payments are due can help you avoid mistakes and unnecessary penalties.

This guide explains how to file a Self-Assessment tax return in the UK, who needs to file, what information you need, and how online tax software like Swiftax can simplify the process.

What Is a Self-Assessment Tax Return?

Self-Assessment is HMRC’s system for collecting Income Tax from individuals whose tax is not fully collected automatically through PAYE.

Instead of tax being deducted directly from your salary or pension, you are responsible for reporting your income, claiming eligible reliefs and expenses, calculating your tax liability, and paying any tax due.

A Self-Assessment tax return may be required if you have income or gains that need to be reported to HMRC, or if HMRC has issued you a notice requiring you to file.

Who Needs to File a Self-Assessment Tax Return?

You may need to complete a Self-Assessment tax return if you:

Are Self-Employed or a Sole Trader

If you run your own business, work as a freelancer, or earn income from trading activities, you will usually need to register for Self-Assessment and report your business income and allowable expenses.

Examples include:

• Freelancers
• Consultants
• Online sellers
• Contractors
• Independent professionals
• Side businesses

Receive Rental Income

If you receive taxable income from property rental, you may need to report it through Self-Assessment.

You may need to declare:

• Rental income received
• Allowable property expenses
• Finance costs where applicable
• Property-related adjustments

The requirement depends on your individual circumstances and whether HMRC requires you to file.

Earn Income from Additional Sources

You may need to file if you receive income that is not fully taxed through PAYE, including:

• Dividend income
• Savings income
• Foreign income
• Capital gains
• Certain pension income
• Other taxable income

Are a Partner in a Business Partnership?

Partners in a business partnership are generally required to complete a Self-Assessment tax return to report their share of partnership profits.

HMRC Has Asked You to File

Even if your circumstances do not appear to require a return, you must submit one if HMRC issues a notice requiring you to complete a Self-Assessment return.

When Do You Need to Register for Self-Assessment?

If you need to file a Self-Assessment return for the first time, you normally need to register with HMRC by:

5 October following the end of the tax year in which you started needing to file.

For example:

If you started self-employment during the 2025/26 tax year, you would normally need to register by 5 October 2026.

After registration, HMRC will provide your:

Unique Taxpayer Reference (UTR)

Your UTR is a 10-digit number used to identify your Self-Assessment account.

Step-by-Step Guide to Filing Your Self-Assessment Tax Return

Step 1: Register with HMRC

If you have never filed a Self-Assessment return before, you need to register with HMRC.

During registration you will provide information such as:

• Personal details
• National Insurance number
• Business details (if applicable)
• Income details

HMRC will send your UTR after registration.

You can register through the official HMRC website.

Step 2: Gather Your Information and Records

Before starting your tax return, collect all relevant documents for the tax year.

You may need:

Income Information

• Self-employment income records
• Sales invoices
• Rental income records
• Employment income details
• P60 or P45
• Dividend statements
• Bank interest statements
• Pension income details
• Foreign income information

Expense Information

If you are self-employed, you may need records of allowable business expenses, such as:

• Office costs
• Software subscriptions
• Professional fees
• Insurance
• Travel expenses
• Advertising costs
• Equipment costs

Only expenses that are wholly and exclusively for business purposes can normally be claimed.

Step 3: Complete Your Tax Return

Your Self-Assessment return includes different sections depending on your circumstances.

Common sections include:

• Personal information
• Employment income
• Self-employment income
• Property income
• Savings and investment income
• Dividend income
• Capital gains
• Tax reliefs and allowances

Make sure all information is complete and accurate before submission.

Step 4: Calculate Your Tax Liability

Once your information has been entered, your tax return will calculate:

• Income Tax due
• National Insurance contributions (where applicable)
• Payments on Account (where applicable)
• Tax already deducted through PAYE

Using a reliable tax software like Swiftax can help reduce calculation errors and ensure figures are prepared correctly.

Step 5: Submit Your Return to HMRC

Most taxpayers submit their Self-Assessment return online.

For the 2025/26 tax year:

• Online filing deadline: 31 January 2027
• Tax payment deadline: 31 January 2027

You should submit your return before the deadline to allow time to resolve any issues.

What Happens After You Submit Your Tax Return?

After successful submission:

• HMRC confirms receipt of your return.
• Your tax calculation becomes available.
• You can review your tax liability.
• You must pay any tax due by the relevant deadline.

Keep copies of your return and supporting records for future reference.

Common Self-Assessment Mistakes to Avoid

Many taxpayers make avoidable errors when completing their returns.

Common mistakes include:

Forgetting to Declare Income

All taxable income must be reported, including income from:

• Freelance work
• Online businesses
• Rental properties
• Investments
• Foreign sources

Claiming Incorrect Expenses

Not every expense is allowable.

Common errors include claiming:

• Personal expenses
• Private purchases
• Costs without supporting records

Missing Tax Deadlines

Late filing can result in penalties and interest charges.

Self-Assessment deadlines include:

• 5 October — Registration deadline
• 31 October — Paper return deadline
• 31 January — Online return and payment deadline

Not Keeping Proper Records

HMRC requires taxpayers to keep sufficient records to support their tax return.

Good record keeping helps you:

• Complete returns accurately.
• Support expense claims.
• Respond to HMRC enquiries.

How Swiftax Makes Self-Assessment Easier

Completing a tax return does not need to be stressful.

Swiftax helps simplify the process with:

• Easy-to-use online tax filing
• Automatic tax calculations
• Cloud-based access
• Simple guided questions
• Secure digital records
• No monthly subscription required

You can prepare your return for free and only pay when you submit.

Frequently Asked Questions

Do I need to file a tax return if I am employed?

Not always. Employees usually pay tax through PAYE, but you may need to file a Self-Assessment return if you have additional taxable income or HMRC requires you to.

Do company directors always need to complete a Self-Assessment return?

No. Being a company director alone does not automatically mean you need to file a Self-Assessment return. It depends on your circumstances and whether HMRC requires one.

How long do I need to keep Self-Assessment records?

You should normally keep records for at least five years after the 31 January submission deadline for the relevant tax year.

Can I file my tax return early?

Yes. You do not need to wait until January. Filing early gives you more time to review your figures and plan for any tax payment due.

File Your Self-Assessment Tax Return with Confidence

Preparing your Self-Assessment tax return early helps you avoid last-minute stress, identify potential issues, and stay compliant with HMRC requirements.

With Swiftax, you can prepare your return online, review your calculations, and submit when you are ready.

Start your Self-Assessment tax return with Swiftax today.

No subscription. Pay only when you submit.

Disclaimer: This article provides general information only and does not constitute tax, accounting, financial, or legal advice. Tax rules and regulations may change, and the application of any information depends on your individual circumstances. Always seek advice from a qualified professional or refer to official guidance before making decisions.