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Filing Your Tax Return: Step by Step

Filing your Self Assessment doesn't have to be stressful. This guide walks you through the online process from start to finish, so you know exactly what to expect before you log in.

Start in April, not January. You'll have 10 months to gather everything, and you won't be competing with millions of last-minute filers crashing the HMRC website.

When to File

The tax year runs from 6 April to 5 April. For the 2025-26 tax year:

• Tax year: 6 April 2025 to 5 April 2026
• Online filing deadline: 31 January 2027
• Paper filing deadline: 31 October 2026 (but file online, it's easier)
• Payment deadline: 31 January 2027 (same as filing)

You can file as soon as the tax year ends on 6 April. The earlier you file, the earlier you know what you owe, and the longer you have to save for it.

What You Need Before You Start

Gather all of this before you touch the HMRC website:

• UTR number - your 10-digit Unique Taxpayer Reference (on previous HMRC letters)
• Government Gateway login - username and password (set up at gov.uk if you haven't already)
• Total income from self-employment for the tax year
• Total expenses broken down by HMRC category (see below)
• Bank interest earned during the year (your bank sends this or it's in your online banking)
• Student loan type (Plan 1, Plan 2, Plan 4 or Postgraduate) if applicable
• Pension contributions you made personally during the year
• P60 or P45 if you also had employment income
• Any other income - rental, dividends, interest from savings over £1,000

Tip for new starters: Your UTR number and Government Gateway login are two different things. The UTR identifies you to HMRC. The Gateway login gets you into the website. You need both.

Step by Step Through the Online Form

1. Log In and Start Your Return

Go to gov.uk/self-assessment-tax-returns. Log in with your Government Gateway ID. Select the correct tax year. HMRC will ask which sections apply to you. Tick "Self-employment" and any others that are relevant.

2. Personal Details

Check your name, address, date of birth and National Insurance number are correct. Update anything that's changed.

3. Self-Employment Pages (SA103)

This is where the main work happens.

Business details: Your trading name (this can just be your own name), business description (e.g. "hairdressing" or "beauty therapy"), business start date, and your accounting period.

Income: Enter your total turnover. This is everything you earned from self-employment before expenses. Include cash, bank transfers, card payments, tips, and any other payments from clients. Do not subtract expenses yet.

Expenses: HMRC breaks expenses into categories:

• Cost of goods (products you buy for treatments)
• Car, van and travel expenses (mileage or actual costs, not both)
• Wages, salaries and other staff costs
• Rent, rates, power and insurance
• Repairs and maintenance of property and equipment
• Phone, fax, stationery and other office costs
• Advertising and business entertainment
• Interest on bank and other loans
• Bank, credit card and other financial charges
• Accountancy, legal and other professional fees
• Other business expenses

Enter totals for each category. You don't need to list every receipt, but you must keep them for 5 years in case HMRC asks.

4. Capital Allowances

If you bought equipment costing over £1,000 (salon chair, UV lamp, laser machine), you may need to claim it as a capital allowance rather than a regular expense. The Annual Investment Allowance lets you deduct the full cost of qualifying equipment in the year you bought it, up to £1,000,000 (2025-26).

5. Net Profit

HMRC calculates this for you: total income minus total expenses minus capital allowances. This is the figure you pay tax and National Insurance on.

6. Other Income Sections

If you ticked other sections (employment income, bank interest, student loan), fill those in now. HMRC guides you through each one.

7. Tax Calculation

Once you've completed all sections, HMRC shows you a tax calculation. This tells you exactly how much Income Tax and National Insurance you owe. Check it makes sense. If you earned £25,000 profit and it says you owe £15,000, something is wrong.

8. Submit

Review everything. Submit. Save or print the confirmation page with your submission reference number.

Common Mistakes

• Wrong tax year. Make sure you're entering figures for the right 12-month period.
• Forgetting cash and tips. All income is taxable, not just what goes through your card machine.
• Claiming personal expenses. Your weekly food shop is not a business expense. Your phone bill is only partly claimable if you use the same phone for personal and business.
• Missing the student loan tick box. If you have a student loan, HMRC needs to know. Forgetting it doesn't make the repayment go away. It just delays it and creates a bigger bill later.
• Mixing up turnover and profit. Turnover is total income before expenses. Profit is what's left after expenses. Enter turnover in the income box, not profit.

Tip for new starters: Screenshot every page as you go through the form. If anything looks wrong later, you'll have a record of exactly what you entered.

What Happens After You Submit

• You get a tax calculation showing what you owe
• Payment deadline is 31 January (same day as filing deadline)
• Payment methods: direct bank transfer (fastest), debit card online, budget payment plan (monthly direct debit through the year)
• If you owe over £1,000, you may also need to make Payments on Account for next year (see related guide)

Payments on Account: The Basics

If your tax bill is over £1,000 and less than 80% of your tax was collected at source (through PAYE), HMRC will ask you to make two advance payments towards next year's bill. Each payment is half of the current year's bill. They're due on 31 January and 31 July. This catches many people off guard in their second year.

What If You Made a Mistake?

You can amend your return within 12 months of the original filing deadline. Log back into your Government Gateway, find the return, and make corrections. HMRC recalculates your bill automatically.

If you discover an error after 12 months, you'll need to write to HMRC or call them.

Who to Contact

• HMRC Self Assessment helpline: 0300 200 3310 (Free)
• HMRC online services helpdesk: 0300 200 3600 (Free)
• TaxAid - free tax help for low-income earners: 0345 120 3779 (Free)
• Tax Help for Older People: 0345 601 3321 (Free)

Sources

• HMRC Self Assessment guidance (gov.uk/self-assessment-tax-returns)
• HMRC expenses categories for Self Assessment (gov.uk)
• Annual Investment Allowance (gov.uk/capital-allowances)

Related Guides

• Self-Assessment for Hairdressers
• Self-Assessment for Beauty Therapists
• Allowable Expenses: What You Can Claim
• Getting and Working with an Accountant
• Payments on Account Explained
• Student Loan Repayments for the Self-Employed

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