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Step-by-Step Guide to Completing Your Self Assessment Tax Return in the UK

Completing your Self Assessment tax return can seem daunting, especially if you are self-employed or have multiple income sources. However, understanding each stage of the process makes it much easier to manage. This step-by-step guide from Stan Lee Accountancy Ltd will help you navigate your Self Assessment tax return smoothly and avoid costly mistakes.

What Is a Self Assessment Tax Return?

A Self Assessment tax return is a system used by HM Revenue and Customs (HMRC) to collect Income Tax from individuals whose income isn’t automatically taxed through PAYE. This includes:

  • The self-employed (sole traders and partners)
  • Company directors
  • Individuals earning additional income (e.g., from property or investments)
  • Those with foreign income or other untaxed earnings

If HMRC has asked you to complete a Self Assessment, or you meet the criteria, it’s your responsibility to file your return correctly and on time.

Step 1: Register for Self Assessment

If you’re self-employed or need to complete a tax return for the first time, you must register with HMRC. You can do this online via the HMRC website.

Once registered, HMRC will send you:

  • A Unique Taxpayer Reference (UTR) number
  • An activation code to access your online account

Make sure you complete registration well before the deadline, as receiving your UTR can take up to 10 working days.

 

Step 2: Gather Your Financial Information

Before starting your tax return, collect all the necessary financial documents. These may include:

  • P60 or P45 forms (if employed during the tax year)
  • Invoices, receipts, and expense records (if self-employed)
  • Bank statements and dividend vouchers
  • Details of property income and expenses

  • Records of savings interest, pensions, or capital gains

Organising these documents in advance will make the filing process much quicker and reduce the risk of errors.

  1. Step 3: Log In and Start Your Tax Return

    Once you have everything ready, log into your HMRC online account. Select the relevant tax year (for example, 6 April 2025 to 5 April 2026) and begin your return.

    You’ll be guided through sections where you’ll need to:

    • Confirm personal details
    • Declare all income sources
    • Claim allowable expenses and reliefs
    • Report capital gains (if applicable)

    Be sure to review each page carefully. Incomplete or inaccurate information can delay processing or trigger an HMRC enquiry.

     

    Step 4: Claim Allowable Expenses

    If you’re self-employed, you can deduct allowable business expenses to reduce your taxable profit. Common examples include:

    • Office supplies and equipment
    • Travel costs and fuel
    • Marketing and advertising
    • Professional fees (such as accountants or solicitors)
    • Work-related phone and internet bills

    Claiming legitimate expenses ensures you only pay tax on your true profits. However, personal or non-business costs cannot be claimed.

     

    Step 5: Review and Submit Your Tax Return

    Once you’ve filled in all relevant sections, review your entries thoroughly. HMRC’s online system will calculate your tax liability automatically based on the information you provide.

    When you’re confident everything is correct, submit your return online. You’ll receive a confirmation message and reference number – keep this for your records.

     

    Step 6: Pay Your Tax Bill

    After submitting your return, HMRC will confirm how much you owe. The payment deadline for online returns is 31 January following the end of the tax year.

    If your tax bill exceeds £1,000, you may also need to make Payments on Account – advance payments towards the next year’s tax.

    Payment can be made via:

    • Online banking or debit card
    • Direct Debit
    • HMRC app
    • At your bank or building society

    Missing the payment deadline could result in interest and penalties, so it’s crucial to pay on time.

     

    Step 7: Keep Records for Future Reference

    By law, you must keep your financial records for at least five years after the submission deadline. Proper record-keeping helps with future returns and protects you in case of an HMRC audit.

How Stan Lee Accountancy Ltd Can Help

Completing your Self Assessment tax return can be time-consuming and stressful. At Stan Lee Accountancy Ltd, our specialist accountants make the process seamless. We handle everything – from registration and expense tracking to tax filing and compliance – ensuring your return is accurate and submitted on time.

With offices in London, United Kingdom, we offer personalised accounting services tailored to your needs.