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How to Complete a Self Assessment Tax Return?

JAFA Accountancy

Updated: 5 days ago


Woman lying on table trying to complete self assessment

Are you stressing about completing your self assessment tax return? In this guide, we will break it down step by step to make filing your tax return easier. 


Gather Essential Information and Documents

To accurately complete your Self Assessment tax return, it's important to gather all how-to-complete-a-self-assessment-tax-returnnecessary information and documents beforehand. Here’s a list:


  1. Basic Identification Information


  • Unique Taxpayer Reference (UTR) - A 10-digit code provided by HM Revenue and Customs (HMRC) upon registration for Self Assessment.


  • National Insurance Number - Found on official documents like payslips, P60s, or letters about tax, pensions, or benefits.


  1. Income Details


Employment Income


  • P60 - Summarises your income and tax paid for the year.

  • P45 - If you changed jobs during the year, this is needed for income and tax details from your previous employer.

  • P11D - Details of benefits-in-kind received from your employer (e.g., a company car).


Self-Employment Income


  • Invoices, bank statements, or bookkeeping records.

  • Income from side hustles or freelancing.


Rental Income


  • Records of rental income and deductible expenses.


Investment Income


  • Dividend certificates.

  • Bank or savings account interest statements.

  • Gains from selling investments, property, or assets (may require reporting for Capital Gains Tax).


Foreign Income


  • Income earned abroad, including dividends, rental income, or wages.


Other Income Sources


  • Pensions (state, workplace, or private).

  • Lump sums or redundancy payments.

  • Trusts or estates income.

  • Any other taxable income.



  1. Expense Records


  • Allowable business expenses (for self-employed) like travel, office supplies, training, utilities for home office, etc.

  • Rental property expenses like maintenance, insurance, and letting agent fees.

  • Professional fees or subscriptions.

  • Contributions to charity under Gift Aid.


  1. Tax Reliefs and Deductions


  • Pension contributions (private or workplace schemes).

  • Contributions to enterprise investment schemes or venture capital trusts.

  • Gift Aid donations.


  1. Special Circumstances


  • Student Loan Repayments - If you’re earning over the repayment threshold.

  • Capital Gains - If you sold shares, property (not your main home), or other investments.

  • Marriage Allowance Transfer - If applicable.



  1. Additional Requirements


  • Child Benefit Payments - If your income is over £50,000 and subject to the High Income Child Benefit Charge.

  • Tax Payments on Account - If required, ensure you know what’s owed and deadlines.

  • Previous Tax Returns - Useful for cross-referencing or if queried by HMRC.


You can find the PDF version of the self assessment tax return forms to help you familiarise yourself with them and understand what information you'll need to have on hand.


Step-by-Step Guide to Completing the Self Assessment Tax Return

Now that you know what documents to gather, make sure you have them with you when completing your self-assessment tax return. Once you have them ready, you can begin completing the tax return by following these easy-to-follow steps.


  1. Register for Self Assessment


If you're self-employed, a partner in a business, or have other untaxed income (e.g., from property rentals or investments), you must register with HMRC.


You can check out this article, where we discuss in detail whether you need to file a self-assessment tax return.


Registration should be completed by 5 October following the end of the tax year in which you had the income.


Visit the HMRC Self Assessment registration page and follow the instructions relevant to your situation.


  1. Log in to Your HMRC Online Account


Once registered, you can use your Government Gateway user ID and password to log in to your HMRC online account.


If you haven't set up an account, you'll need to create one using your UTR and personal details.


  1. Complete the Relevant Sections of the Tax Return


Start by confirming or updating your personal information to make sure it’s accurate. The HMRC system will then customise your tax return based on your situation, showing only the sections you need to complete.


Declare all income, including employment (using figures from your P60 or P45), self-employment (with total income and allowable expenses), and other sources like property rentals, savings, investments, or foreign income.


Include details for deductions and reliefs, such as pension contributions and charitable donations. If you have specific income or gains, like capital gains, complete the supplementary pages to cover everything accurately.


  1. Review and Submit Your Tax Return


Before submitting your tax return, you should review all the information carefully to make sure it’s correct. Check your income, expenses, and deductions to avoid mistakes. 


The HMRC system will calculate your tax automatically based on the details you’ve entered. Once you’re happy everything is accurate, you can submit your return online and you will get an instant confirmation from HMRC.


If you have any questions or confusion, you can always contact HMRC


What We Think

Completing a Self Assessment tax return can feel overwhelming, especially if you’re new to the process or juggling multiple sources of income. 


In our experience, the system itself isn’t overly complicated, but it can be time-consuming and confusing if you’re unsure which sections apply to your circumstances. 


HMRC provides some useful guidance, but it often lacks the practical, step-by-step clarity many people need.


As for HMRC penalties for late submissions or payments, they can feel harsh, especially for first-time filers or those who genuinely struggle to meet deadlines.


How JAFA Can Help

We know how stressful filing a Self Assessment tax return can be, especially if you’re unsure where to start. We use AI technology for bookkeeping to help you organise your finances throughout the year, so everything you need is ready when it’s time to file.


If HMRC’s guidance feels unclear, we break things down into simple, step-by-step instructions. We help you identify allowable expenses, avoid mistakes, and make sure your tax return is accurate.


We also send reminders and update you on what you owe, helping you avoid late fees and penalties.


Contact our expert accountants in Birmingham, UK, either by booking a FREE discovery call or calling us on +44 121 227 6277


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