How to fill in a self-assessment tax return: 2025 Guide

Table of Content:

• What is a Self-Assessment Tax Return?
• Why Self-Assessment Tax return is important?
• Who Needs to File a Self-Assessment Tax Return?
• Will HMRC Send Me a Tax Return?
• Self-Assessment Tax Return Deadlines
• How Do Self-Employed Tax Returns Differ?
• Which Tax Return Forms Do You Fill in for Self-Employed Income?
• What If I’m a Sole Trader with a Limited Company?
• Submit Your Tax Return
• Self-Assessment Frequently Asked Questions (FAQs)

What is a Self-Assessment Tax Return? 

A Self-Assessment tax return is an annual declaration to HM Revenue & Customs (HMRC) that allows individuals to report their income, gains, and expenses to calculate how much tax they owe for the year. While many taxpayers have taxes automatically deducted through PAYE (Pay As You Earn) from salaries or pensions, Self-Assessment is required for those with additional or untaxed income streams. 

This system ensures HMRC collects the correct amount of tax on income such as: 

• Self-employment profits (freelancers, contractors, sole traders) 

• Rental income from property investments 

• Capital gains from selling assets like stocks, bonds, or second homes 

• Dividends exceeding your tax-free allowance 

• Foreign income or earnings from overseas investments 

• Side hustles or gig economy work (e.g., freelance writing, ridesharing) 

You may also need to file a Self-Assessment return if you: 

• Earn over £100,000 annually 

• Claim specific tax reliefs (e.g., pension contributions, charitable donations) 

• Are a company director (even if you only receive dividends) 

• Need to repay Child Benefit via the High-Income Child Benefit Charge 

The process involves submitting either a paper form or, more commonly, an online tax return through HMRC’s portal. Once filed, HMRC calculates your final tax liability or refund based on the information provided.

Why Self-Assessment Tax return is important? 

Filing accurately and on time ensures compliance with tax laws, avoids penalties, and helps you claim legitimate deductions. Missing the deadline or underreporting income can trigger fines, interest charges, or even HMRC investigations.

For first-time filers, HMRC provides tools like the Self-Assessment registration checker to confirm whether you need to submit a return.

Who Needs to File a Self-Assessment Tax Return? 

Over 12.1 million individuals were required to submit a Self-Assessment tax return for the previous tax year. While self-employed workers must file annually to pay income tax and National Insurance on their profits, many others also fall under this requirement. 

You must complete a Self-Assessment return if you: 

• Earned £100,000 or more as an employee or pensioner in the last tax year. 

• Received £10,000 or more from savings interest or investments (note: you must also declare savings interest above your personal savings allowance and dividends exceeding the dividend allowance). 

• Had untaxed income of £2,500 or more, such as tips, commission, or freelance earnings. 

• Need to claim tax relief on pension contributions as a higher- or additional-rate taxpayer. 

• Owe capital gains tax from selling assets (e.g., property, shares) at a profit. 

• Claim Child Benefit while you or your partner’s income exceeds the High Income Benefit Charge threshold. 

• Receive taxable foreign income or live abroad but earn UK income. 

• Receive state pension payments exceeding your personal allowance (if it’s your sole income). 

• Are a business partner, limited company director, trustee of a pension scheme/trust, executor of an estate, a Lloyd’s of London “name,” or a minister of religion. 

• Received an HMRC P800 form indicating underpaid tax from the previous year that remains unpaid. 

Some individuals must file a return and pay via PAYE, such as those with private pensions, investment income, taxable capital gains, or a side business alongside employment. Limited company directors must also submit both a personal tax return and a company tax return. 

Will HMRC Send Me a Tax Return? 

HMRC typically issues a tax return if you: 

• Have untaxed income from investments, property, or overseas sources. 

• Made capital gains above the annual exempt amount (£6,000 in 2023-24; £3,000 in 2024-25). 

• Filed a return last year. 

However, do not wait for HMRC to contact you if you know you owe tax. It is your legal responsibility to declare all taxable income annually. If HMRC sends you a tax return, you must submit it – even if you owe nothing. 

How to Register for Self-Assessment ?

If you’re filing a tax return for the first time, you must register with HMRC’s Self-Assessment system. Follow these steps carefully to ensure a smooth process: 

1. Register with HMRC 

• Online Registration: Visit HMRC’s registration portal and select the option that applies to you: 

• Self-employed/sole trader 
• Partnership (for business partnerships) 
• Not self-employed (for landlords, investors, or those with untaxed income) 

• Provide your National Insurance number, contact details, and business information (if applicable). 

2. Receive Your Unique Taxpayer Reference (UTR) 

• After registering, HMRC will mail your 10-digit UTR within 10 working days. This number is critical for all future filings. 

• Lost your UTR? Call HMRC’s Self-Assessment helpline at 0300 200 3310. 

3. Activate Your Government Gateway Account 

• Once you receive your UTR, HMRC will send a Government Gateway activation code by post. 

• Act promptly: The code expires after 28 days. Use it to finalize your online account setup. 

4. Complete Your Account Setup 

• Log in to your Government Gateway account using your new credentials. 

• Ensure all personal/business details are accurate before proceeding. 

5. Submit Your Tax Return 

• Only after completing steps 1-4 will you gain access to HMRC’s online portal to file your return. 

⚠️ Important Notes: 

• Allow up to 20 working days for the entire process. Avoid waiting until the deadline (31 January). 

• If registering close to the 5 October deadline, notify HMRC immediately to avoid penalties. 

• Keep all HMRC letters secure—they contain essential login details and reference numbers. 

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How to Fill in a Self-Assessment Tax Return

Filing your Self-Assessment tax return requires careful attention to detail, whether you choose the online or paper method. Here’s a breakdown of the process and key information you’ll need: 

Online vs. Paper Returns 

• Online Filing: HMRC’s digital portal guides you through relevant sections automatically. You’ll only need to complete parts that apply to your circumstances (e.g., self-employment, rental income). 

• Paper Returns: You must manually select the correct forms. Most filers start with the SA100 main form, but supplementary pages (e.g., SA103 for self-employment, SA105 for property income) may be required. 

Simplified SA200 Form: 

• This simplified, four-page form is sent by HMRC to a limited group of taxpayers, typically employees, pensioners, or self-employed people with a turnover under £85,000. 

• It cannot be requested or downloaded, and if you don’t receive one, you must complete the standard SA100. 

• At just four pages, it’s shorter but not optional – HMRC decides eligibility and sends it directly to you. 

Key Details to Include

1. Income

• Taxed/Untaxed Income: Self-employment profits, savings interest, dividends, and capital gains. 
• State Pension: Total amount received, including any lump sum. 
• Private Pensions: Gross amounts from annuities or lump-sum payments. 
• Benefits: Taxable benefits (e.g., Carer’s Allowance, Jobseeker’s Allowance). 
• Other Income: Non-investment income (e.g., freelance work) and related allowable expenses (e.g., equipment, travel). 

2. Deductions & Reliefs 

• Pension Contributions: Post-tax payments to registered schemes. 
• Charitable Donations: Gift Aid donations (include charity reference numbers if applicable). 
• Blind Person’s Allowance: Tick if claiming this tax-free allowance. 

3. Special Circumstances 

• Student Loans: Repayments deducted via PAYE. Student Loans: Repayments deducted via PAYE. 
• High Income Child Benefit Charge: Required if you or your partner earn over £50,000 (2023-24) or £60,000 (2024-25) and claim Child Benefit. 
• Marriage Allowance: Transfer part of your £12,570 personal allowance to a spouse if your income is below this threshold. 

Documents to Prepare 

Before starting, gather: 

Mandatory: 

• National Insurance number 
• UTR (Unique Taxpayer Reference) 
• P60/P45 forms (employment income) 
• Records of income (invoices, bank statements) 
• Expense receipts (business costs, mileage logs) 
• Pension/charity contribution proofs 

Additional (If Applicable): 

• Tenancy agreements (for landlords) 
• Dividend vouchers 
• Capital gains calculations 

Pro Tip: Double-check figures against your records to avoid errors. If filing on paper, allow extra time for postal delays – HMRC must receive your return by 31 October to avoid penalties. 

By staying organized and methodical, you’ll streamline the process and reduce the risk of mistakes. 

Self-Assessment Tax Return Deadlines 

Understanding Self-Assessment deadlines is critical to avoiding HMRC penalties and staying compliant with HMRC. Here’s what you need to know: 

Tax Year Basics:

Self-Assessment tax calculations are based on the UK tax year, which runs from 6 April to 5 April of the following year. Your tax return and payment for the 2024-25 tax year are due in 2026, as outlined below.

Key Deadlines for 2024/25 Tax Returns

Important Notes 

• Payments on Account: If you make advance payments (common for self-employed filers), these are due by 31 January 2026 and 31 July 2026. These count toward your total tax bill. 

• Interest Charges: Late payments accrue interest at Bank of England base rate + 4% (as of 6 April 2025). 

• Penalty Escalation: 
  • 3 months late: Additional £10 daily fines (up to £900) 
  • 6 months late: 5% of tax owed or £300 (whichever is higher) 
  • 12 months late: Further 5% or £300 charge 

Why These Dates Matter 

• 31 January 2026 is the most critical deadline – missing it triggers both filing and payment penalties. 

• Paper filers have an earlier cutoff (31 October 2025) due to processing times. 

Pro Tip: Even if you can’t pay in full by 31 January, submit your return anyway to avoid the £100 filing penalty. Use HMRC’s Time to Pay scheme if struggling with payments.

Mark these dates in your calendar and set reminders – HMRC does not offer extensions!

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Avoid Late Filing Penalties 

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How Do Self-Employed Tax Returns Differ?

Self-employed tax returns require careful attention to income, expenses, and unique tax rules that differ from standard employment. Here’s what sets them apart: 

1. Tax Period: Accounting Dates vs. Basis Periods 

• Accounting Period: While you can choose any 12-month period for your business accounts, most sole traders align their year-end with 31 March to 5 April to sync with the UK tax year (6 April – 5 April). 

• Basis Period Rules (From April 2025): 
  • All unincorporated businesses (sole traders/partnerships) must now use 6 April to 5 April as their basis period for tax calculations, regardless of their chosen accounting dates. 
  • 2024-25 Transitional Year: Businesses with non-aligned accounting periods had to report profits for a period longer than 12 months to adjust to the new system. 
  • Simplification: Aligning your accounting period with the tax year (6 April – 5 April) streamlines filings and avoids complex overlap calculations.

2. Payment Schedule: Overlap Profits 

Self-employed tax payments follow a unique timeline: 

• Year 1: Taxed on profits from your start date to the end of the tax year. 
  Example: If you started on 5 February 2025, you’d pay tax on profits from 5 Feb – 5 April 2026. 

• Year 2: Taxed on 12 months of profits to your accounting date. 
  Example: Profits from 5 Feb 2025 – 4 Feb 2026 are taxed, creating overlap profits (double taxation of 5 Feb – 5 April 2025). 

• Year 3+: Taxed annually on your accounting period. 
  Overlap Relief: Claim back overlap profits only when you cease trading. 

3. Payments on Account 

After your first year, HMRC requires advance tax payments for the current tax year: 

• Due Dates: 
  • 31 January: 50% of prior year’s tax bill + balancing payment for last year. 
  • 31 July: Remaining 50%. 

• Calculation:
  Based on your previous year’s tax liability. 
  Example: If you owed £5,000 in 2024-25, you’d pay £2,500 on 31 Jan 2026 and £2,500 on 31 Jul 2026. 

Key Notes: 

• Payments on account do not apply in your first year.
• You can reduce payments if your current-year income drops (notify HMRC promptly).

Why This Matters 

• Cash Flow: Payments on account spread costs but require disciplined budgeting. 

• Overlap Complexity: Misaligned accounting periods create temporary double taxation. 

• Compliance: Basis period reforms aim to simplify filings but require careful adjustment. 

Pro Tip: Use accounting software to track profits, expenses, and overlap relief. If closing your business, claim overlap relief to recover overpaid tax. 

By understanding these rules, self-employed taxpayers can avoid surprises and optimize their tax strategy. 

Which Tax Return Forms Do You Fill in for Self-Employed Income? 

When filing your Self-Assessment tax return as a self-employed individual, you must use specific forms to declare your income accurately. Here’s what you need to know: 

1. Core Forms for Self-Employed Filers 

• SA100: The main tax return form everyone must complete. It covers personal details, employment income, and basic tax calculations. 

• SA103: A supplementary form for self-employment income. Choose between: 
  • SA103S (Short Version): For simpler cases with turnover ≤ £85,000 (2024-25 tax year) and no complex adjustments (e.g., accounting date changes). 
  • SA103F (Full Version): Required if your turnover exceeds £85,000, your accounting period doesn’t align with HMRC’s basis period (6 April – 5 April), or you have complex income streams. 

2. Key Rules for Choosing SA103S or SA103F

• First-Year Filers: Most cannot use SA103S if their accounting period (e.g., 1 Jan – 31 Dec) doesn’t match HMRC’s basis period. 

• Basis Period Rules: From April 2024, all unincorporated businesses must use 6 April – 5 April as their basis period. If your accounting dates differ, SA103F is mandatory. 

• Complications: Use SA103F for: 
  • Loss claims 
  • Averaging farming or creative industry profits 
  • Changes in accounting dates 

⚠️ Warning: Selecting the wrong form could invalidate your return. Double-check eligibility using HMRC’s SA103 guidance.

3. How to File 

• Online: The system automatically directs you to the correct sections based on your answers. 

• Paper Returns: Manually attach SA103S or SA103F to your SA100. 

Pro Tips 

• Turnover Calculation: Include all business income before deducting expenses. 

• Record-Keeping: Maintain digital records (invoices, receipts) to simplify form completion. 

• First-Year Filers: Use SA103F if uncertain – it’s safer for complex setups. 

By using the correct forms, you ensure compliance and avoid delays or penalties. 

What If I’m a Sole Trader with a Limited Company or Partnership? 

Navigating tax returns becomes more complex if you operate as a partnership or hold a directorship in a limited company. Here’s how to proceed: 

For Partnerships 

• Form SA104: Mandatory for partnerships. Choose between: 
  • SA104S (Short Version): For straightforward partnerships with turnover ≤ £85,000. 
  • SA104F (Full Version): Required for complex cases, such as partnerships with multiple income streams or international operations. 

• Responsibilities: Each partner must file their own SA100, along with the partnership’s SA104. 

For Limited Company Directors 

• Form SA102: Directors must complete this employment supplement to declare: 
  • Salary, bonuses, and benefits (e.g., company car, health insurance). 
  • Dividends received (reported separately under “Dividend Income”). 

• Note: Even if you’re a director and sole shareholder, you must file both a personal tax return (SA100) and a corporate tax return for the company. 

How to Keep Your Records 

Maintaining accurate records is non-negotiable for tax compliance. Here’s what you need to know: 

What Records to Keep 

• Income Proof: Invoices, sales ledgers, bank statements. 

• Expense Evidence: Receipts, mileage logs, utility bills (for home office claims). 

• Employment Details: P60, P11D (for directors). 

• Partnership/Limited Company: Profit-sharing agreements, shareholder registers, company accounts. 

How Long to Keep Them 

• Minimum: 5 years after the 31 January submission deadline. 

• Exceptions: Keep records for 6 years if claiming losses or submitting late.

Digital vs. Physical Storage 

• Digital: Use accounting software (e.g., QuickBooks, Xero) for automatic backups and HMRC compliance under Making Tax Digital (MTD). 

• Physical: Organize folders by tax year and category (income/expenses). 

Why It Matters 

HMRC can audit records up to 20 years retrospectively if they suspect fraud. Proper documentation: 

• Avoids penalties (up to £3,000 per year for poor record-keeping). 

• Simplifies responses to HMRC enquiries. 

• Supports accurate tax calculations. 

Pro Tips 

1. Digitize Receipts: Use apps like Dext or Expensify to scan and categorize receipts. 

2. Separate Accounts: Maintain distinct bank accounts for business/personal finances. 

3. Annual Review: Reconcile records yearly to catch errors early. 

For detailed guidance, refer to HMRC’s record-keeping rules. 

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Submit Your Tax Return 

Filing your 2024-25 tax return is streamlined with The Infinity Groups, which simplifies the process by: 

• Calculating your tax liability  

• Highlighting tax-saving opportunities (e.g., allowable expenses, reliefs) 

• Submitting your return directly to HMRC 

This service is ideal for self-employed filers, landlords, and those with multiple income streams. 

Tax Returns If Your Circumstances Change 

Life changes can impact your tax obligations. Here’s what to do: 

1. New Untaxed Income? 
   • Notify HMRC by 5 October 2025 (after the 2024-25 tax year ends).
   • Examples: Starting a side hustle, rental property, or freelance work.
   • HMRC will confirm if you need to file a return.
     
2. No Longer Need to File? 

• Contact HMRC to close your Self-Assessment account if you: 
  • Stop being self-employed 
  • No longer earn untaxed income
  • Fall below the filing threshold  

• Call 0300 200 3310 or update your status via your HMRC online account. 

Self-Assessment Frequently Asked Questions (FAQs)

Navigating the UK tax system can be challenging. To simplify the process, we’ve compiled answers to the most frequently asked questions about filing your self-assessment tax return. 

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Struggling with your tax return? Avoid mistakes and penalties by letting our experts handle it for you. Book a free consultation today! 

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