Navigating the Self Assessment tax return process can feel overwhelming at first, especially if you’ve recently become self-employed, started earning rental income, or have other income streams not taxed at source. The Self Assessment system is how HMRC collects Income Tax from individuals who are not taxed automatically through PAYE. Understanding what’s required, including which forms and supplementary pages to complete, is crucial to staying compliant and avoiding penalties.
Who Needs to Complete a Self Assessment Tax Return?
Self Assessment is not required for everyone. Typically, you’ll need to submit a tax return if you fall into one or more of the following categories:
- You’re self-employed as a sole trader and earned more than £1,000 (before expenses)
- You’re a partner in a business partnership
- You earned income from renting out property
- You received income from savings, investments, or dividends
- You need to claim certain tax reliefs or allowances
- You earned foreign income
- You made capital gains that need to be reported
- You received Child Benefit and your income is over £60,000
Even if HMRC hasn’t sent you a notice to complete a return, you’re still legally obliged to do so if any of the above applies to your situation.
Registering for Self Assessment
Before submitting your first Self Assessment tax return, you must register with HMRC. This process differs slightly depending on your circumstances. If you’re self-employed, you must register for both Self Assessment and Class 2 National Insurance. If you’re not self-employed but need to report income or capital gains, you register using a different online form. Once registered, you’ll receive a Unique Taxpayer Reference (UTR), which you’ll use to file returns and manage your tax account.
The SA100: The Core Tax Return Form
The SA100 is the main form used in Self Assessment. It is eight pages long and covers a wide range of income and tax reliefs. Most individuals will need to complete this form each year if they fall into the categories requiring Self Assessment.
The SA100 includes sections for:
- Employment income
- Pension income
- Interest from savings and dividends
- Benefits received (such as Child Benefit if income exceeds the threshold)
- Student loan repayments
- Gift Aid donations
- Pension contributions
- Marriage Allowance claims
- Blind Person’s Allowance claims
Filing this form accurately is vital, as errors can lead to delays in processing and potential penalties.
SA200: The Short Tax Return
The SA200 is a simplified version of the SA100. It is only sent out by HMRC to taxpayers who meet specific criteria. This form is shorter and more straightforward but cannot be used unless HMRC instructs you to do so. If you receive an SA200, it typically means your tax affairs are relatively simple and do not involve multiple sources of income.
What Are Supplementary Pages?
The SA100 is supported by supplementary pages designed to report additional or more complex income types. These pages ensure that HMRC has a comprehensive view of all your income and can accurately assess the tax owed. Each type of income that doesn’t fall under the general categories in the SA100 has its own corresponding supplementary page.
Failing to complete the necessary supplementary pages is a common mistake and can result in underreporting income, leading to fines and interest charges. Supplementary pages must be filed at the same time as your SA100.
Commonly Used Supplementary Pages
SA102: Employment
This page is for those who are employed or serve as company directors. It is used to report income from jobs, including salary, benefits, bonuses, and expenses reimbursed by your employer. A separate SA102 must be completed for each employment or directorship.
Details required include:
- Employer’s name and PAYE reference
- Gross pay and tax deducted
- Benefits and expenses received
- Contributions to employer pension schemes
SA103S and SA103F: Self-Employment
Sole traders must complete either the SA103S (short) or SA103F (full) form depending on the annual turnover of their business.
- SA103S is for businesses with turnover below the VAT registration threshold, which is £90,000 for the 2024/25 tax year
- SA103F is for businesses with turnover above the threshold or with more complex accounts
Information reported includes:
- Business name and description
- Income and allowable business expenses
- Capital allowances
- Adjustments to profits
- Class 4 National Insurance contributions
Each business must have a separate SA103 form.
SA104S and SA104F: Partnerships
If you are in a business partnership, you need to complete either the SA104S (short) or SA104F (full) form.
- Use SA104S if you are only reporting your share of partnership trading income and simple interest income
- Use SA104F if the partnership income is more complex
You’ll need to provide:
- Partnership name and tax reference
- Your share of income and tax credits
- Any deductions or reliefs applicable
Each partnership interest must have a separate SA104 form.
SA105: Property Income
This form is used to report rental income from properties in the UK, furnished holiday lettings in the UK or European Economic Area (EEA), and premiums from leases. If you rent out property, this page captures all income and associated allowable expenses.
Information to report includes:
- Total rental income
- Expenses such as mortgage interest, maintenance, letting agent fees
- Profit or loss on each property
- Capital allowances if furnished holiday lettings apply
This form is crucial for landlords to accurately report their property earnings.
SA106: Foreign Income
You’ll need to complete this if you have:
- Earnings from overseas employment or self-employment
- Interest or dividends from foreign investments
- Rental income from overseas properties
- Foreign pensions
You can also use this page to claim Foreign Tax Credit Relief, which helps avoid being taxed twice on the same income.
SA108: Capital Gains
This page is required if you’ve sold or disposed of:
- Property that isn’t your main home
- Shares or other investments
- Business assets
If your gains exceed the annual tax-free Capital Gains Tax allowance (£3,000 in 2024/25), you must report the details using SA108. Information needed includes the proceeds from the sale, allowable costs, and any reliefs or exemptions.
SA109: Residence, Remittance, and Domicile
This form is necessary if:
- You are a non-UK resident with UK income
- You’re claiming the remittance basis for overseas income
- You want to claim personal allowances as a non-resident
This page ensures your residency and domicile status are properly recorded so that HMRC can apply the correct tax rules.
When and How to File Your Self Assessment Tax Return
You can file your return either online or by paper. Paper returns must be submitted by October 31 following the end of the tax year, while online returns are due by January 31. Filing online is generally faster, more secure, and comes with helpful error-checking tools.
You’ll also need to pay any tax owed by January 31. If your tax bill is more than £1,000, you might be required to make payments on account for the following year, with payments due in January and July.
Late filing or payment triggers automatic penalties:
- £100 for missing the deadline (even if no tax is due)
- Additional penalties after three, six, and twelve months
- Interest on unpaid tax
Understanding the timeline and sticking to it is essential for staying compliant.
Understanding the SA302 Tax Calculation
After filing your tax return, you can access an SA302, which is a summary of your income and the tax HMRC has calculated you owe. This document is often required when applying for mortgages or loans as proof of income.
SA302s are available about 72 hours after your return is submitted. They can be downloaded through your online HMRC account or through third-party tax software. They include details such as:
- Total income
- Taxable income
- Total tax due
- National Insurance contributions
Having a clear record of your SA302s can be helpful for both financial planning and official verification purposes.
What If HMRC Doesn’t Send You a Return?
It’s a common misconception that HMRC will always let you know when you need to file a return. In reality, the responsibility lies with you. If you meet the criteria requiring a Self Assessment return, you must register and file—even if you haven’t been contacted by HMRC.
Failure to do so can result in significant penalties. Always review your income sources annually and assess whether you need to file. Even if no tax is ultimately due, you must complete and submit a return if your circumstances require it.
Organising Your Records and Documents
Once you understand the Self Assessment process and know which forms you need, the next step is gathering the right records. This is essential for completing your tax return accurately and claiming the tax reliefs and allowances available to you.
Organised records not only make tax season easier but also ensure you stay compliant with HMRC’s requirements. Poor record-keeping can lead to underpayment or overpayment of tax, and in worst cases, penalties for incorrect submissions.
What Records You Need to Keep
The exact records you need will depend on your income sources. However, everyone filing a Self Assessment return should keep the following as applicable:
- Bank statements for business and personal accounts
- Invoices issued to clients or customers
- Receipts for business-related purchases
- Rental agreements and property management statements
- Dividend vouchers and investment statements
- Payslips, P45s, and P60s from employment
- Pension income and contribution statements
- Records of any benefits or allowances claimed
You must keep records for at least five years after the 31 January submission deadline of the relevant tax year. So for the 2024/25 tax return due by 31 January 2026, records must be kept until at least January 2031.
Digital vs Paper Records
HMRC accepts both digital and paper records. However, digital records are easier to manage, search, and back up. Scanned copies of receipts, cloud-based storage for financial statements, and spreadsheets for income tracking can save time and reduce errors.
Under Making Tax Digital, many taxpayers will eventually be required to keep digital records and submit updates to HMRC through approved software. Preparing early by digitising your record-keeping can ease this transition.
Tracking Self-Employment Income and Expenses
If you are self-employed, it’s essential to maintain a clear record of income and allowable business expenses. This enables you to calculate your taxable profit accurately and claim deductions where eligible.
Examples of allowable business expenses include:
- Office supplies and equipment
- Travel costs related to your business
- Business premises costs, such as rent and utilities
- Advertising and marketing expenses
- Professional fees like accountants or legal advice
- Software subscriptions for business use
- Mobile phone or broadband costs (if used for business)
You must be able to prove each expense with a receipt or other documentation in case HMRC requests evidence.
Claiming Simplified Expenses
For some expenses, HMRC allows you to use simplified or flat-rate expenses. These are especially helpful for self-employed individuals who work from home or use their personal vehicle for business purposes.
Working from Home
If you work from home, you can claim a flat rate depending on the number of hours you work there per month. You do not need to calculate actual expenses for utilities, council tax, or mortgage interest if using this method.
Business Vehicle Use
You can claim flat rates per mile for using your personal vehicle for business trips:
- 45p per mile for the first 10,000 miles
- 25p per mile thereafter
This method simplifies the process and removes the need to track exact fuel and maintenance costs.
Income from Property: What to Track
If you earn income from UK property, keeping detailed records is equally important. You must track:
- Rental income received, including deposits retained
- Mortgage interest (not capital repayments)
- Maintenance and repair costs
- Letting agent fees
- Insurance premiums
- Council tax and utility bills (if paid by you)
- Service charges and ground rents
Ensure you distinguish between capital and revenue expenditure. Capital expenses (like improvements to a property) are not immediately deductible but may be claimed later for capital gains tax purposes.
Joint Property Ownership
If you own a rental property with another person, each owner must report their share of the income and expenses. By default, HMRC assumes equal ownership, but this can be adjusted with a formal declaration if the actual ownership split differs.
Understanding Allowances and Reliefs
Maximising allowances and reliefs can reduce your tax bill. Some are automatically applied, while others must be claimed through your return.
Personal Allowance
Most taxpayers can earn a certain amount each year before paying Income Tax. For 2024/25, the standard Personal Allowance is £12,570.
Trading Allowance
If you have income from self-employment under £1,000, you do not need to register for Self Assessment or report this income. If you earn more, you can still deduct the first £1,000 as a trading allowance if you do not claim actual expenses.
Property Allowance
A similar £1,000 allowance applies to property income. You can claim this instead of actual expenses if it results in a higher deduction.
Marriage Allowance
This allows a non-taxpaying spouse to transfer a portion of their unused Personal Allowance to their spouse, reducing the latter’s tax bill.
Blind Person’s Allowance
Taxpayers registered blind or severely sight-impaired can claim an additional allowance.
Pension Contributions
Contributions to private pensions can qualify for tax relief. You must include them in your return to receive higher-rate relief if you’re a higher or additional rate taxpayer.
Gift Aid
Charitable donations made under Gift Aid increase the value of donations and offer tax benefits. Higher and additional rate taxpayers can claim the difference between their rate and the basic rate through Self Assessment.
Calculating Taxable Income
Once you have total income and allowable deductions, you can calculate your taxable income. HMRC uses this figure to determine how much tax you owe.
The main tax bands for 2024/25 are:
- Basic rate: 20% on income from £12,571 to £50,270
- Higher rate: 40% on income from £50,271 to £125,140
- Additional rate: 45% on income over £125,140
Other taxes may apply, such as:
- Class 2 and Class 4 National Insurance for the self-employed
- Capital Gains Tax for asset sales
- Dividend tax on income from shares
- Savings tax if interest income exceeds allowances
Using Estimated Figures
In some cases, you may not have exact figures by the filing deadline. HMRC allows estimated or provisional figures, as long as you mark them clearly and explain why they were used. You must update these as soon as final figures are available.
Avoiding Common Mistakes
Errors on your tax return can lead to fines or overpaying tax. Avoid these common mistakes:
- Omitting small income sources like bank interest or freelance income
- Entering gross figures where net is required (or vice versa)
- Failing to include all relevant supplementary pages
- Claiming non-allowable expenses
- Forgetting to declare foreign income
- Missing the deadline and incurring penalties
Take time to double-check all figures and ensure every income source and deduction is accounted for.
Filing Deadlines and Penalties
Key dates to remember:
- 5 October: Deadline to register for Self Assessment
- 31 October: Deadline for paper returns
- 31 January: Deadline for online returns and tax payment
- 31 July: Deadline for second payment on account (if applicable)
Penalties for missing deadlines:
- £100 automatic fine for filing late
- Additional daily penalties after three months
- 5% of tax due after six months
- Interest on unpaid tax
Make sure to submit early to avoid stress, identify any issues, and give yourself time to pay any tax owed.
Payments on Account
If your tax bill is more than £1,000, you may be required to make advance payments for the next tax year. These are called payments on account and are due in two instalments:
- First payment by 31 January
- Second payment by 31 July
Each payment is half your previous year’s tax bill. If you expect your income to decrease, you can apply to reduce payments on account, but underpaying without reason may lead to interest charges.
Keeping Up with Changing Rules
Tax rules change regularly. For example, the VAT registration threshold, income tax bands, and allowable deductions can shift each year. Keep up to date by regularly checking HMRC updates and guidance, especially before filing your return.
Certain one-off reliefs and pandemic-related measures may also affect your return, so it’s important to read the notes for each form and verify current figures.
When to Seek Professional Help
While many individuals can manage their own tax returns, certain situations can benefit from professional support. Consider seeking help if:
- You have foreign income or are non-resident
- You have complex investments or multiple income sources
- You are selling property or business assets
- You need help reducing your tax liability legally
A professional can also review your return for errors or offer advice on how to manage your finances more tax-efficiently in future years.
Getting Ready to File Your Return
After gathering your records and understanding the forms, the final step in the Self Assessment journey is completing your return. This phase requires attention to detail and a structured approach to ensure your return is accurate, complete, and submitted on time.
Before you begin filling out your tax return, make sure you have your Government Gateway user ID and password, your Unique Taxpayer Reference (UTR), and all the necessary documentation to support your entries.
Choosing the Right Submission Method
There are two main ways to file your Self Assessment return:
- Online through HMRC’s digital services
- By post using paper forms
Most people choose the online method because it offers convenience, automatic calculations, and later deadlines. However, if you are more comfortable with paper or have very specific needs, postal submission is still available until 31 October following the end of the tax year.
Logging into Your HMRC Account
To file online, sign into your personal tax account using your Government Gateway credentials. If you don’t have an account, you’ll need to create one and verify your identity. Once signed in, navigate to the Self Assessment section and choose to start a new return.
Make sure you are filing for the correct tax year. For example, if you’re reporting income earned between 6 April 2024 and 5 April 2025, you’re completing the 2024/25 return.
Starting the SA100 Main Return
Begin with the SA100 form, the core of your tax return. You’ll be asked to confirm your personal details, such as:
- Name and address
- Date of birth
- Marital status
- National Insurance number
Next, the return will ask whether you’ve received income from specific sources. Your answers determine which supplementary pages you’ll need to fill in.
Completing the Supplementary Pages
Supplementary pages are added depending on the income you report. You only need to complete the pages relevant to your circumstances.
Employment (SA102)
If you were employed during the tax year, fill in the SA102 page. Enter details from your P60 and P45, including:
- Total pay received
- Income tax deducted
- Benefits in kind from a P11D
- Student loan deductions (if not already handled by your employer)
Include each job separately using a new SA102 for each employment.
Self-Employment (SA103S or SA103F)
If you’re self-employed, use the short (SA103S) or full (SA103F) form, depending on your turnover and business complexity. You’ll report:
- Business income
- Allowable expenses
- Capital allowances (if applicable)
- Losses carried forward or used against other income
Property Income (SA105)
For landlords or those receiving income from UK property, SA105 allows you to report:
- Rental income
- Allowable expenses
- Mortgage interest (not repayments)
- Periods of vacancy
- Profit or loss from each property
Foreign Income (SA106)
Report any overseas income, such as foreign employment, pensions, or rental income. You may also use this form to claim relief for foreign tax already paid on that income.
Capital Gains (SA108)
Use this page to report gains from selling shares, property, or other investments. Include:
- Description of the asset
- Date acquired and sold
- Proceeds and acquisition cost
- Associated costs such as legal fees
- Applicable reliefs (like Private Residence Relief)
Residence and Domicile (SA109)
Complete this if you:
- Lived outside the UK during the tax year
- Claim the remittance basis
- Are non-domiciled or need to claim personal allowances as a non-resident
These entries can be complex, so read the form guidance carefully.
Claiming Reliefs and Allowances
Within the SA100, you can also claim tax reliefs and adjustments, including:
- Blind Person’s Allowance
- Marriage Allowance transfer
- Charitable donations under Gift Aid
- Relief for pension contributions
- Adjustments for Child Benefit if your income exceeds £60,000
Declare any tax overpaid, and indicate whether you want it refunded or offset against future liabilities.
Reviewing the Tax Calculation
Once all pages are completed, the online system automatically calculates your tax bill based on the information you’ve provided. Review this calculation carefully, paying attention to:
- Taxable income total
- Allowances claimed
- Income tax due at each band
- National Insurance contributions
- Capital gains tax (if applicable)
- Payments on account due
If something looks incorrect, go back and review your figures. A small entry error can lead to a significant change in tax owed.
Submitting Your Return
When you’re confident the return is accurate, you can submit it online. HMRC will issue an on-screen confirmation and email receipt. This confirms that your return was successfully filed.
Be sure to print or save a copy of the return and the confirmation for your records. Keep all supporting documents and receipts in case HMRC later asks for verification.
What Happens After You File
After submission, HMRC may:
- Issue a tax bill (if unpaid tax is owed)
- Confirm your refund (if overpaid)
- Adjust your tax code for employment income
- Request more information if something doesn’t add up
If your tax bill is due, you must pay it by 31 January following the end of the tax year. Late payments incur interest and penalties.
Making Payments to HMRC
You can pay your Self Assessment bill using various methods:
- Online bank transfer
- Direct Debit
- Debit or corporate credit card
- Through your personal tax account
- At your bank or building society (if you have a paying-in slip)
Always allow a few days for payments to clear. Missed deadlines lead to automatic penalties and daily interest.
Amending Your Return
If you realise you made a mistake or forgot to include something, you can amend your return online within 12 months of the original deadline. For the 2024/25 tax year, you can make corrections up to 31 January 2027.
To amend a return:
- Log in to your account
- Select the relevant tax year
- Choose “Amend Self Assessment return”
- Update the information and resubmit
HMRC will recalculate your tax and issue a revised bill or refund where applicable.
HMRC Compliance Checks
Sometimes, HMRC carries out compliance checks to verify the information on your return. This doesn’t mean you’ve done something wrong. They may ask for:
- Proof of income (invoices, receipts)
- Bank statements
- Contracts or tenancy agreements
Always respond promptly and accurately. Keeping thorough records helps protect you in case of an audit.
Dealing with Refunds
If you’ve overpaid tax, HMRC will process your refund after checking your return. This typically takes 5 to 10 working days. You can choose to:
- Have the refund paid directly into your bank account
- Offset it against your next tax bill
Monitor your personal tax account to check the status of any refund due.
Managing Future Obligations
Once you’ve completed your first Self Assessment return, it’s wise to put systems in place to make future filings easier. Consider:
- Keeping monthly records rather than doing it all at year-end
- Setting calendar reminders for key deadlines
- Reviewing your tax code and allowances annually
- Saving for your next tax bill regularly
If your circumstances change—such as stopping self-employment or ceasing rental activities—inform HMRC so they can update your record and remove the need for future returns.
Registering for VAT or PAYE
As your business grows, you may exceed thresholds that trigger other obligations. For example:
- VAT registration is mandatory if your turnover exceeds £90,000 in 2024/25
- You must register for PAYE if you take on employees
Both of these bring additional reporting requirements. Plan ahead to ensure you stay compliant as your situation evolves.
Avoiding Penalties and Staying Compliant
The best way to avoid problems with your tax return is to:
- File early, not just on time
- Keep comprehensive and accurate records
- Double-check every entry before submission
- Respond promptly to any communication from HMRC
Penalties can be avoided through diligence and organisation. If you’re ever unsure about your obligations, HMRC provides guidance online, or you can consult a tax professional for support.
Planning Ahead for Tax Efficiency
Beyond compliance, Self Assessment gives you an opportunity to think about how to organise your finances more tax-efficiently. Consider:
- Contributing more to pensions
- Using tax-free savings options like ISAs
- Splitting income with a spouse where legally allowed
- Making charitable donations
Good tax planning is legal and encouraged. Taking advantage of available reliefs can reduce your overall tax burden while staying within the rules.
Conclusion
Filing a Self Assessment tax return may seem overwhelming at first, especially if you’re newly self-employed, receiving rental income, or have multiple sources of taxable income. But with the right understanding, preparation, and tools, the process becomes more manageable and even empowering.
Through this guide, you’ve explored the essentials from understanding the SA100 form and supplementary pages to knowing what records to keep, how to accurately complete your return, and what to expect after submission. You’ve learned about the importance of reporting all relevant income, claiming allowances and reliefs where applicable, and meeting deadlines to avoid penalties and interest charges.
Keeping detailed records, staying organised throughout the year, and planning ahead for future tax obligations can transform Self Assessment from a stressful deadline into a structured financial routine. Whether you’re a sole trader, landlord, investor, or someone earning foreign income, understanding your tax responsibilities gives you more control over your finances and helps ensure you stay compliant with HMRC.
Most importantly, remember that Self Assessment is not just about paying tax, it’s also an opportunity to optimize your tax position, claim what you’re entitled to, and build better habits around your personal and business finances. With consistent attention and a willingness to learn, each year will become easier and more straightforward than the last.