Every January, millions of UK taxpayers face the same deadline: 31 January for filing Self Assessment tax returns online. For many, this process is shrouded in confusion, stress, and the fear of getting something wrong. The good news? Filing your Self Assessment doesn’t have to be overwhelming.
This comprehensive guide from Flux Accounting team will walk you through everything you need to know about Self Assessment tax returns—from understanding whether you need to file one, to gathering the right information, navigating the online system, calculating what you owe, and ensuring you claim every allowance and relief you’re entitled to.
Whether you’re filing for the first time or looking to improve your understanding of the process, this guide will demystify Self Assessment and help you approach it with confidence.
What is Self Assessment and Who Needs to File?
Self Assessment is HMRC’s system for collecting Income Tax directly from individuals who have income that isn’t taxed at source through PAYE (Pay As You Earn). Rather than HMRC calculating your tax bill, you “self-assess” your income and calculate what you owe.
Who Must File a Self Assessment Tax Return?
You must complete a Self Assessment tax return if, in the last tax year (6 April to 5 April), you were:
Self-Employed or in a Partnership:
- A sole trader with income over £1,000
- A partner in a business partnership
Company Directors:
- Unless it’s a non-profit organisation and you didn’t receive payment
High Earners:
- Your income was over £100,000
- You or your partner received Child Benefit and either of you earned over £50,000 (High Income Child Benefit Charge)
Additional Income:
- Rental income over £1,000 (before expenses)
- Tips and commission not taxed through PAYE over £2,500
- Income from savings, investments, or dividends over your allowances
- Foreign income that needs to be declared
Capital Gains:
- You sold assets and your total gains exceeded the annual exempt amount (£3,000 for 2024/25)
Other Situations:
- You’re a trustee of a trust or registered pension scheme
- You received income from abroad
- You want to claim certain tax reliefs
- Your State Pension income was over your Personal Allowance
- HMRC has asked you to file a return
Even if you don’t strictly need to file, you might choose to if you want to claim back tax you’ve overpaid or claim certain tax reliefs.
Who Doesn’t Need to File?
If you’re employed and all your tax is collected through PAYE, with no additional income sources, you typically don’t need to file a Self Assessment. HMRC and your employer handle everything automatically.
Not sure if you need to file? We offer free consultations where we review your circumstances and provide clear guidance. Don’t risk penalties by missing a required return—let us confirm your obligations.
Important Deadlines You Must Know
Missing Self Assessment deadlines triggers automatic penalties, so understanding the timeline is crucial.
Key Dates for the 2024/25 Tax Year
| Tax year ends | 5 April 2025 |
| New tax year begins (2025/26) | 6 April 2025 |
| Deadline to register for Self Assessment if you’re filing for the first time and weren’t self-employed | 5 October 2025 |
| Deadline for paper tax returns (we recommend online filing instead) | 31 October 2025 |
| Deadline to file online if you want HMRC to collect tax owed through your PAYE tax code (only if you owe less than £3,000 and already pay tax through PAYE) | 30 December 2025 |
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31 January 2026 |
| Deadline for second payment on account (if applicable) | 31 July 2026 |
Penalties for Late Filing
HMRC issues automatic penalties for late returns:
- 1 day late: £100 fixed penalty (even if you owe no tax)
- 3 months late: £10 per day for up to 90 days (£900 maximum)
- 6 months late: £300 or 5% of tax owed (whichever is higher)
- 12 months late: Additional £300 or 5% of tax owed (whichever is higher)
Plus, you’ll pay interest on any tax paid late (currently 7.75% per year) and additional penalties of 5% of the tax owed at 30 days, 6 months, and 12 months after the deadline.
Missing the deadline is expensive—a return that’s 6 months late with £5,000 tax owed could cost you over £1,000 in penalties alone.
Registering for Self Assessment
If you’ve never filed a Self Assessment before, you must register with HMRC before you can submit your return.
How to Register
Online Registration: Visit the official site here and complete the online form. You’ll need:
- Your National Insurance number
- Your address and contact details
- Details about your income (what you do, when you started)
By Phone: Call HMRC’s Self Assessment helpline on 0300 200 3310
Timeline for Registration
HMRC will post you:
- Unique Taxpayer Reference (UTR) – arrives within 10 days
- Activation code for HMRC’s online services – arrives within 7 days
You need both before you can file online, so register well before the deadline. If you’re newly self-employed, register by 5 October following the tax year you started.
Setting Up Your Government Gateway Account
Once you have your UTR and activation code, create or access your Government Gateway account at gov.uk. This is your portal for all HMRC online services, including filing your return.
Gathering the Information You Need
Before starting your return, collect all relevant documents and information. Being organised makes the process much faster and ensures accuracy.
Essential Documents for Everyone
- National Insurance number
- Unique Taxpayer Reference (UTR)
- Previous year’s tax return (if you filed one)
- P60 from employer (if employed)
- P45 from employer (if you left a job during the year)
- Bank statements showing interest received
- Investment statements showing dividends
- Pension contribution details
For Self-Employed Individuals
- Complete business records for the tax year
- Income records: Invoices, sales records, bank statements showing business income
- Expense records: Receipts, invoices, bank statements for business expenses
- Mileage log if claiming vehicle expenses
- Home office calculations if working from home
- Asset purchases for capital allowances
- Stock/inventory valuations at year-end
For Landlords
- Rental income records from all properties
- Mortgage interest statements
- Property expense receipts: repairs, maintenance, insurance, letting fees
- Tenant details and rental agreements
For Other Income Sources
- Dividend vouchers or statements
- Interest statements from banks and building societies
- Foreign income documentation
- Capital gains records: Purchase and sale documentation for assets sold
- Pension income statements
Step-by-Step Guide to Filing Self Assessment Return Online
Ready to complete your return? Here’s the process broken down into manageable steps.
Step 1: Log into Your Government Gateway Account
Go to https://www.gov.uk/log-in-file-self-assessment-tax-return and sign in with your Government Gateway credentials. You’ll need your User ID, password, and UTR.
Step 2: Select the Correct Tax Year
Ensure you’re completing the return for the correct tax year. For the January 2026 deadline, you’re filing for 2024/25 (6 April 2024 to 5 April 2025).
Step 3: Complete Your Personal Details
Verify your personal information is correct:
- Name and address
- Date of birth
- National Insurance number
- Contact details
Update anything that’s changed.
Step 4: Employment Income Section
If you were employed during the tax year:
Enter details from your P60/P45:
- Total pay received
- Tax already paid
- Employer details and PAYE reference
Include employment benefits:
- Company car
- Medical insurance
- Any other benefits (check your P11D form)
The online system calculates whether you owe more tax or are due a refund based on this information.
Step 5: Self-Employment Section
For sole traders and partners, this is the most detailed section:
Business Details:
- Business name and description
- Start date
- Accounting period dates
- Business address
Income: Enter your total business turnover (sales/income before expenses)
Allowable Expenses: List all legitimate business expenses in the appropriate categories:
- Cost of goods sold/stock
- Staff costs
- Premises costs (rent, rates, utilities)
- Motor expenses
- Travel and subsistence
- Advertising and marketing
- Professional fees (accountancy, legal)
- Telephone and internet
- Office supplies
- Insurance
- Bank charges and interest
- Depreciation (capital allowances)
- Other business expenses
Capital Allowances: Claim tax relief on business assets like equipment, machinery, and vehicles
Net Profit/Loss: The system calculates your profit (income minus expenses), which determines your tax bill
Simplified Expenses: Small businesses can use flat rates for certain expenses instead of calculating actual costs:
- Motor vehicle expenses based on business mileage
- Working from home based on hours worked
- Living in business premises
Consider whether simplified expenses or actual expenses result in a better outcome—it varies by business.
Step 6: UK Property Income
If you receive rental income:
Property Details:
- Number of properties
- Type (residential, furnished holiday lettings, etc.)
Income: Total rent received during the tax year
Allowable Expenses:
- Mortgage interest (restricted to 20% tax relief)
- Repairs and maintenance
- Insurance
- Letting agent fees
- Legal and professional costs
- Service charges and ground rent
- Council tax and utilities you pay
- Other property expenses
Property Allowance: If your rental income is under £1,000, you can claim the property allowance instead of actual expenses, making your rental income tax-free.
Step 7: UK Interest and Dividends
Bank Interest: Enter interest from all UK bank accounts and building societies. Most people have a Personal Savings Allowance (£1,000 for basic rate taxpayers, £500 for higher rate) making this tax-free.
Dividend Income: Enter dividends from UK companies and funds. You have a Dividend Allowance (£500 for 2024/25) before tax applies.
The system tells you if you owe tax on these income sources based on your total income.
Step 8: Capital Gains
If you sold assets during the year:
Assets Requiring Declaration:
- Property (other than your main home)
- Shares and investments
- Business assets
- High-value possessions (over £6,000)
For Each Asset:
- Purchase price and date
- Sale price and date
- Costs of purchase and sale (legal fees, agent fees)
- Improvement costs
Capital Gains Tax Calculation:
- Total gains from all assets
- Minus Annual Exempt Amount (£3,000 for 2024/25)
- Tax on remaining gains at 10% or 20% (or 18%/24% for residential property)
Step 9: Pension Contributions
Claim tax relief on pension contributions:
- Personal pension contributions (that haven’t received tax relief at source)
- Employer contributions to personal pensions
This can significantly reduce your tax bill and is often overlooked.
Step 10: Gift Aid Donations
Claim higher rate tax relief on Gift Aid donations to charities. Basic rate relief goes to the charity, but higher and additional rate taxpayers can claim the difference.
Step 11: Other Information
Include any other relevant information:
- Student loan repayments
- High Income Child Benefit Charge
- Marriage Allowance claims
- Blind Person’s Allowance
- Foreign income
- Other tax reliefs
Step 12: Review and Submit
Check Everything Carefully:
- All figures are accurate
- You’ve claimed all allowable expenses and reliefs
- Your calculations look reasonable
- Personal details are correct
View Your Calculation: Before submitting, review HMRC’s calculation showing:
- Total income
- Total tax due
- Tax already paid
- Balance to pay or refund due
Submit Your Return: Once you’re satisfied everything is correct, submit your return. You’ll receive a submission receipt with a reference number—save this.
Our clients never worry about getting Self Assessment wrong because we handle everything. We review all your records, identify every legitimate deduction, complete your return accurately, and submit it on time. You get maximum refunds or minimum liability, with zero stress.
Common Self Assessment Mistakes to Avoid
Even experienced taxpayers make errors. Here are the most common mistakes and how to avoid them.
1. Missing the Deadline
Set multiple reminders starting in December. Don’t wait until the last minute—technical issues or missing information can derail late submissions.
2. Not Keeping Adequate Records
HMRC can investigate your returns up to 6 years back. Without proper records, you can’t defend your claims and may face penalties for unsubstantiated expenses.
3. Mixing Personal and Business Expenses
Only genuine business expenses are allowable. Personal spending run through your business account doesn’t become deductible just because it’s in the business account.
4. Claiming Non-Allowable Expenses
Common mistakes:
- Personal clothing (unless it’s protective wear or costume)
- Commuting from home to regular workplace
- Entertainment for yourself
- Gym memberships (usually)
- Fines and penalties
5. Incorrect Classification
Ensure income and expenses are in the correct categories. Misclassification can trigger HMRC reviews.
6. Not Declaring All Income
HMRC receives information from many sources—banks, employers, investment platforms. If you don’t declare income they know about, you’ll face penalties.
7. Arithmetic Errors
Double-check all calculations. The online system calculates automatically, but errors in the figures you enter cause problems.
8. Forgetting About Payment on Account
Don’t be caught off-guard by the January bill that’s often much higher than you expected due to payments on account.
9. Not Claiming Available Reliefs
Leaving money on the table by not claiming allowances and reliefs you’re entitled to.
10. Ignoring HMRC Correspondence
If HMRC writes to you with questions or requests, respond promptly. Ignoring them makes situations worse and can trigger investigations.
What Happens After You File?
Immediate Confirmation
Upon submission, you’ll receive a submission receipt with a unique reference number. Save this—it’s proof you filed on time if any issues arise.
Checking Your Calculation
Log back into your account to view your:
- Tax calculation
- Amount owed or refund due
- Payment on account amounts
- Payment deadlines
Paying Your Tax Bill
Online/Phone Banking: Use HMRC’s bank details with your 11-character payment reference (shown in your account)
Debit/Credit Card: Pay directly through your Government Gateway account (note: credit cards incur a fee)
Direct Debit: Set up a Direct Debit at least 5 working days before the deadline
CHAPS: For same-day payment (fees apply)
Refunds
If you’re owed a refund, HMRC typically processes it within 5 weeks. Ensure your bank details are correct in your account.
HMRC Enquiries
HMRC can open an enquiry into your return within 12 months of submission. They might:
- Request additional information
- Check specific aspects of your return
- Conduct a full compliance check
Most enquiries result from:
- Random selection
- Returns that look unusual compared to similar businesses
- Significant year-on-year changes
- Information not matching data HMRC holds
Having proper records and accurate returns means enquiries are straightforward to resolve.
Amending Your Tax Return
Made a mistake or forgot something? You can amend your return within 12 months of the filing deadline (by 31 January 2027 for the 2024/25 return).
How to Amend
Log into your Government Gateway account and select to amend your return. Make the necessary changes and resubmit. HMRC will recalculate your tax bill.
If Your Tax Bill Increases
Pay the additional amount immediately to avoid interest charges.
If You’re Due a Refund
HMRC will process the refund, though it may take longer than initial refunds.
Amendments After 12 Months
After the 12-month window, you can’t amend your return through the online system. You must write to HMRC explaining the error and asking for a correction. HMRC has discretion over whether to allow late amendments.
Getting Professional Help
Self Assessment might seem straightforward on the surface, but the complexity increases significantly with:
- Multiple income sources
- Self-employment or partnerships
- Property income
- Capital gains
- Foreign income
- Complex financial arrangements
When to Consider Professional Help
You Should Consider an Accountant If:
- You’re newly self-employed and unfamiliar with business expenses
- Your financial situation is complex
- You want to ensure you’re claiming all available reliefs
- You’re facing an HMRC enquiry
- You don’t have time to handle it yourself
- You’re concerned about making costly mistakes
- Your tax bill is substantial and optimisation could save significant amounts
How Can We Help?
Our Self Assessment service provides complete peace of mind. We handle everything from gathering information to filing your return, ensuring accuracy and maximising your refund or minimising your liability. Most clients save more in tax optimisation than our fees cost—and they eliminate the stress entirely. When choosing us, you get:
- Complete preparation and filing of your tax return
- Review of all income sources and expenses
- Identification of all eligible reliefs and allowances
- Tax planning advice to reduce future bills
- Payment on account management
- Unlimited support for HMRC queries
- Representation in enquiries or investigations
- Secure online document sharing portal
- Fixed, transparent fees agreed upfront
Conclusion
Self Assessment doesn’t have to be the annual source of stress and confusion it is for many taxpayers. With proper preparation, good record-keeping throughout the year, and understanding of the process, you can approach your tax return with confidence.
The key takeaways:
- Start early – Don’t wait until January
- Keep meticulous records – Throughout the year, not just at tax time
- Understand deadlines – Missing them is expensive
- Claim everything you’re entitled to – Don’t leave money on the table
- Get help when needed – Professional support often pays for itself
Whether you choose to file yourself or work with an accountant, the most important thing is ensuring your return is accurate, complete, and submitted on time. Your financial peace of mind is worth the effort.
