How to File a Self Assessment Tax Return
This guide will help you fill in a Self Assessment tax return yourself online.
How to File a Self Assessment Tax Return in the UK (2025/26)
Filing your Self Assessment tax return might seem daunting, but with the right guidance, it can be a straightforward process. Whether you're self-employed, a landlord, or receiving untaxed income, this comprehensive guide explains everything you need to know for the 2025/26 tax year.
What Is a Self Assessment Tax Return?
A Self Assessment tax return is how individuals report income that HMRC doesn’t already know about. It’s most commonly required by:
Self-employed individuals and sole traders
Company directors
Landlords earning rental income
People with income from savings, investments, or abroad
Those earning over £100,000 annually
If HMRC has contacted you or if your circumstances match the above, you're likely required to file.
Key Deadlines for Self Assessment (2025/26)
| Deadline Type | Date |
|---|---|
| Register for Self Assessment | 5 October 2025 |
| Paper tax return deadline | 31 October 2025 |
| Online tax return deadline | 31 January 2026 |
| Payment due date | 31 January 2026 |
| Second payment on account (if applicable) | 31 July 2026 |
Missing deadlines may result in penalties and interest charges.
Step-by-Step Guide: How to File a Self Assessment Tax Return
Step 1: Register with HMRC
If you’ve never filed before, register via HMRC online:
Self-employed: Register as a sole trader
Other income: Register for Self Assessment for non-trading income
Once registered, you’ll receive:
A Unique Taxpayer Reference (UTR)
An activation code to set up your online account
Step 2: Set Up Your Personal Tax Account
Go to HMRC’s Government Gateway and sign in or create an account. This account allows you to file, view, and manage your tax affairs.
Step 3: Gather the Information You’ll Need
You’ll need to provide:
Your UTR and National Insurance number
Records of income (self-employment, dividends, pensions, rental income)
Business expenses or tax-deductible costs
P60 or P45 if employed
Details of pension contributions or charitable donations
Bank interest earned
Step 4: Complete the Return Online
Navigate to your HMRC online account and follow the Self Assessment prompts:
Fill in the sections relevant to your income sources
Declare any reliefs, allowances, or deductions
HMRC will calculate the tax owed (or refund due) as you go
Step 5: Review and Submit
Double-check your return before submission:
Ensure all figures are accurate
Save a copy of the confirmation receipt
Once submitted, HMRC will confirm the amount owed or refunded.
Step 6: Pay Any Tax Due
You must pay any tax by 31 January 2026. If your tax bill exceeds £1,000, you may be required to make payments on account, with the second instalment due 31 July 2026.
Who Doesn’t Need to File?
You typically don’t need to file a return if:
You're fully taxed through PAYE with no additional income
Your only income is from state benefits
You have savings or dividends below the annual allowances
If you receive a notice to file but believe you don’t need to, contact HMRC promptly to avoid penalties.
Common Mistakes to Avoid
Missing the deadline (incurs automatic £100 penalty)
Entering incorrect figures (e.g. gross vs. net)
Forgetting to declare all income sources
Not claiming allowable expenses
Failing to budget for payment on account
Expert Tips to Make Filing Easier
Keep digital records: Use accounting software or HMRC-recognised apps to stay organised.
Claim all deductions: Include legitimate business expenses, pension contributions, and charitable donations.
Use a tax professional: If your situation is complex, an accountant can help reduce errors and maximise reliefs.
✅ Common Expenses You Can Claim on a Self Assessment Tax Return
When you're self-employed or earning untaxed income, HMRC allows you to deduct reasonable business expenses from your income before calculating tax. Claiming allowable expenses helps reduce your taxable profit—ensuring you only pay tax on your actual earnings. Below is a breakdown of the most common deductible expenses for the 2025/26 tax year.
| Expense Category | Examples of Allowable Costs |
|---|---|
| Office Costs | Stationery, printing, computer software, postage |
| Business Premises | Rent, business rates, utilities, property insurance |
| Travel Expenses | Fuel, train fares, vehicle insurance, hotel stays (business travel only) |
| Staff Costs | Employee wages, subcontractor fees, staff training |
| Marketing | Website costs, online ads, business cards, social media promotion |
| Professional Fees | Accountant fees, legal advice, consultancy |
| Use of Home (Simplified Method) | Flat rate based on hours worked from home |
| Phone and Internet | Business-related portion only |
Summary: Key Takeaways
Register early – by 5 October if you’re newly self-employed
File online by 31 January 2026 for the 2025/26 tax year
Accurate records are essential for avoiding fines
Use HMRC's online services or seek professional help if unsure
Frequently Asked Questions
Do I have to file a tax return if I’m employed?
Only if you earn other income (e.g. side business, rental, dividends) or if you’re requested by HMRC.
What if I make a mistake?
You can amend your return within 12 months of the 31 January deadline.
Can I file a return on paper?
Yes, but only until 31 October. Online filing is available until 31 January.
What happens if I miss the deadline?
You’ll face a £100 penalty, increasing over time. Interest is also charged on unpaid tax.