Introduction
Filing your first Self Assessment tax return can feel overwhelming, but with the right preparation, it's straightforward. This guide walks you through everything you need to know as a first-time filer.
Who Needs to File?
You must file a Self Assessment tax return if:
- You earned more than £1,000 from self-employment
- You're a company director
- You received income from property rentals
- You had untaxed income (tips, commission, foreign income)
- You or your partner received Child Benefit and income exceeded £60,000
- You earned income from savings, investments, or dividends above allowances
- You had capital gains above the annual exempt amount
Step 1: Register with HMRC
When to Register
Register by 5 October following the end of the tax year in which you became self-employed.
For 2025/26 (6 April 2025 to 5 April 2026), register by 5 October 2026.
How to Register
- Create a Government Gateway account at gov.uk
- Register for Self Assessment online
- Select the correct category (sole trader, partnership, etc.)
- Receive your Unique Taxpayer Reference (UTR) by post within 10 working days
- Activate your UTR online
Step 2: Gather Your Records
Essential Documents
- Government Gateway login details
- UTR number
- National Insurance number
- Records of all income
- Business expenses receipts
- Bank statements
Income Records
- Invoices issued to clients
- P60 or P45 (if you had PAYE employment)
- Bank interest statements
- Dividend vouchers
- Rental income records
Step 3: Understanding the Tax Year
The UK tax year runs from 6 April to 5 April.
For 2025/26:
- Period: 6 April 2025 to 5 April 2026
- Online filing deadline: 31 January 2027
- Payment deadline: 31 January 2027
Step 4: Calculate Your Profit
Total Business Income - Allowable Business Expenses = Taxable Profit
Allowable Expenses
- Office supplies and equipment
- Business travel costs
- Professional fees
- Marketing and advertising
- Training directly related to your business
- Proportion of home costs if working from home
Step 5: Filing Your Return
Forms You'll Complete
SA100 (Main Return): Personal details, other income, tax calculation.
SA103S or SA103F (Self-Employment): Self-employment supplementary pages.
Filing Methods
Online Filing (Recommended):
- Deadline: 31 January
- Instant confirmation
- Immediate tax calculation
Paper Filing:
- Deadline: 31 October (three months earlier)
- Slower processing
Step 6: Pay Your Tax Bill
Payment Deadline
Tax for 2025/26 is due by 31 January 2027.
What You'll Pay
- Income Tax on profits above Personal Allowance (£12,570)
- Class 2 NI (£3.45/week if profits exceed £12,570)
- Class 4 NI (6% on profits £12,570-£50,270, 2% above)
Payment on Account
If your tax bill exceeds £1,000, HMRC requires payments on account - advance payments toward next year's tax.
Common Mistakes to Avoid
- Missing Registration Deadline - Register as soon as you start self-employment
- Claiming Personal Expenses - Only claim genuine business expenses
- Forgetting Income Sources - Report all income including cash payments
- Poor Record Keeping - Keep receipts for at least 5 years
- Last-Minute Filing - File early to avoid errors
- Not Setting Money Aside - Save 25-30% of profits for tax
FAQs
What if I miss the 31 January deadline?
Automatic £100 penalty, with additional penalties if outstanding. File as soon as possible.
Can I amend my return?
Yes, within 12 months of the original filing deadline.
Do I need to submit receipts?
No, but keep them for at least 5 years in case of HMRC enquiry.
What if I can't afford my tax bill?
Contact HMRC for a Time to Pay arrangement.
Conclusion
Filing your first tax return becomes manageable with preparation. Register promptly, keep good records, and give yourself time before the deadline.