Filing Your First Self Assessment Tax Return

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.

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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

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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

1. Create a Government Gateway account at gov.uk
2. Register for Self Assessment online
3. Select the correct category (sole trader, partnership, etc.)
4. Receive your Unique Taxpayer Reference (UTR) by post within 10 working days
5. Activate your UTR online

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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

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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

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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

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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

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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.

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Common Mistakes to Avoid

1. Missing Registration Deadline - Register as soon as you start self-employment
2. Claiming Personal Expenses - Only claim genuine business expenses
3. Forgetting Income Sources - Report all income including cash payments
4. Poor Record Keeping - Keep receipts for at least 5 years
5. Last-Minute Filing - File early to avoid errors
6. Not Setting Money Aside - Save 25-30% of profits for tax

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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.

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Conclusion

Filing your first tax return becomes manageable with preparation. Register promptly, keep good records, and give yourself time before the deadline.