The Ultimate Guide to Self Assessment Tax Returns
Introduction
Whether you're a tradesperson, freelancer, creative, consultant or contractor, Self Assessment is the system HMRC uses to collect Income Tax from people who don’t pay tax through PAYE. For new sole traders, the process can feel overwhelming — especially the first time.
This guide walks you through everything you need to know:
- what Self Assessment actually is
- when and how to register
- which forms to complete
- how to calculate your tax
- key deadlines
- penalties
- and how digital tools like QTax make filing far easier
If you want an even more detailed breakdown of specific areas, you can explore our related guides, such as How Self Assessment Works or Allowable Expenses for Sole Traders.
What Is Self Assessment?
Self Assessment is HMRC’s process for collecting Income Tax and National Insurance from individuals whose income isn’t automatically taxed.
Who needs to file?
You must file a Self Assessment tax return if you:
- are a sole trader earning more than £1,000 per year
- receive income outside PAYE (e.g., freelance, rental, dividends, foreign income)
- are a partner in a partnership
- need to claim certain reliefs (e.g., tax relief on pension contributions)
Why sole traders file tax returns
Unlike employees, sole traders do not have tax deducted automatically.
So you’ll need to:
- declare your business income
- declare your allowable expenses
- report any additional income
- calculate tax and National Insurance owed
For a walkthrough of the form sections, see our Self Assessment Guide.
How to Register for Self Assessment
If you haven't registered yet, you must do so by 5 October following the end of the tax year in which you started trading.
Steps to register:
- Create a Government Gateway account
- Register for Self Assessment as a sole trader
- Receive your UTR (Unique Taxpayer Reference)
- Activate your HMRC account
For a full step-by-step process, see our Guide to Registering as Self-Employed.
What You Need Before You Start Filing
To complete your tax return, you’ll need:
Personal details
- UTR number
- National Insurance number
- Government Gateway login
Business records
- Total income from self-employment
- Allowable expenses
- Invoices or records of work done
- Mileage logs
- Bank statements (if useful for checking totals)
Other income (if relevant)
- Employment income (e.g., P60, P45)
- Dividends
- Savings interest
- Rental income
Our Record Keeping Guide covers this in more detail.
Key Parts of a Self Assessment Tax Return
You will generally complete two forms:
SA100 — The Main Tax Return
Covers:
- personal income
- pensions
- charity donations
- student loans
- tax reliefs
SA103 — Self-Employment Section
Most sole traders complete the short version (SA103S).
You’ll enter:
- total income
- allowable business expenses
- adjustments (if any)
- your final taxable profit
If your business is more complex or above the VAT threshold, you may need the full SA103F.
How to Complete Your Self Assessment: Step-by-Step
1. Enter your income
Provide your total income from self-employment for the tax year (6 April to 5 April).
If you had PAYE employment alongside self-employment, enter the details from your P60 or P45.
2. Add your allowable expenses
These reduce your taxable profit.
Common examples include:
- travel for work
- tools and materials
- home office costs
- professional memberships
- phone and internet usage
Our Allowable Expenses Guide explains what you can and cannot claim.
3. Add additional income (if relevant)
This may include:
- savings interest
- dividends
- rental profits
- foreign income
4. Review your summary
HMRC will calculate:
- Income Tax
- Class 2 National Insurance
- Class 4 National Insurance
- If payments on account apply
5. Submit your return
If filing online, you can submit directly using digital software.
QTax offers a simple, guided submission flow — learn more on our Product Tour.
Key Deadlines You Must Know
5 October — Register for Self Assessment
If this is your first year being self-employed.
31 October — Paper tax return deadline
Unless posting a paper form, you will file online.
31 January — Online filing deadline
And the deadline to pay your tax bill.
31 July — Second payment on account (if applicable)
This only applies if your tax bill was >£1,000.
See more in our Tax Deadlines Guide.
Penalties for Missing Deadlines
Late filing penalties
- £100 immediately
- Additional daily penalties after 3 months
- Further penalties at 6 and 12 months
Late payment penalties
- Daily interest
- Additional charges at 30 days, 6 months and 12 months
Missing deadlines can get expensive quickly — which is why reminders matter.
How to Calculate Your Self-Employed Tax
Your tax is based on: Taxable profit = Income − Allowable expenses
HMRC then applies:
- Income Tax bands
- Class 2 National Insurance
- Class 4 National Insurance
- Personal Allowance
If you're unsure what your tax bill might look like, we cover this in detail in our article on Calculating Your Self-Employed Tax Bill.
How QTax Makes Self Assessment Easy
QTax is built specifically for UK sole traders and is designed to remove the confusion from tax returns.
✔ Step-by-step guidance
No jargon — just simple questions.
✔ Expense guidance
Helpful explanations of what can be claimed.
✔ Smart checks
We highlight common mistakes and missing information.
✔ Direct HMRC submission
Submit your tax return digitally through QTax.
✔ Built for sole traders
Focused on simplicity and speed, not bloated features.
See the full experience in the Product Tour.
Pricing is simple too — details on our Pricing page.
FAQs
Do I need an accountant to file Self Assessment?
No. Many sole traders file on their own using digital tools.
Can I file early?
Yes — HMRC opens online filing in April.
Can I amend my return after filing?
Yes — you can amend most returns within 12 months.
Conclusion
Self Assessment doesn’t need to be confusing or time-consuming. With the right guidance and a simple digital tool, you can file your return quickly, correctly and with far less stress.
QTax is built specifically for sole traders — helping you understand what to include, avoid common mistakes and submit your tax return with confidence.
Ready to file your Self Assessment the easy way?
Start your 30-day free trial with QTax.