How to File Your Self Assessment Tax Return: Step by Step
Once you know you need to file a Self Assessment return, the actual process is more straightforward than it looks. This guide from Filing Accounts UK walks through the mechanics of filing, from registration to payment. If you’re not yet sure whether you need to file at all, see our guide on who needs to file a Self Assessment return first.
At Filing Accounts, we file Self Assessment returns for individuals and sole traders across the UK.
Step 1: Register (If You Haven’t Already)
If this is your first return, register with HMRC by 5 October following the end of the tax year you need to report. Once registered, HMRC posts your Unique Taxpayer Reference (UTR), usually within 2–3 weeks — you can’t file without it, so don’t leave this until the last minute.
Step 2: Gather Your Documents
- Your UTR and National Insurance number
- Details of all UK taxable income — employment, self-employment, rental, dividends, pensions, savings interest
- Records of any expenses you plan to claim
- Pension contribution records
Step 3: Complete the Return
File online via your HMRC account, using the SA100 main form plus any relevant supplementary pages — SA103 for self-employment income, SA105 for rental income, and so on depending on your circumstances. HMRC’s online guidance notes walk through each section as you go.
Step 4: Submit by the Deadline
| Deadline | Date |
|---|---|
| Register (if new) | 5 October |
| Paper return | 31 October |
| Online return and payment | 31 January |
You can save your progress online and return to it later before final submission — useful if you’re waiting on a specific figure or document.
Step 5: Pay What You Owe
Once submitted, HMRC calculates your tax due. Pay online by 31 January — see our guide on how to pay HMRC correctly for payment references and methods. If your bill exceeds £1,000 and less than 80% of your tax was already collected via PAYE, you’ll likely also need to make Payments on Account toward next year’s bill.
Step 6: Keep Your Records
Keep all supporting records — invoices, receipts, bank statements — for at least 5 years after the 31 January submission deadline for that tax year. HMRC can request evidence of any figure on your return within that window.
Common Mistakes to Avoid
Registering Too Late
Your UTR can take 2–3 weeks to arrive by post — register well before the October deadline, not at the last minute.
Forgetting Small Amounts of Untaxed Income
Savings interest, small dividends, and casual income all need declaring alongside your main income.
Not Budgeting for Payments on Account
A first tax bill can be larger than expected once Payments on Account are included — plan for this rather than being caught out.
Frequently Asked Questions
Which forms do I need alongside the SA100?
It depends on your income — SA103 for self-employment, SA105 for property income, and other supplementary pages for specific income types.
Can I save my return and finish it later?
Yes, HMRC’s online service lets you save progress and return before final submission.
How long should I keep my records?
At least 5 years after the 31 January deadline for the relevant tax year.
Need Help Filing Your Self Assessment Return? Talk to Filing Accounts UK
At Filing Accounts, we handle Self Assessment registration and filing for individuals, sole traders, and landlords across the UK.
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