How to File a Self Assessment Tax Return in the UK (Step-by-Step Guide)

If you’re self-employed (sole trader), a company director, or earn income outside of PAYE, you’ll probably need to file a Self Assessment tax return.

But the process can feel confusing, and with strict HMRC deadlines, mistakes can be costly.

In this guide, I’ll walk you through how to file a self assessment tax return in the UK step by step, explain common pitfalls, and show you when it’s worth using an accountant.

Who Needs to File a Self Assessment Tax Return?

You must complete a Self Assessment if you:

• Are self-employed (sole trader or freelancer)
• Are a partner in a partnership
• Are a company director with untaxed income
• Earn rental income, dividends, or overseas income
• Have untaxed savings or investment income
• Earn more than £150,000 if your income is solely taxed through PAYE (for 2023–24 tax year)

💡 Note: From the 2024–25 tax year, this £150,000 PAYE threshold will be removed, but other HMRC criteria still apply.

👉 Unsure if you need to file? HMRC has an eligibility checker but if you’re still not certain, an accountant can confirm.

Key Deadlines for Self Assessment Tax Returns in the UK

• 5 October – register for Self Assessment (first-time filers)
• 31 October – paper return deadline
• 31 January – online filing & payment deadline (previous tax year)

Miss the deadline and you face:

• £100 initial late filing penalty
• Daily penalties after 3 months
• Interest on late payments

Table: PAYE-Only Thresholds

Step-by-Step Guide: How to File Your Self Assessment Tax Return Online

Step 1: Register with HMRC

• Register online via GOV.UK if it’s your first time.
• HMRC will send you a Unique Taxpayer Reference (UTR) and login details.

Step 2: Gather Your Records

• Income: invoices, payslips, dividend vouchers, rental income
• Expenses: receipts, mileage logs, business costs
• Other income: savings interest, overseas income

👉 Good bookkeeping makes this step simple.

Step 3: Log Into HMRC Online

• Use your Government Gateway ID
• Navigate to “Self Assessment”

Step 4: Fill In the Return

• Enter your income, expenses, and reliefs
• Check allowances (e.g., trading allowance, marriage allowance, other tax relief if relevant)

Step 5: Submit and Pay

• Submit online by 31 January
• Pay tax owed by the same deadline (or set up a payment plan)

Common Self Assessment Mistakes to Avoid

• Missing out on allowable expenses (overpaying tax)
• Forgetting additional income (risking HMRC penalties)
• Leaving it too late, last-minute rush = errors
• Not keeping digital records (mandatory under Making Tax Digital)

Do You Need an Accountant for Self Assessment?

You can file it yourself. But many business owners choose an accountant because:

• It saves hours of admin stress
• Reduces errors and avoids penalties
• Ensures you claim every allowable expense
• Provides peace of mind with HMRC compliance

👉 At Skynet Accounting, we handle Self Assessment Tax Returns for individuals and businesses so you can focus on running your business.

FAQs About Filing a Self Assessment Tax Return

How do I file a self assessment tax return in the UK?
Register with HMRC, gather your records, log into your account, complete the return, and submit by 31 January.

Can I do my self assessment myself?
Yes, but many prefer an accountant to save time and avoid mistakes.

What happens if I miss the tax return deadline?
You face penalties: £100 immediately, then daily charges after 3 months, plus interest on late payments.

Do I need an accountant for self assessment?
Not legally, but accountants often save more in tax reliefs than they cost.

Who needs to file if I earn high income?

• 2023–24: PAYE-only earners above £150,000 must file
• 2024–25 onward: Threshold removed, but other triggers (self-employment, dividends, rental income) still apply

Conclusion

Filing a Self Assessment tax return isn’t impossible but it’s easy to make mistakes or miss deadlines.

With specialist support, you’ll not only stay compliant but also maximise tax efficiency.

And of course, if you’d like a chat about how we can take away the stress of managing your accounts, taxes, finances, and production costs so you get to keep more of what you earn and focus on growing your business, just book a call with us.

Written by Yesim Tilley ACMA, CGMA

www.skynetaccounting.co.uk