If you’re filing a Self Assessment tax return in the UK for the first time, it can feel overwhelming. Whether you’re self-employed, earning rental income, or receiving dividends, completing the HMRC form correctly is crucial to avoid penalties or overpaying tax.

This simple, professional guide walks you through how to fill in a Self Assessment tax return online from registration to final submission based on HMRC best practices and expert tax advice.

Step 1: Register for Self Assessment (If You Haven’t Already)

Before you can file, you must register with HMRC.

✅ Who must register: Self-employed individuals, landlords, company directors, freelancers, investors earning over allowances, or people with additional income not taxed at source.

✅ How to register:

1. Visit HMRC’s website: GOV.UK → Register for Self Assessment.

2. Select your status (Self-employed or registering just to file a return).

3. Create or log into your Government Gateway account.

4. HMRC will send your Unique Taxpayer Reference (UTR) in the post.

5. You’ll also receive an activation code for your online account.

Deadline to register: By 5 October following the end of the tax year you need to report.

Step 2: Gather All the Documents You’ll Need

You’ll need to report all taxable income and any claimable expenses.

 Have these ready:

If you’re claiming expenses, collect receipts, mileage logs, utility splits, and software invoices.

Step 3: Log In to Your HMRC Online Account

1. Go to GOV.UK and log in using your Government Gateway ID.

2. Select ‘Complete your Self Assessment tax return’ for the relevant tax year.

3. Confirm your personal and contact details before starting.

Step 4: Fill In the Main Tax Return (SA100 Form)

The SA100 is the core part of your return. You’ll answer questions about:

1.  Personal details
2. Employment income (if applicable)
3.  Pension and benefits
4. Interest from banks or building societies
    Dividends
5. Charitable donations or tax reliefs
6. You will also be asked if you need to fill additional supplementary pages (e.g. self-employment, property, capital gains).

Step 5: Complete the Relevant Supplementary Sections

1️⃣ Self-employment (SA103S or SA103F)

For sole traders. Short form (SA103S) is used for income under £85,000.
You must:

• Enter turnover (total income)

• Deduct allowable expenses

• Enter profit or loss

• Declare Class 2 and Class 4 National Insurance contributions

2️⃣ Property Income (SA105)

For landlords renting out UK property. Include rental income, expenses, and net profit.

3️⃣ Partnerships (SA104)

Applicable if you are in a partnership (alongside a separate return for the business).

4️⃣ Capital Gains (SA108)

If you sold assets like property, shares, or investments.

5️⃣ Foreign Income (SA106)

For overseas earnings, dividends, or rental payments.

Step 6: Enter Expenses and Claim Allowances

Only claim expenses that are “wholly and exclusively” for business use. These may include:

✅ Travel and fuel
✅ Office costs
✅ Advertising and marketing
✅ Tools, software, subscriptions
✅ Accountancy fees
✅ Work-from-home allowance

Landlords can also claim repairs, insurance, agent fees, and mortgage interest relief (limited).

Correctly claiming expenses helps reduce your taxable profit.

Step 7: Review the Tax Calculation Summary

Once all sections are completed, HMRC will show a summary of:

✅ Tax owed
✅ National Insurance due
✅ Student loan repayments (if applicable)
✅ Payments on account (advance payments for the next year)

Review carefully. If anything looks incorrect, go back and check the relevant sections.

Step 8: Submit Your Tax Return to HMRC

✅ Click ‘Submit’ online.
✅ Make note of your confirmation reference number.
✅ Save a copy of your return and calculation.

Deadline to submit online: 31 January after the tax year ends (e.g., 2024/25 return deadline = 31 January 2026).

Step 9: Pay Your Tax Bill

Your tax payment is also due by 31 January.
If payment on account applies, you must also make an advance payment for the next tax year.

Payment can be made via:

• Bank transfer (Faster Payments)

• Direct debit

• Debit card

• Online banking

• HMRC payment plan (if needed)

Step 10: Keep Records for at Least 5 Years

HMRC may review your return in the future, so it’s a legal requirement to retain financial records.

Common Mistakes to Avoid When Filing Self Assessment

Frequently Asked Questions (FAQs)

Do I need an accountant to file?

No, but it’s recommended if you have multiple income sources or complex deductions.

Can I amend my return?

Yes, within 12 months of the original deadline.

What happens if I owe no tax?

You still need to file if required, or you will face penalties.

What if I cannot pay on time?

You can apply for a Time to Pay arrangement with HMRC.

Final Thoughts: Filing Smart Helps You Save Time and Money

Filling in a Self Assessment tax return becomes easier once you understand the process and stay organised. With accurate record-keeping, awareness of expenses, and timely filing, you can take control of your tax responsibilities confidently. If unsure at any stage, seeking advice from a tax professional can help you avoid costly errors.