How Do I File an Income Tax Return in The UK?

HMRC or HM Revenue and Customs is Gov UK Taxes, customs, and payments authority that collects money to pay for public services. They have a self-assessment system to collect taxes. But who has the time? 

For many small business owners and freelancers in the UK, filing a self-assessment tax return can feel overwhelming. Especially if you’re juggling client work, invoices, and deadlines, all at the same time. 

But here’s the good news. 

With smart preparation, the right approach, and expert guidance, it becomes a simple, manageable task. In fact, filing early not only gives you peace of mind, but may also help secure any refunds sooner and keep HMRC penalties at bay.

But not to worry. This blog will guide you through the self-assessment income tax return process, which includes: 

• Important dates
• Documents you need to gather 
• Steps to file your return
• Mistakes to avoid

Let’s keep reading. 

What is Self Assessment? 

If you live in the UK and receive income that isn’t taxed at source (such as self-employment, rental properties, dividends, or overseas earnings), you may be required to complete a Self Assessment tax return. This process lets you declare your taxable income and capital gains to HM Revenue & Customs (HMRC) for the relevant tax year.

To do this, you’ll use the SA100 form, which is the main document for filing a Self Assessment tax return. It includes sections for various income sources, tax reliefs, and allowances. Whether you file online or by post, the SA100 helps HMRC (and you) work out how much tax you owe—or if you’re due a refund.

Understanding how to properly complete the SA100 form is crucial for ensuring you pay the right amount of tax and avoid penalties.

If you’re new to the process, we can help you to accurately prepare Self Assessment tax returns. At TAJ Accountants, we assist you with bookkeeping, accountancy, payroll, or tax advisory services.

Do You Need to File an Income Tax Return in The UK?

The Government of the UK have defined specific criteria that make you eligible for tax filing, under which your annual income is classified. 

However, you need to submit a Self Assessment tax return if you are:

• Self‑employed or a sole trader, earning more than £1,000 in turnover.
• Earning income from rentals, dividends, investments, or foreign sources.
• A company director, even if you don’t draw a salary.
• Receiving untaxed income (e.g. from freelance gigs or consultancy).
• Earning over £150,000 or liable for the High Income Child Benefit Charge.
• Being asked directly by HMRC to file one.

Note: Even if you expect zero tax due, not filing on time incurs immediate fines. In recent years, more than 600,000 people were fined despite having no tax to pay.

If you’re unsure whether you need to register, our team can assess your circumstances and, if required, register you with HMRC and get your UTR in place.

What Documents Do You Need to File Your Tax Return? 

 To file your tax return accurately, you will need the following: 

• Your Unique Taxpayer Reference (UTR) and National Insurance number.
• Proof of all income streams:

• Self‑employment turnover (invoices, bank statements)
• PAYE income (P60/P45 forms)
• Rental income and investment dividends
• Any foreign or miscellaneous income

• Records of expenses and receipts (business-related, allowable costs)
• Details of pensions, benefits, charitable gifts, or student loan repayments
• A copy of your previous year’s tax return (if applicable)

Organising all these papers/documents in advance speeds up the filing process and reduces errors.

Tax Filing Deadlines in The UK

Late returns face escalating fines from an immediate £100 up to 100% of your tax bill. Late or missed payments incur interest (currently around 8%).

Steps to File Your Income Tax Return

Here’s how TAJ Accountants guides you through each step:

Step 1: Register for Self Assessment

If this is your first time filing, you must register with HMRC by 5 October. You’ll get a UTR by post within 10 days (21 days if you live abroad).

Step 2: Gather Your Financial Documents

Use our checklist above to collect everything in one place.

Step 3: Log In to HMRC Online

Use your Government Gateway credentials to access your tax account. If you prefer, you can use approved accounting software like FreeAgent, Xero, or file through an accountant (like us!).

Step 4: Fill in Your Return

You’ll need to complete the sections that apply to your income types and enter details for deductions and reliefs.

Step 5: Submit Before the Deadline

Submit your return online by 31 Jaunary and make any payments due.

Step 6: Pay Any Tax Owed or Claim a Refund

You can pay via debit/credit card, Direct Debit, or bank transfer. If HMRC owes you a refund, you’ll typically get it within 5–10 working days.

Please note: If you’re due a refund, you must submit early so that you receive it faster.

If you owe more than £1,000 in tax, expect a second Payment on Account by 31 July.

With TAJ Accountants, you won’t have to worry about forms, numbers, or deadlines—we do it all for you. We handle all of this on your behalf using fully HMRC-compliant software and making sure every detail is checked.

Common Mistakes During Self-Assessment Tax Return Filing (and How You Can Avoid Them) 

As most small business owners as well as individuals don’t have much time to work on their bookkeeping on a daily basis, it’s very common to make a mistake during tax filing. 

Here are common mistakes you should avoid during tax filing: 

• Entering the wrong tax year
• Not reporting all income sources (especially side gigs or bank interest)
• Forgetting to claim valid business expenses
• Misplacing your UTR or not registering in time
• Filing without reviewing—many pay more tax than necessary!

How TAJ Accountants Help You With Self-Assessment Tax Return 

Filing a tax return may be unavoidable—but it doesn’t have to be stressful. With the right preparation and expert help, you can file on time, claim everything you’re entitled to, and avoid penalties. Let TAJ Accountants simplify the process for you—so you can focus on running your business.

Here’s how we help UK residents file stress-free, accurate returns:

✅ HMRC registration and UTR support

✅ Expense tracking and tailored deductions

✅ Full Self Assessment filing and submission

✅ Post-filing support and HMRC communication

✅ Trusted by hundreds of self-employed professionals in London and across the UK

✅ Fixed pricing and no hidden costs

✅ Remote or in-person service—you choose

Bonus: We also help with Making Tax Digital readiness and VAT filing if you’re over the threshold.

Book your free Self Assessment consultation today and file your return with total confidence.

Frequently Asked Questions 

1. How do I file my income tax return online in the UK?

You can file your return through HMRC’s online portal using your Government Gateway ID, or you can hire an accountant like TAJ Accountants to file it for you.

2. What documents do I need to file a Self Assessment tax return in the UK?

You’ll need your UTR, National Insurance number, income records (self-employment, PAYE, rental), expense receipts, and pension/student loan information.

3. Do I need to file a tax return if I’m employed in the UK?

Only if you have additional income (freelance, rental, dividends) or HMRC tells you to. Most PAYE-only employees don’t need to file.

4. How much does it cost to file a Self Assessment tax return in the UK?

Costs vary, but at TAJ Accountants we offer fixed, affordable packages starting from £150 for basic filings. Book a consultation for an exact quote.

5. What happens if I miss the 31 January deadline?

You’ll face an instant £100 penalty, with additional charges for further delay. Late payments accrue daily interest.