Introduction

If you’re self-employed, a sole trader, or earn income outside your PAYE salary, you’re likely required to file a self-assessment tax return with HMRC. While the process may seem overwhelming at first, understanding the self-assessment tax form, key deadlines, and how the self-assessment tax year works can make things much simpler.

In this blog, we’ll cover everything you need to know about self-assessment tax return filing in the UK — from who needs to file to how to avoid penalties.

What is Self-Assessment Tax?

Self-assessment tax is the system used by HMRC (HM Revenue and Customs) to collect income tax from individuals who don’t have it automatically deducted from their income.

This includes people who:

• Are self-employed or sole traders
• Are business partners or company directors
• Have rental income
• Receive dividends or foreign income
• Earn over £100,000
• Need to pay capital gains tax

Instead of relying on an employer to deduct tax, you must report your income and expenses using the self-assessment tax form and pay any tax owed directly to HMRC.

Who Needs to File a Self Assessment Tax Return?

You’ll usually need to file if, during the tax year, you:

• Worked as a freelancer, contractor or self-employed person
• Earned more than £1,000 from self-employment
• Received untaxed income such as rent or investment returns
• Claimed child benefit and earned over £50,000
• Need to report capital gains from selling property, shares or other assets

If you’re unsure, HMRC offers an online tool to check whether you need to complete a tax return.

Understanding the Self-Assessment Tax Year

The self-assessment tax year runs from 6 April to 5 April of the following year. For example, the 2024/25 tax year runs from 6 April 2024 to 5 April 2025.

You’ll file your tax return after the tax year ends, so for the 2024/25 tax year, returns are due by:

• 31 October 2025 for paper returns

• 31 January 2026 for online submissions

These deadlines are critical — miss them, and you’ll face automatic penalties.

Registering for Self-Assessment

If you’re filing for the first time, you need to register for self-assessment through HMRC’s website. This must be done by 5 October, following the end of the tax year in which you earned income.

Once registered, you’ll receive a Unique Taxpayer Reference (UTR) and be able to access your online tax account.

Completing the Self-Assessment Tax Form

The self-assessment tax form includes several sections, depending on your circumstances. The basic return is called SA100, and there are supplementary pages for:

• Self-employment (SA103)
• Property income (SA105)
• Capital gains (SA108)
• Foreign income (SA106)

You’ll need to declare:

• All sources of income (employed, self-employed, savings, rental, dividends)
• Business expenses
• Any tax reliefs or allowances
• Contributions to pensions or charitable donations

Gather records such as invoices, receipts, interest statements, and P60/P45 forms before you start.

Submitting Your Tax Return

There are two ways to file:

1. Online via HMRC

This is the most popular method. Once logged into your HMRC account, you can complete your return, calculate your tax bill instantly, and submit securely.

2. Paper Return

You can request a paper version if needed, but the deadline is earlier (31 October), and calculations are not automated.

Paying Your Self-Assessment Tax

Tax must be paid by 31 January following the end of the tax year. This includes:

• Any tax you owe for the previous tax year
• Your first “payment on account” for the next tax year, if applicable

Payments on account are advance payments towards your next year’s tax bill, usually required if your bill is over £1,000.

A second payment on account is due on 31 July.

Avoiding Penalties

Missing deadlines or submitting incorrect information can be costly. Here’s what to watch out for:

• Late filing: £100 fixed penalty, even if no tax is due
• Over 3 months late: £10 daily fines, up to £900
• Late payment: 5% surcharge on the unpaid tax
• Incorrect returns: Penalties range from 15% to 100% of unpaid tax, depending on the reason

Always double-check your information and file on time to stay penalty-free.

Top Tips for Easy Self-Assessment Filing

• Start early: Don’t wait until January; gather your documents as soon as the tax year ends.
• Use software or a professional: Accounting software or a tax advisor can help simplify the process.
• Keep good records: Save receipts, invoices, and bank statements throughout the year.
• Review allowances: Don’t forget to claim allowable expenses or tax reliefs.
• Set reminders: Mark key deadlines in your calendar or sign up for HMRC alerts.

Common Mistakes to Avoid

• Late Registration: Missing the October 5 registration means NY next can’t file.
• Incorrect UTR: Using someone else’s reference delays processing.
• Incomplete Records: Guessing figures increases error risk and HMRC queries.
• Overlooking Reliefs: Failing to claim pension or Gift Aid reduces your refund.
• Ignoring Penalties: Even small lateness triggers fines and interest.

Stay organised, use reminders, and consult HMRC guidance or a tax adviser if stuck.

Conclusion

Filing your Self Assessment tax return may seem daunting, but by following these steps—registering on time, gathering accurate records, choosing the right filing method, and claiming all eligible reliefs—you’ll complete your return smoothly. Meet key deadlines to avoid penalties, pay what you owe, and keep copies of your submission. A well-managed Self Assessment helps you stay on good terms with HMRC, ensures you pay the correct tax, and may even result in a welcome refund. Use this guide as your roadmap, and take charge of your UK tax return with confidence.

Final Thoughts

Filing a self-assessment tax return doesn’t have to be stressful. By understanding the self-assessment tax year, registering on time, and using the correct self-assessment tax form, you can take control of your taxes and avoid penalties.

Whether you file on your own or work with a tax professional, staying organised and informed is the key to hassle-free self-assessment tax return filing.