Self Assessment tax deadline today: Who needs to register with HMRC by 5 October?

Anyone who needs to complete a Self Assessment tax return for the first time in the 2025/26 tax year has until the end of today (5 October) to tell HMRC, or risk penalties later down the line.

Many people focus on the 31 January tax return deadline, but 5 October is an important date for anyone who has never filed a Self Assessment return before or previously left the system and now needs to file again.

The deadline is particularly important for people who have started earning money outside their usual job, including newly self-employed workers and some people making money from a side hustle, property or investments.

It's not a deadline for paying your tax, if you need to complete a Self Assessment return for the 2025/26 tax year, you normally have until 31 January 2027 to submit it online and pay any tax owed.

Who needs to register for Self Assessment by 5 October?

You must tell HMRC by 5 October 2026 if you need to complete a tax return for the 2025/26 tax year and either:

• Have never sent a Self Assessment tax return before

• Or you registered for Self Assessment previously but did not need to send a return for the 2024/25 tax year.

The 2025/26 tax year ran from 6 April 2025 to 5 April 2026.

There are several reasons why you might need to file a return.

• You are newly self-employed

If you worked for yourself as a sole trader and earned more than £1,000 in gross trading income during the tax year, you generally need to register for Self Assessment.

The £1,000 threshold is based on your gross income before expenses, rather than the amount of profit you made.

This could catch out people who started a small business, freelanced alongside their main job or began a side hustle during the year.

• You have untaxed income

You may also need to file a tax return if you received income that wasn't taxed through PAYE.

For example:

• Money from renting out property or land

• Tips and commission

• Savings interest

• Dividends

• Foreign income

• Other taxable UK income.

• You have a High Income Child Benefit Charge

You may need to complete a Self Assessment return if you are liable for the High Income Child Benefit Charge and don't pay it through PAYE.

This is an important one to check if your income has increased and you or your partner receive Child Benefit.

• You are a business partner

If you became a partner in a business partnership during the 2025/26 tax year, you may need to register for Self Assessment.

• You need to pay Capital Gains Tax

You may also need to submit a tax return if you had to pay Capital Gains Tax after selling or disposing of something that had increased in value.

Do I need to register for a tax return if I sell things online?

Simply selling on items that you no longer want or need does not automatically mean you owe tax or need to complete Self Assessment.

But if you are buying or making goods with the intention of selling them for a profit, this can amount to trading income.

If your gross trading income is more than £1,000, you generally need to register and declare it.

This could therefore be relevant to people making money through online selling, including those who have turned a side hustle into a regular source of income.

What happens if you miss the 5 October deadline?

HMRC says you could get a penalty if you don't tell it by 5 October when you are required to register.

You don't have to complete your entire 2025/26 tax return today.

Once registered, you can complete your return after the end of the tax year.

For most people filing online, the deadline for submitting the 2025/26 return and paying the tax due is 31 January 2027.

Registering doesn't take lon.

How to register for your tax return

Once you've registered, HMRC will issue you with a Unique Taxpayer Reference (UTR), which you'll need when completing your tax return.

You can then start preparing your 2025/26 return.

For anyone leaving this until the last minute, HMRC launched an improved Self Assessment registration service in September 2026.

The new online system includes pre-populated information, the ability to save and return to an application, online support and confirmation by email or text.

HMRC says people using the new service can receive their UTR in their online account within 72 hours, rather than waiting up to 15 days for it to arrive by post.

Register online here.