Self assessment tax return for 2025

New year, new taxes.

Now that we’ve taken down the Christmas decorations and put out the champagne to celebrate the arrival of 2025, it’s time to tackle something a little less festive but just as important: your Self Assessment Tax Return.

Although this is usually required if you are self-employed, new laws introduced last year mean more people may be likely to submit one. If you took on a side hustle, earned income from renting property, or received tax-free income from investments, you may need to file a return, even if you’ve never had to do so before.

We’ll detail everything you need to know – from understanding what counts as taxable income to how to submit your tax return so you can stay on the right side of HMRC.

What is a self-assessment tax return?

A self-assessment tax return is a way of reporting income and any tax you owe to HMRC. This is especially important if you are a individual entrepreneur or self-employed, as there is no employer to automatically deduct your tax via the PAYE (Pay As You Earn) scheme. It is therefore up to you to declare your income and pay the correct amount of tax.

The deadline for submitting your self-assessment tax return is January 31. However, if you exceed this deadline, you will have to pay a £100 late fee if your tax return is up to 3 months late. The penalties only get heavier the longer you delay. After 6 months you could face an additional fine and after 12 months you could be assessed even higher fees.

It is also important to produce on time if you need to income taxas HMRC will start charging interest on any unpaid tax once the deadline passes.

What is the ancillary activities tax?

Secondary activities have become an essential way for people to increase their income alongside their main job. After all, 43% of British adults started a parallel activity in 2024, with 65% of Millennials and Generation Z plan to continue them in 2025.

However, if you run a side business yourself, you will be required to pay a self-assessment tax return if you win £1,000 or more in one year – otherwise known as Secondary tax. Even if you don’t reach this threshold, you may still need to register for the self-assessment in case you reach this amount.

So why was this introduced?

The Side Hustle Tax is not an official tax, but HMRC requires payment of income tax to ensure that untaxed income from side hustles is correctly declared and taxed in accordance with applicable laws.

The rise of the gig economy, particularly through platforms like EtsyUber and Airbnb, has created new opportunities for additional income, but it has also led to significant untaxed income. This is called a “tax gap,” and as more and more people earn money through side hustles and don’t always report it, this gap has widened. So, to solve this problem, HMRC ensures that non-profit income is declared and taxed correctly so that everyone contributes fairly.

Do I need to submit a self-assessment tax return?

Not sure whether or not you should submit a self-assessment? You will be responsible if:

• You are self-employed as a sole trader, freelancer or entrepreneur
• You have additional untaxed income, such as rental income, ancillary income over £1,000 or investment income.
• You had to pay capital gains tax when you sold something that increased in value
• You are a business manager
• If you earn more than £60,000 in the 2024-25 tax year and need to pay High Income Child Benefit.

If you still feel stuck, we recommend seeking professional advice. an accountant. They will be able to tell you whether you are responsible for self-assessment by assessing your specific financial situation, reviewing your sources of income and determining whether you need to file a tax return based on your situation.

How to submit a self-assessment tax return

A self-assessment tax return can be filed online or via an SA100 form, which you can download from government website. Alternatively, you can request one by contacting HMRC directly if you do not have internet access.

To submit your tax return by self-assessment online, simply:

• Go to Self-Assessment Login Page and enter your Government Gateway user ID
• Click on the option to complete your self assessment tax return and select ‘Start now’
• Enter your personal information (including your name, unique taxpayer reference (UTR) and date of birth)
• Enter details of your income, expenses and any other relevant information.
• Submit the return online via the HMRC portal

For a more detailed explanation, check out our guide on complete tax returnswhich will guide you through each step and ensure you cover all the essential elements. We also recommend speaking with a tax professional for personalized advice, particularly regarding any new legislative changes and how they might affect side businesses or individuals with other additional income.