7 steps to self-assessment success for first-time filers

If you are newly self-employed, started a lucrative side hustle recently, or have any untaxed income in 2024-25, you may now need to pay taxes using self-assessment.

You can file a tax return at any point between 6 April and the final cut-off date of 31 January. That deadline might seem like ages from now, but the process can be daunting if you've never done it before. 

Here, Which? offers seven steps to help rookie self-assessment customers successfully submit their first tax return.

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1. Check if you need to file

Anyone with untaxed income from 2024-25 may need to complete a self-assessment form.

In a Which? survey of 1,269 members in November 2024, just under a third said they would be submitting a self-assessment tax return. Among them:

• 15% planned to file tax returns because of money made from self-employment. 
• 18% need to pay tax on state pension income that exceeds their personal allowance
• 57% need to pay tax on savings interest or investment income

Other reasons why you might need to file include money made from tips and commissions, lodger income, and even claiming tax relief on charitable donations.

If you're unsure about whether you need to file a tax return, you can use HMRC's online tool to check or contact HMRC directly. 

• Find out more: how to fill in a self-assessment tax return

2. Register as soon as possible

There are many dates and deadlines to watch out for throughout the year, but the most important is the cut-off for registering with HMRC. You have until 5 October 2025 to let the tax office know you need to start using self-assessment. 

The first thing to do is to set up a government gateway account if you don't already have one. Once that's sorted, registering for self-assessment is relatively straightforward. HMRC's online guide takes you through it step-by-step.

The sooner you get started, the better. After registering, you will need to wait for your 10-digit Unique Taxpayer Reference (UTR) number to arrive. That can take up to 10 working days if you're in the UK or 21 days if you're abroad. Without this number you won't be able to set up your online tax account. 

Once that's sorted, the next hurdle is to request an activation code for your account. That can take another 10 days to come through.

• Find out more: important deadlines for 2025 tax returns

3. Make a note of deadlines 

Registering with HMRC isn't the only date you need to remember. Make sure you mark the following in your calendar:

• 31 October 2025: If you want to file a paper return, make sure you get it to the tax office by Halloween. 
• 31 January 2026: You have until midnight on this day to get your online tax return to HMRC. This is also the date you need to pay your bill by. If you have been self-employed for a full year, you may be asked to start paying tax in advance through the ‘payments on account’ arrangement. The first instalment is due on this date, too.
• 6 April 2026: The first day of the new financial year is when any new tax rates and regulations announced in the Chancellor’s Autumn Statement and Spring Budget will come into force. It's also the first date that you can file your self-assessment for the previous financial year.
• 31 July 2026: If you're self-employed and use payments on account, this is the date when the second instalment is due. 

4. Get your paperwork together

Before making a start, get together all the relevant documents and information you need to complete the self-assessment form. 

That includes your P60 form (if you’re employed), relevant receipts and invoices, bills, bank statements, tenancy agreements, student loan statements, and details of any benefits you’ve received.

It's also a good idea to go through your current and savings account statements for the relevant financial year, and make a list of all the different places your money is coming from. That way, you'll have a better idea of which income sources you need to declare and which taxes may be due. 

• Find out more: how to calculate your tax bill

5. Don't forget allowances and expenses

You may be able to reduce your bill to HMRC by claiming various reliefs and allowances when you file your return. This can range from business trips to office running costs such as stationery. 

You can also claim tax relief on the cost of running a business premises, including energy. If your workplace is at home, then you can also claim a proportion of your bills for the time you are working.

The rules around what you can and cannot claim as expenses can be confusing. If you're in any doubt, HMRC's website has more information.

There are also tax breaks for higher-rate and additional taxpayers who made Gift Aid declarations when donating to charity, as well as tax relief on pension contributions using self-assessment.

If you’re declaring capital gains – after selling shares or an investment property, for example – you can reduce your tax bill by offsetting any losses made during the financial year you're filing for.

• Find out more: self-employed expenses

6. Seek help if you need it

If you don't think you can pay by the deadline of 31 January, get in touch with HMRC as soon as possible.

You may be able to set up a Time to Pay arrangement that lets you pay in instalments over an agreed period. You’ll pay interest on anything owed after the deadline, but at a lower rate than the late payment rate.

7. Use a tax calculator

If you're not a maths whizz, the GoSimpleTax calculator can tot up the tax bill for you. It also suggests expenses and allowances that you might have forgotten.

You can declare income from a range of sources, and even use the tool to submit your return directly to HMRC once you're satisfied that everything's been included.