Self-assessment deadline looms: file your tax return now and avoid £100 fine 

4 January 2025

As the deadline for online self-assessment tax returns approaches at midnight on 31 January 2025, there’s still time to file and pay any tax you owe for the 2023/24 tax year. 

In 2024, 11.5 million people submitted their self-assessment returns on time, with 778,068 people filing on 31 January, according to HMRC. However, an estimated 1.1 million people missed it. 

While tax returns can be stressful, it’s vital to be prepared and file on time to avoid costly penalties and for peace of mind. 

If you haven’t completed your self-assessment tax return yet, there’s still time and these handy tips from Unbiased, the UK's leading platform for financial advice enquiries, can help. 

Tips to help you submit your tax return on time 

1. Do I need to file a self-assessment tax return? 

While millions need to file a tax return in the UK, you may not necessarily be one of them. 

For example, those who are self-employed, have more than one job or those accessing their pension while working may have to complete a tax return, but this isn’t a comprehensive list. 

If you’re unsure whether to complete a self-assessment tax return, use this handy tool to find out.  

2. Have your documents ready 

It’s a good idea to keep your financial documents and records in a safe place, such as a dedicated filing cabinet. You’ll need to have your P60 or P45, a P11D (if you had one) and your national insurance number. 

You also need records of your income, including earnings and income from other things such as rent, investments and interest, as well as records of any expenses. It’s worth stressing that you must file a self-assessment return if you exceed your annual trading allowance of £1,000.  

Also, make sure you have your unique taxpayer reference (UTR), as HMRC will need this – if you haven’t registered as self-employed, you must do this to receive your UTR.  

3. Set aside time for your tax return 

It can be tempting to put off completing your self-assessment return, but it’s important that you don’t, as filing it late or with errors can result in a penalty from HMRC. 

It’s worth setting aside some time every year to review your documents and income over the previous tax year. You then have time to ensure everything is correct and query anything before filing your tax return. 

4. Avoid common mistakes 

There are many mistakes that can be easy to make and are worth avoiding. 

For example, always ensure you tick the right boxes when completing your tax return, and make sure you claim any allowable expenses you’re entitled to. 

If you’re unsure if you can claim any allowable expenses, it’s a good idea to do your research or consult a qualified tax accountant.  

5. Don’t forget about pension tax relief 

If you’re a basic-rate taxpayer, you automatically get 20% tax relief. 

However, if you’re a higher or additional-rate taxpayer, you may need to claim the extra 20% or 25% via your tax return, which can be repaid via a rebate, change in tax code or a reduced tax bill. 

You can claim pension tax relief for up to four previous tax years.  

6. Are you eligible for the marriage allowance? 

The marriage allowance allows you to transfer £1,260 of your personal allowance to your husband, wife or civil partner if you earn under £12,570 a year. 

This can reduce their tax bill by up to £252 in a tax year, and you can backdate your claim to 5 April 2020. You can apply via self-assessment if you’re already registered and have sent tax returns, or online or via post. 

7. Consult an accountant 

If you need support with your self-assessment tax return, consider using a qualified accountant. 

An accountant can help you file your tax return and claim any allowances you’re eligible for, as well as reduce the likelihood of any errors and avoid a late submission.  

Unbiased quickly connects individuals with qualified accountants, financial advisers, mortgage and insurance brokers. 

What if I make a mistake on my tax return? 

If you make an error on your self-assessment tax return, you can update it within 12 months of the deadline. 

This may impact your tax bill, so you may have to pay more or request a refund.  

What if I cannot pay my tax bill on time? 

If you cannot pay your tax bill on time, you can set up a payment plan, provided you have filed your latest tax return, owe £30,000 or less, are within 60 days of the payment deadline and don’t have anything outstanding with HMRC.  

“Filing your self-assessment tax return can be stressful, but it doesn’t need to be,” says Karen Barrett, chief executive and founder of Unbiased.   

“By ensuring you have all your documents ready and setting aside time beforehand, and claiming any allowances you’re eligible for, you can take the stress out of your self-assessment return. 

“However, if it’s your first time filing your self-assessment or your finances are complex, it’s worth considering consulting a qualified accountant.”