When is the tax return deadline? Why leaving it until January could cost you time and money

Ercan Demiralay FCCA explains when you need to file your tax return, the necessity for preparation, and the pitfalls of leaving it till January.

Do you complete a self assessment tax return each year?

Or have you recently:

• Started a business or side hustle? 
• Taken dividends for the first time?
• Earned income outside of Pay As You Earn (PAYE)?

If so, there’s one date you need to be aware of and it's also one you really shouldn't leave until the last minute.

31 January is the official deadline for submitting your tax return and paying what you owe. 

Given the end of the tax year is 5 April, 9 months may sound like a lot of time to submit it, but the reality in our experience is that a lot of people leave it until December or January. That's often where the problems start. 

This can result in:

• Unexpected tax bills
• Potential missed opportunities to reduce your liability
• Unnecessary stress in the lead up to the deadline

That's why we've written this blog post, to help you understand:

• When you need to prepare and file
• What happens when you leave it too late
• The process and how to stay ahead of it

Quick links to sections covered in this blog post include:

1. When is the UK tax return deadline? 

If you're completing a self assessment tax return in the UK, the deadline for this is 31 January following the end of the tax year. 

The 2025/26 tax year ended on 5 April 2026, so this means the tax return submission deadline is 31 January 2027. 

2. Where many people can go wrong

People know the deadline but end up leaving a lot of work until January. We've come across this many times with new clients and we often hear the same things:

'I didn't realise it would be this much.' 

'I wish I'd started earlier.'

In fact the latest Making Tax Digital for Income Tax Self Assessment initiative means it's more important than ever to record income, transactions, and expenses as they occur through the appropriate software. 

3. The key tax return deadlines you need to know

It's important to understand that filing a tax return is a process which means there's more than 1 important date. Be sure to note the following for your diary:

4. What happens if you miss the filing deadline? 

If you fail to file on time, or don't pay on time then things can get expensive quickly. 

• An automatic penalty of £100 (even if not tax is due)
• Additional penalties applied after 3, 6, and 12 months
• Interest charged on the unpaid tax

For many people it's not just the issue of fines and penalties, it's the stress and lack of control over the situation that arises from missing the deadline. 

5. Why leaving your tax return until January causes problems

The 9 month gap between the end of the tax year and the tax return deadline feels like plenty of time doesn't it?

However, we find that thinking is what tends to lead to things potentially going wrong. If it leads to you not starting the work until January then this risks:

a) You not knowing what you owe

Only completing your return in January means you'll find out your tax bill at the equivalent of the last minute. When this happens we can see £10,000 and £20,000+ bills with no time left to plan or budget for the amount owing.

b) You lose out on potential opportunities to reduce your tax bill

Tax planning can't really happen in January, it's just too close to the deadline with too little time left to co-ordinate and plan effectively. So by that point most of the options available to you will likely have passed. 

c) Mistakes become more likely

If you're in a rush to meet a deadline then this can easily lead to errors. If you submit a return with errors then this risks:

• Triggering HRMC enquiries
• Leading to penalties and fines
• Costing you time and money

d) Being reactive, not proactive

Think of it like this, it's a mindset matter. If you're leaving things close to the deadline then you're not really in control are you? Instead you're reacting under pressure. 

6. An alternative, better way to approach your tax return

We find that the clients and people who handle this best work with us early on and don't wait for January. They treat the tax return as exactly what it is, a process and not just a deadline. 

This then means:

• Being organised early on
• Understanding your position and tax liability in advance
• Planning for making payment

7. When should you start the tax return process?

Definitely not January. Ideally you'll start as soon as the tax year ends in April. This is especially the case if your financial affairs are complicated. As a general rule, the earlier you start:

• The more control you have over the process
• The more tax planning options you'll likely have
• The fewer surprises you're likely to face

8. An example and the real deadline

Maybe you've had a strong year culminating in higher profits in your business. You then extract this out of the company through more dividends than previous years. Let's also say that you also sold a second property in that same tax year. 

If you wait until January you'd likely discover a tax bill of potentially £15,000 or more with just a few weeks to pay it. But if you'd started earlier then you could've planned, budgeted, and maybe even reduced the tax bill.

This goes to show that the official deadline may be 31 January but in reality it's much earlier if you want to:

• Avoid stress
• Stay in control of the process
• Potentially reduce your liability

9. What should you do next?

If you haven't started thinking about your tax return yet then now is the time!

Ultimately the biggest problems we see don't come from people ignoring things, rather them thinking they had more time. 

The content of this post was created on 25/01/2017 and updated on 07/05/2026.

Please be aware that information provided by this blog is subject to regular legal and regulatory change. We recommend that you do not take any information held within our website or guides (eBooks) as a definitive guide to the law on the relevant matter being discussed. We suggest your course of action should be to seek legal or professional advice where necessary rather than relying on the content supplied by the author(s) of this blog.