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If you’re self-employed or have other undeclared income, such as from renting out property, you’ll need to submit Self Assessment tax returns. This can be a little daunting, especially if you haven’t done one before, and the fact that mistakes can lead to hefty fines and penalties, even if the mistake was totally innocent.

Unfortunately, errors on tax returns can be all too easy to make, so in this guide we go over some of the most common ones that we see (and how to avoid them or put things right).

Missing tax deadlines

Forgetting about Payments on Account

Misplacing your Unique Taxpayer Reference (UTR) number

Not claiming allowable expenses

Incomplete or missing information

Entering the wrong figures

Not declaring payments into your pension

Forgetting to include supplementary pages

Not keeping proper records

Not learning lessons for next year

What should I do if I make a mistake on my tax return?

There are certain fixed deadlines throughout the year you need to be aware of here. Most people file their tax return online, but some people still go down the paper route.

So, a tax return for the 2022/23 tax year must be submitted online by 31st January 2024, but a paper return should be submitted by 31st October 2023.

If you don’t submit your tax return before the deadline, HMRC will consider it to be a late submission, and penalties will apply.

Our tip? Set reminders! In several different places if you need to. As well as helping you to get your return in on time, there are other benefits for submitting sooner rather than later.

If your tax bill is more than £1,000, you’ll need to make an advance payment towards next year’s bill. Known as making Payments on Account, the advance payment is worked out by halving this year’s bill, so it can be a pretty nasty shock if you weren’t expecting it.

Read our blog to learn more about Payments on Account.

When you sign up for Self Assessment, HMRC will send you a Unique Taxpayer Reference (UTR) number. You need this to complete your tax return in order for HMRC to identify your tax records correctly.

Your UTR should be kept safe, but don’t worry if you lose it! It’s normally shown on previous tax returns, or you could sign in to your Personal Tax Account. If you’re still stuck, use HMRC’s general enquiries page to get in touch for a reminder or a replacement.

Businesses pay tax on the profits that they make, not the total income, so deducting allowable business expenses ultimately lowers your tax bill.

Some people are nervous about claiming tax relief on their expenses, or they’re not too sure what they’re allowed to include. Do some research, or check in with an accountant or reliable advisor, just don’t miss out!

Good record-keeping is essential for filling out your tax return accurately, but we all know that life happens and sometimes things get lost.

Obviously, the best solution is pre-empting this with robust bookkeeping processes (or a time-machine to nip back before that receipt goes in the washing machine). Failing that, don’t give up hope.

You may be able to submit a tax return using provisional figures.

If your paperwork turns up later, you can adjust your tax return for up to one year after the submission deadline has passed. Just be aware that this is definitely a last resort though.

Forgetting to sign and date your paper tax return

Completing your tax return online is the quickest, easiest, and most secure way of doing things. If you do decide to submit your tax return on paper, don’t forget to sign and date it before submission.

It sounds obvious, but many people forget to do this every year and it’s a simple mistake to make. A photocopy won’t work either, it needs to be original.

Make sure you enter in all your numbers correctly, as it’s so easy to make a mistake or even a simple typo. While HMRC won’t usually issue a penalty if you’ve taken ‘reasonable care’ in completing your return, it really isn’t a risk worth taking. Those typos can get expensive!

When you complete your tax return online, many of the calculations will be done for you, but you’ll still need to double-check everything – final responsibility lies with you, even if you use an accountant.

If you make any private pension contributions during the tax year, these need to be declared too. Not only is this key information for your tax return, but you’ll also get tax relief on any contributions that you make.


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Any untaxed income that hasn’t been declared in your main tax return needs to be included in supplementary pages. This would include things like:

This is just a small list of the most common ones and isn’t exhaustive. Again, these are quite complex topics which is where an accountant can really help.

Keeping up-to-date, accurate records is essential. It makes it far easier to complete your tax return correctly, and you can use this data throughout the year to monitor your finances and make more effective business decisions.

We go into more detail about records you might need for Self Assessment in our Tax Return Checklist, or take a look at our bookkeeping guide to learn more about keeping financial records.

Your tax return is finally done, and it’s so tempting to simply kick back and forget about it until next year. But there are still things you can do to make the process that bit smoother next time (you’ll thank yourself later).

First off, consider putting any investments into an ISA. This can be more tax-efficient, plus you won’t need to declare it as UK income or pay Capital Gains Tax next time round.

Think about upping your pension payments too if you can. This could help reduce your tax bill in addition to making things financially easier later in life. Bear in mind though that pension rules can change often so it all depends on your circumstances. Generally, you won’t be able to access any money you’ve contributed to your pension until you reach 55 (57 from 2028). It’s worth seeking advice from a regulated financial advisor if you’re not sure.

One final tip for next year is to consider putting all your pensions, savings and investments together in the same place. It keeps things simple with less paperwork and can make it easier to see where tax efficiencies can be made.

However, you may need to pay exit fees or lose certain benefits in doing this, so again it’s worth speaking to a professional before you decide. Everyone’s situation is slightly different here, so don’t attempt to go it alone if you’re not clear. But a bit of advance preparation before your next tax return is due can go a long way.

Any mistakes on your tax return can be corrected, usually up to 12 months following the submission deadline. It’s known as ‘making an amendment’.

For example, you have until 31st January 2024 to file your tax return online for the 2022/23 tax year. Once you hit submit, you can then make an amendment up until 12 months afterwards – so 31st January 2025.

Just be aware that if you do make any errors, you might need to pay interest if you don’t pay enough tax. It’s best to act quickly on this!

The fact is that filing tax returns isn’t the most fun activity in the world but getting it right is really important. That’s why hiring an experienced accountant makes sense, even if you’ve filed several times before. Not only can accountants give you that extra peace of mind, but they can also find ways to (legally) reduce your tax bill as far as possible. Because let’s face it, in these tricky financial times nobody wants to pay more than they have to.

Getting your tax return right first time can save you a serious financial headache later on. Talk to one of our team and call 020 3355 4047, and get an instant online quote.

About The Author

Dean Salmon

I'm an AAT and ACA qualified Chartered Accountant with over 13 years experience working with businesses, contractors and sole traders. I also love watching live music, and quizzes! Learn more about Dean.

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