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

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 use the paper forms; they have different deadlines for submission!

• The deadline for submitting a paper return is 31st October following the end of the tax year which it relates to
• If you’re filing online, the deadline for submission is 31st January following the end of the tax year

So, a tax return for the 2025/26 tax year must be submitted online by 31st January 2027, but a paper return should be submitted by 31st October 2026.

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.
 

Forgetting about Payments on Account

You’ll need to make advance payments towards the following year’s bill if the amount you owed for the year before it was more than £1,000. Known as making Payments on Account, the advance payments can be a pretty nasty shock if you’re not expecting them.

Misplacing your Unique Taxpayer Reference (UTR) number

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.

Not claiming allowable expenses

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!

Incomplete or missing information

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.

 
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.

Entering the wrong figures

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.

Not declaring payments into your pension

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.

Forgetting to include supplementary pages

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:

• Income from share schemes or property
• A life insurance pay-out
• Earnings from overseas that you’ve already paid tax on, or that aren’t taxable in the UK
• Post cessation receipts
• Loss relief claims
• Certain employment deductions
• Stock dividends, non-qualifying distributions, or close company loans written-off
• Compensation payments or other lump sums from your employer
• Information about any bonuses or perks which are classed as taxable benefits in kind
• Taxable lump sums from pension schemes based overseas
• Age-related Married Couple’s Allowance claims
• Other tax reliefs that haven’t already been included

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.

Not keeping proper records

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.

Managing pensions and investments

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.

Increasing your pension payments

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.

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

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 2027 to file your tax return online for the 2025/26 tax year. Once you hit submit, you can then make an amendment up until 12 months after the deadline (the deadline, not the date you submitted) – so 31st January 2028.

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

 
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.