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Get started10 tax return mistakes to avoid this January
More than 12 million people are expected to file a self-assessment for the 2024-25 tax year
With the 31 January deadline looming, more than 5.6 million people have yet to file their self-assessment tax returns.
According to HMRC, more than 6.36 million people have already submitted their returns, with 91,500 doing so over the festive period. For everyone else, the final rush is now underway.
Unfortunately, last-minute number-crunching can lead to mistakes, which could see you paying more tax than you need to – or landing yourself with a fine from HMRC.
To help, Which? has highlighted 10 common self-assessment errors and how to avoid them.
1. Failing to submit by the deadline
The self-assessment deadline is 31 January, and you must submit your return by 23:59 to avoid a penalty.
Unless you have a ‘reasonable excuse’ or an agreed a delay with HMRC, the penalties for filing late are:
- One day late: £100 fine
- Up to three months late: £10 a day, capped at 90 days (plus the initial £100 fine)
- Six months late: £300 or 5% of the tax due, whichever is higher
- 12 months late: £300 or 5% of the tax due, whichever is higher
After you see how much tax you owe, you need to click the ‘submit’ button. It's easy to forget, but your forms aren't filed until you do this. You'll know you're finished when you get a confirmation email, so always check your inbox to confirm you received it.
- Find out more: how to fill in a self-assessment tax return
2. Not paying tax on time
As well as filing your tax return, you must also pay any tax you owe by 31 January.
Your tax bill doesn't come out automatically. There are several ways to pay your tax bill, but timing matters – Faster Payments, Chaps and debit or business credit card payments usually reach HMRC the same or next day, while Bacs and cheques can take several days. HMRC doesn't accept personal credit cards.
If you owe a relatively small amount of tax and are employed or receive a pension through PAYE, HMRC may be able to collect what you owe through your tax code instead, spreading the cost across future pay packets.
If you pay your tax late, HMRC will charge interest from the date the payment was due. This is currently 7.75%, based on the Bank of England base rate plus four percentage points.
You could also face additional penalties:
- After 30 days: 5% of the tax outstanding
- After six months: a further 5%
- After 12 months: a further 5%
If you can’t pay the full amount by the January deadline, then you can set up a payment plan online. You can do this if:
- you have already filed your tax return
- the deadline was less than 60 days ago
- you owe £30,000 or less.
Find out more: income tax calculator
3. Ignoring pension contributions
When filling out your self-assessment, you need to remember to include any pension contributions you've made. If you're a basic-rate taxpayer, you get 20% tax relief on the money you put into the pension pot, and your pension provider usually claims the relief automatically.
If you're a higher or additional-rate taxpayer, you can claim 40% and 45% respectively. Your pension provider will claim 20% for you, but you'll need to claim the other 20% or 25% through self-assessment.
If you don’t include it, you’ll miss out on the relief and pay more tax than you need to.
- Find out more: pension tax calculator
4. Not declaring income or capital gains
All taxable income needs to be declared as part of self-assessment; this means you’ll need to report all sources of income you have. This can include dividends from investments, rent from buy-to-let property and interest on cash savings.
However, any interest or investment returns through an Isa are tax-free, so you don’t have to declare these.
With capital gains, you’ll need to declare them on your tax return if your gains for the year (before deducting any losses) are more than the annual exemption amount. This sits at £3,000 for the 2025-26 and 2024-25 tax years.
If you're already registered for self-assessment, you must report it if the total amount you sold an asset for was more than £50,000 – even if your actual profit was less than £3,000.
There's a special process for capital gains tax for 2024-25, because the rates changed partway through the year. This is explained on the government website.
If you fail to declare any income or gains when submitting your tax return, you could be stung with a penalty.
- Find out more: how to calculate your tax bill
5. Not being aware of ‘payments on account’
When you pay your tax, you might have to pay for next year at the same time. This is called ‘payments on account’.
If this is your first time filing, your bill on 31 January might be larger than you expect. This is because you’ll need to pay your full tax bill for last year, plus half of your estimated bill for the next year, too. You’ll then pay the other half by the end of July.
If you think you will earn much less money this year, you can ask HMRC if you can pay a lower amount on account.
- Find out more: tax reliefs – how to reduce your tax bill
6. Getting your tax code wrong
Your tax code affects how much tax is taken from the income you receive through PAYE, such as wages or a pension.
If your tax code is wrong, the ‘tax already paid’ figures you enter on your self-assessment return could be inaccurate. This could leave you paying too much tax or owing more than you expected.
It’s your responsibility to check that your tax code is correct. If you spot a mistake, contact HMRC as soon as possible.
- Find out more: tax codes and what they mean
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Join Which? Money7. Forgetting to declare charity donations
Giving money to charity can help to cut your tax bill, so you should mention it on your self-assessment tax return. Under Gift Aid, the charity gets extra money from the government when you donate.
For every £100 you give, the charity can claim an extra £25. If you pay a higher rate of tax, you can also claim additional tax relief for yourself through self-assessment. This means the charity receives more, without it costing you any extra.
However, you’ll need to have paid enough tax that year to cover what the charity is claiming back. If you forget to include Gift Aid donations, you’ll miss out on the tax relief.
- Find out more: online tax returns explained
8. Submitting incorrect figures
Mistakes such as mistyped numbers or miscalculations are common, particularly when people rush to meet the deadline.
If you realise you’ve made an error after submitting your return, you can amend it online. You usually have until 12 months after the filing deadline to make changes. HMRC will then recalculate your bill and either ask you to pay more tax or issue a refund.
Penalties for inaccuracies depend on the circumstances. If you took reasonable care and made an honest mistake, you’re unlikely to be fined. Penalties are more likely where errors are due to carelessness or deliberate under-reporting.
9. Poor record keeping
If you’re self-employed, you can usually reduce your tax bill by claiming allowable business expenses. These are costs incurred wholly and exclusively for your work.
You need to keep records and evidence of these expenses. Without them, you could miss out on relief and end up paying more tax than necessary.
By law, you must keep records for at least five years after the 31 January deadline for the relevant tax year. Many advisers recommend keeping them for around six years to be safe.
- Find out more: late tax returns and penalties for mistakes
10. Overlooking the child benefit tax charge
Under the high income child benefit tax charge (HICBC), if you or your partner earns more than £60,000 a year, you have to start repaying child benefit. The charge is tapered, so the more you earn, the more you need to pay back. Once your income reaches £80,000, you have to repay the full amount.
Previously, the only way to pay the charge was through self-assessment, even if you were employed. However, this changed in September 2025, and some people can now repay it through PAYE instead.
You can use the new service if you’re employed and don’t need to file a self-assessment return for any other reason. If you do need to submit a return – for example, because you have taxable savings interest – you’ll still need to pay the charge through self-assessment.
If you’re eligible, you’ll need to opt in to have the charge collected through PAYE. If it’s your first time paying, you can sign up using the Gov.uk website. If you already pay through self-assessment but want to switch to PAYE, you’ll need to contact HMRC to make the change.
You have until 31 January 2026 to opt in for the 2024-25 tax year, which ended on 5 April 2025. Once you opt in, HMRC will change your tax code so the charge is taken automatically from your salary each month.
If you opt in partway through the year, the remaining payments will be spread over the months left until 5 April, meaning your monthly deductions will be higher.
