3 June 2025 –
Filing your self-assessment tax return can feel like a daunting task, especially when you’re juggling the day-to-day demands of running a business or managing personal finances. While some mistakes are harmless, others can result in penalties, overpayments, or missed opportunities for tax savings.
Below are just five examples of some of the mistakes that can be made when submitting a tax return—and what you should be thinking about to avoid these in future.
1. Missing the Filing Deadline
The deadline for filing your online self-assessment tax return is 31 January following the end of the tax year. For example, for the tax year ending 5 April 2025, the deadline is 31 January 2026. (If you are filing a paper return, the deadline is three months earlier).
Late submissions can result in automatic penalties, even if you don’t owe any tax.
Useful tips:
Set reminders in your calendar well in advance, and aim to complete your return as soon as possible after 5 April, while the year is still fresh in your mind.
If someone else is preparing your return on your behalf, make sure that you provide any information they need promptly, and to the required level of detail.
Submitting your tax return early will also provide you more time to pay any tax owed, by the 31 January.
2. Not Including All Income Sources
Business owners and landlords may be aware that they need to report their self-employment or property income via self assessment, but many don’t realise they need to report other taxable income sources—such as:
- Employment income
- Job seekers allowance
- Pension income
- Bank interest
- Dividends
- Capital gains
- Foreign income
Useful tips:
Think of your tax return as a complete financial snapshot of the year, not just a report of your main source of income.
Include sources of income even when tax has already been deducted, such as through PAYE.
3. Not claiming for Use of Home
If you work from home, you may be entitled to claim for household bills, and possibly rent or mortgage interest.
A flat rate or a “reasonable” method of allocation can be used to calculate the total claimable amount.
If not using a flat rate, the use of home calculation is not always straightforward and can have unintended tax consequences if not thought through.
Useful tips:
If you own your home, and use a room exclusively for business, this could result in you paying a higher amount of Capital Gains Tax in the event of a house sale, so it may be beneficial to use the room for personal and business use.
Bear in mind that company directors and employees can’t claim back any proportion of rent, mortgage interest, or council tax from their companies.
Consider speaking to an Accountant to take care of the calculation for you.
4. Missing out on Tax Relief
Higher or additional rate taxpayers may be able to claim tax relief where they have donated to a UK charity, or paid into a personal pension plan.
Remember donations could include contributions made towards fundraising events, as well as unwanted clothes or items given to charity shops.
Useful tips:
If your total earnings are > £50,270, download your bank statements in excel, and search for any charitable donations paid, or pension contributions made.
Search for emails received from charity shops confirming how much your donated goods were sold for.
Double check that the charity is UK based and registered with the Charity Commission or another regulator if applicable.
5. Paying too much Tax during the Year
If you are in self assessment, you may need to make payments on account towards the next tax year, in January and July, unless the amount of tax you owed was less than £1,000 in the previous tax year, or you paid more than 80% of tax outside of self assessment (such as through PAYE).
Payments on account will generally be calculated as half of the tax you owed last year through self assessment.
However, if you know that your income in the next tax year is going to be lower than last year, for example because you have ceased your sole trader business, or sold your rental property, you can apply to reduce your payments on account.
Useful tips:
If you think your self assessment tax liability will be lower in the next tax year, you should calculate what you think it will be, and apply to reduce your payments on account accordingly. This can be done either within the self assessment tax return itself, or online at a later date.
You must claim by 31 January after the end of the tax year.
If you end up making higher payments on account than needed, rest assured this will be taken into account within your next self assessment tax calculation, and any overpaid tax will be refunded.
Need Help with Your Tax Return?
Whether you’re a sole trader, landlord, company director or someone with more complex tax affairs, Zip Accounting can help take the pressure off. From making sure your return is accurate, to identifying ways to save tax—we’ve got you covered.
👉 Get in touch today to see how we can help.
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