Self Assessment Tax Return Errors

You breathe a sigh of relief upon clicking ‘submit’ on your Self Assessment. But then later down the line you discover it contains an error… Or worse – multiple mistakes. With the rush to get it…

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Last Updated: 29th March 2022

You breathe a sigh of relief upon clicking ‘submit’ on your Self Assessment. But then later down the line you discover it contains an error… Or worse – multiple mistakes. With the rush to get it completed, inaccuracies are common amongst taxpayers.

To avoid this, it’s always worth learning from the blunders of others. Here, we’ve gathered the most frequent tax return errors. Take note, and not only will your Self Assessment be mistake-free, but you’ll be able to ensure you pay no more than you owe.


Sometimes people are shocked by their total bill, but they haven’t actually taken advantage of all the ways to be tax-efficient. Claiming on expenses is often overlooked, for example. Small expenditures such as travel and marketing can add up to a lot, and offsetting these means you can significantly reduce the amount of tax you need to pay.

Others do the opposite. With such a wide variety of expenses, they end up trying to claim on ones that aren’t permitted, and so their calculations are too low. There are specific rules around what is classed as allowable – knowing these before you submit the Self Assessment will come in handy.


Another common error is putting the tax return to the back of your mind completely. When it comes to the submission date, this means scrambling around trying to find the necessary information. When this can’t be recovered, vital data is missing and that leads to inaccurate calculations.

Even those who have kept all their records might face issues. If receipts have simply been left and exposed to sunlight, then they may have become discoloured and the numbers may be unreadable. HMRC need this proof, so avoid this situation by simply snapping photographs of your records at the time they were issued.


Perhaps it’s not a case of a lack of records, but that some information is just not being relayed to HMRC. Depending on your circumstances, there may be specific details that you need to put in your tax return – if you have other sources of income, for example. Neglecting to declare that you receive foreign income is actually a criminal offence.

You should also tell HMRC if your business expenses consist of any private use. If you use a vehicle, the personal usage should show an adjustment to the amount claimed, otherwise they might deem it suspicious.


Of course, you need to ensure that the details you do provide are accurate. Perhaps a figure has been entered into the wrong box, causing incorrect calculations and either under or overpaying on tax. Or maybe you’ve made estimates instead of working out the sums.

Understanding the nitty-gritty of tax isn’t straightforward for those who aren’t experts in this area, but HMRC don’t take kindly to false information – it needs to be in full, with evidence to prove it.

It is worth noting that, if you do realise you have made a mistake, you can submit an updated tax return to HMRC with the correct figures. Any changes must be submitted within 12 months of the original filing deadline. So, for the 2020/21 tax return, amendments must be given to HMRC before 31st January 2023. Please note that if the correction results in you needing to pay extra tax, then this must be made immediately and you may be charged interest.


If you choose the right Self Assessment software, then the incorrect information problem is alleviated. GoSimpleTax works out the calculations so you don’t have to get your head around them, and it offers helpful tips too. You’ll be able to reduce the tax you owe with saving recommendations, and know that you’ve entered all the correct details.

With our tax return software, you can complete the Self Assessment swiftly. This means you can avoid making the ultimate mistake: not filing it on time, and being subject to HMRC’s penalty system.

Take advantage of GoSimpleTax today with a free trial.

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Blog content is for information purposes and over time may become outdated, although we do strive to keep it current. It's written to help you understand your Tax's and is not to be relied upon as professional accounting, tax and legal advice due to differences in everyone's circumstances. For additional help please contact our support team or HMRC.

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