Filing your Income Tax Self Assessment (ITSA) can be a daunting task – whether it be your first time or your twenty-first, there is always room for error. In this article, we narrow down our top 5 common mistakes made when filling in these essential forms.

sitting meditatively with laptop

If you’re new to Self Assessment, why not also take a look at our top tips for financial record-keeping, and also familiarise yourself with HMRC’s ongoing Making Tax Digital initiative? For now, check out these top 5 ITSA errors!

Under-reporting income streams

HMRC is currently on a mission to reduce the so-called "tax-gap": the difference between the amount of tax that theoretically should be paid, and the tax that is paid in reality. A big part of this is due to preventable error in reporting income streams.

The UK-based Institute for Fiscal Studies conducted a revealing study in 2017, showing that 36% of Self Assessment returns included some level of under-reporting; this figure rose to 60% among the self-employed (sole traders).

With this in mind, don’t forget to declare the entirety of your:

  • Salary (including tips and bonuses if you are an employee)
  • Earnings from savings and investments
  • Overseas income and pensions
  • Income from rentals/holiday properties
  • Other relevant income streams (check HMRC guidelines thoroughly!)

Despite this frequent error, the tax gap in the year 2021–2022 fell to an all-time low (4.8%), and it is predicted to continue on a downward trend with the introduction of Making Tax Digital, scheduled to come into full effect for most taxpayers in 2026 (check our guide here to prepare).

Under-reporting allowable expenses

If you’re self-employed, then this one is certainly for you! We all know that businesses incur running costs. However, there are many cases in which these costs are exempt from taxation. Due to a lack of familiarity, confidence, or experience, many individuals overlook eligible expense claims, leading to an unnecessarily inflated tax bill.

It's advisable to research the allowable expenses you're entitled to or consult with a tax advisor or recognised authority such as AbraTax to ensure you don't overpay on your tax obligations. For now, though, here are some potential allowable expenses to consider:

  • Travel expenses – for example, fuel or public transport fares
  • Repairs and maintenance of business premises
  • Rent, utility bills, and property insurance for your business
  • Stationery – including postage, printing, and some computer software

Mistakenly marking wrong sections

Human error can always occur – particularly if you aren't familiar with the Self Assessment form or if you aren't diligent while filling it out. To avoid any errors, consult HMRC's guidelines on ITSA or seek specialised assistance from an HMRC-recognised third party (‘bridging service’) such as AbraTax.

Always verify that you're entering the most accurate information and have selected the correct options for you and your business before submitting.

Forgetting additional information

An easily achieved error in filing your Income Tax Self Assessment is failing to provide information on additional, less-common income streams, deductions, or tax reliefs. If you’re still filing your taxes in paper format, these are not included in the standard SA100 form for Self Assessment, but instead must be declared in a separate form titled SA101.

Additional income streams can include:

  • Earning interest on UK government bonds (‘gilts’)
  • UK tax relief after paying tax abroad
  • Business receipts taxed as the income of a previous year
  • Married Couple’s Allowance
  • Employment lump sums

If self-employed (e.g., running a tax-applicable side gig), don’t forget also to fill in a separate SA102S (for annual turnover below the VAT threshold) or SA103F (for annual turnover above the VAT threshold) if making your tax return offline.

working at desk

Not submitting online (or not using AbraTax!)

Arguably the most significant error in the Self Assessment sphere is not moving online – or at least not preparing to do so. Since HMRC announced the Making Tax Digital (MTD) scheme in 2015 (see our guide here), it has been a dead cert that we will all need to move our finances digital and submit tax returns online.

Although VAT returns are already exclusively online, the first hard deadline for Self Assessment is set to arrive in 2026. The benefits of filing your tax digitally are that it is simpler, less time-consuming, and less prone to error: who wants to be filling in paper forms for such an important task?

Furthermore, HMRC-recognised bodies such as AbraTax are literally designed to make your life easier: why not jump on the bandwagon – you’ll have to jump on soon, anyway!

Disclaimer: We aim to offer educational articles on our blog, focusing on tax-related topics. However, it's important to note that over time, the relevancy of this content might diminish, and we cannot guarantee accuracy. While these articles serve as a tool for enhancing tax knowledge, they are not a replacement for expert advice in accounting, taxation, or legal matters, given the unique nature of each individual's situation. Should you require personalized assistance, we encourage contacting HM Revenue and Customs (HMRC).