Self-assessment tax returns are a crucial part of managing personal and business finances in the UK, yet they’re also a minefield of potential errors that can lead to HMRC inquiries, missed reliefs, or unexpected tax bills.

In her session, From Confusion to Clarity: Tackling Self-Assessment for Clients, Samantha Mitcham gave examples of some of the most common mistakes seen in practice and tips on how to avoid them.

1. Relying Solely on Last Year’s Data

One recurring mistake is comparing current year information with the previous year's return without conducting an API pull from HMRC. This tool allows software to retrieve real-time data directly from HMRC systems, catching discrepancies such as new employment sources or PAYE changes that the client may have forgotten to mention.

2. Overlooking Tax Code Notices

Tax codes offer insight into allowances and deductions that affect how much tax a client pays. If not checked properly, especially for codes deviating from the standard 1257L, underpayments or adjustments from previous years may be missed. Always request the most recent coding notice from clients and review it thoroughly.

3. Forgetting the High Income Child Benefit Charge

This often-overlooked charge affects clients or their partners earning over £50,000 who claim child benefit. Key to avoiding errors here is understanding the household income and having details such as children's dates of birth. A smart onboarding form can prompt clients to provide this information upfront.

4. Ignoring Student Loan Repayments

Whether it’s due to slow communication from the Student Loan Company or oversight during the review, this box is often left unticked. Ask clients annually whether a student loan is due, even if they were previously exempt.

5. Missing Additional Income Sources

Seemingly minor income such as bank interest, dividends, pension drawdowns, or capital gains can easily be missed. Without a comprehensive checklist, these can slip through the cracks. Encourage clients to disclose any new financial activity—even if it seems insignificant.

6. Assuming Payments on Account Have Been Made

Tax software may assume payments on account were paid, but clients often miss or partially pay them due to cash flow issues. Always check actual payments via the HMRC portal to ensure accurate balance calculations.

7. Not Reassessing Future Payments on Account

Clients’ circumstances can change dramatically—from going on maternity leave to reduced income. Discuss projected earnings for the next tax year to determine if payments on account need to be reduced, helping to avoid unnecessary payments or future penalties.

8. Overlooking Pension Contributions and Gift Aid

Private pension contributions and gift aid donations can increase a taxpayer’s basic rate band, resulting in reduced higher-rate tax. Clients often confuse workplace pensions with private ones, so be specific in your queries. Gift aid, in particular, is commonly misunderstood and underclaimed.

9. Failing to Account for Brought Forward Losses

Capital losses, sole trader losses, and finance costs can be carried forward, but software isn’t foolproof. Manually review previous returns to ensure nothing is left behind—this could significantly reduce a client’s liability.

10. Not Questioning Income Gaps

Treat the tax return like a jigsaw puzzle. If there’s a gap in employment or trading activity, find out why. This might lead to identifying benefits received, missed pre-trading expenses, or income that should be disclosed. Always seek a narrative to close unexplained periods.

Accuracy in self-assessment is more than just data entry—it requires inquisitiveness, systems, and communication. Building structured onboarding processes, using checklists, and making use of technological tools like HMRC API integration can significantly reduce errors. Most importantly, open dialogue with clients and an eye for the small details can make all the difference.

For the full session, please click here. In this course, Samantha Mitcham covers the following topics:

• What creates pressure and chaos during self-assessment season - and how to fix it
• How to standardise your process, from onboarding to filing
• Common errors when completing self-assessment tax returns
• How MTD will change self-assessment and what to do now to get ahead
• Practical advice on pricing, packaging, and positioning your self-assessment services

The contents of this article are meant as a guide only and are not a substitute for professional advice. The author/s accept no responsibility for any action taken, or refrained from, as a result of the material contained in this document. Specific advice should be obtained before acting or refraining from acting, in connection with the matters dealt with in this article. The information at the time of publishing was accurate and could be subject to final changes.