7 Self Assessment Tax Mistakes To Avoid

Sophie Graham, a Personal Finance Expert at Sunny, says: “Self-assessment can feel overwhelming, especially if you only deal with it once a year. Most mistakes aren’t about doing anything wrong on purpose, but about missing details or leaving things too late. Taking a bit of time to prepare and double-check your return can make the whole process far less stressful.”

1. Leaving your tax return until the last minute

“Procrastination is one of the biggest causes of self-assessment stress. Leaving everything until January increases the risk of errors, missing information or technical issues. Filing earlier gives you time to gather documents, ask questions and make corrections if needed. It also means the task isn’t hanging over you for weeks. Submitting early can also bring peace of mind, knowing your return is already taken care of.”

2. Forgetting to declare all sources of income

“It’s easy to focus on your main income and forget smaller or irregular amounts. Side hustles, freelance work, or rental income all need to be declared. Missing income can lead to unexpected tax bills or penalties later on. Keeping a simple record throughout the year can help avoid this. Having everything in one place makes completing your return much more straightforward.”

3. Missing out on allowable expenses

“Many people pay more tax than necessary by not claiming legitimate expenses. Costs such as professional fees, office supplies or a portion of household bills for those working from home may be allowable. Understanding what you can and can’t claim makes a real difference. If you’re unsure, checking HMRC guidance or seeking advice can be worthwhile. Claiming correctly ensures you’re not paying more tax than you need to.”

4. Making simple calculations or data entry errors

“Small mistakes like entering the wrong figures, using incorrect dates or mixing up gross and net amounts are surprisingly common. These errors can delay processing or trigger follow-up queries from HMRC. Taking time to review your return carefully before submitting can prevent avoidable problems. A second look is often all it takes. Using accounting software or HMRC’s online checks can also help reduce mistakes.”

5. Forgetting to budget for the tax bill

“Submitting your return doesn’t mean the cost is taken care of. Many people underestimate how much they’ll need to pay or forget to set the money aside. Planning for the bill in advance helps avoid a last-minute scramble. Spreading savings across the year can make payments far more manageable. This approach can also prevent the need to rely on credit to cover the cost.”

6. Overlooking payments on account

“Payments on account often catch people out, especially if it’s their first time dealing with them. These advance payments towards the next tax year can significantly increase what’s due in January. Understanding whether they apply to you helps prevent surprises. Factoring them into your planning makes cash flow easier to manage. Reviewing previous tax bills can help you anticipate whether payments on account are likely.”

7. Missing the deadline altogether

“Failing to submit your return or pay your tax on time can lead to automatic penalties. Even if you can’t pay the full amount straight away, submitting your return by the deadline is crucial. HMRC offers payment plans for those who need them. Acting early gives you more options and peace of mind. Communicating with HMRC sooner rather than later can help limit additional charges.”

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