Filing your first self assessment tax return can feel daunting. For many people, it’s the first time they’ve had to report income themselves, without an employer or payroll system doing it for them.
It’s also why self assessment mistakes are so common, particularly among first-time filers. The rules aren’t always obvious, the terminology can be confusing, and HMRC doesn’t always explain why something is required, just that it is.
The good news is that most mistakes are completely avoidable once you know what to look out for.
Below are the most common self-assessment errors first-time filers make, why they happen, and how to avoid unnecessary stress, penalties, and unexpected tax bills.
1. Not Realising You Need to File at All
One of the biggest HMRC issues we see is people simply not realising they were required to submit a tax return.
This commonly affects people who:
Start freelancing or self-employment
Earn side income alongside PAYE work
Receive rental or overseas income
Because tax isn’t deducted automatically, HMRC expects you to report it yourself.
If you’re unsure whether Self Assessment applies to you, this guide on whether you need to file a Self Assessment tax return breaks it down clearly.
2. Leaving Registration Too Late
Before you can file a tax return, you must be registered for self assessment, something many first-time filers don’t realise until it’s nearly deadline day.
Late registration can lead to:
Missed deadlines
Rushed or incomplete returns
Avoidable penalties
HMRC deadlines come around quickly, especially in January, when panic searches peak.
3. Missing Income (Without Realising It)
Many first-time filers assume only their “main” income counts.
In reality, all taxable income must be declared, including:
Side hustles
Freelance or gig work
Online selling
Small additional earnings
Missing income, even unintentionally, can trigger HMRC follow-ups later.
This often links directly to unexpected tax bills, which we covered in our guide on why tax bills end up higher than expected.
4. Not Understanding What Expenses You Can Claim
Another common self-assessment mistake is either:
Not claiming any expenses at all
Claiming expenses that aren’t actually allowable
First-time filers often worry about “getting it wrong”, so they underclaim, which means paying more tax than necessary.
Others unknowingly include personal costs that HMRC doesn’t allow.
Understanding what qualifies as a legitimate business expense makes a significant difference to your final tax calculation.
5. Forgetting About Payments on Account
Payments on Account catch many people off guard in their first or second year.
Instead of just paying what you owe for the year, HMRC may ask you to:
Pay part of next year’s tax in advance
This often leads to confusion, especially when the bill looks much higher than expected.
6. Relying on Estimates or Guesswork
When records aren’t properly kept, it’s tempting to “roughly estimate” figures.
This can cause problems if:
Income is understated
Expenses are overstated
Figures don’t align with bank records
HMRC expects accurate reporting, and inconsistencies can raise flags later.
7. Missing the Deadline (Or Filing at the Last Minute)
January is the busiest month for self assessment, and filing late dramatically increases the risk of errors.
Late submission can result in:
Automatic penalties
Interest on unpaid tax
Unnecessary stress
Even if you can’t pay immediately, filing on time is always better than missing the deadline altogether.
8. Misunderstanding the Tax Calculation
Many people submit their return correctly, but don’t fully understand the tax calculation HMRC produces.
This can make it hard to spot:
Errors
Adjustments from previous years
Student loan repayments
Payments on Account
Which is why people often only question things after receiving a large bill.
9. Not Checking HMRC’s Final Calculation
HMRC can amend returns or apply corrections after submission.
If you don’t review the final calculation carefully, you may miss:
Corrections
Changes to allowances
Additional charges
Always read the calculation, not just the amount due.
10. Trying to Handle Everything Alone
Self Assessment doesn’t have to be stressful, but first-time filers often assume they should be able to manage it themselves.
In reality, even a simple review by a professional can:
Prevent common tax return mistakes
Ensure allowances and expenses are claimed correctly
Provide reassurance that everything is compliant
For those who want support, Platinum Accountancy Services offers professional self assessment and tax return support, helping clients file accurately and confidently without last-minute panic.
How to Avoid Problems Going Forward
Understanding the rules, knowing what to declare, and getting the right level of support can make the process of filing self assessment far less stressful.
If you’re unsure about your return, worried about HMRC issues, or simply want peace of mind, Platinum Accountancy Services can help. Get in touch to discuss your Self Assessment and ensure everything is handled correctly.
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