How to avoid common errors on your Self-Assessment Tax Return

How to avoid common errors on your Self-Assessment Tax Return

Filing your self-assessment tax return probably isn’t your favourite task of the year. But if you’re self-employed, earn rental income, or receive untaxed income, it’s your responsibility to let HMRC know what you owe.

While it’s a straightforward process for some, it’s also easy to get wrong. And unfortunately, even small mistakes can lead to delays, penalties, or an unexpected tax bill.

Here’s how to avoid the most common errors and get your self-assessment right first time.

1. Leaving it too late

Putting it off until January is one of the biggest mistakes people make. Rushing leads to forgotten figures, missing paperwork, and stressful evenings spent trawling through receipts.

If you submit late, HMRC can charge a penalty even if you don’t owe any tax. That’s money down the drain for no good reason.

How to avoid it:
Start early. You can submit your return from as soon as the tax year ends on 5 April. Give yourself time to gather everything and, if needed, seek advice from an accountant.

2. Incorrect personal information

It might sound obvious, but plenty of people submit returns with the wrong National Insurance number or Unique Taxpayer Reference (UTR). Others forget to update HMRC if they’ve moved or changed their name.

Why it matters:
Errors like these can lead to delays or make it harder to access your records later on.

Tip:
Before submitting your return, double-check that your personal details match what HMRC has on file.

3. Missing income sources

Many people forget to include all of their income. It’s not just about what you’ve earned from your main job or freelance work. HMRC also wants to know about things like:

  • Rental income
  • Dividends
  • Interest on savings
  • Capital gains from selling shares or property
  • Foreign income
  • Side hustles or casual work

Tip:
 Review all your bank statements and paperwork from the year to make sure nothing is missed.

4. Confusing turnover with profit

Turnover is the total amount you’ve earned. Profit is what’s left after you’ve deducted allowable business expenses. Mixing these up can result in incorrect tax calculations.

Tip:
Be clear on the difference. If you’re using accounting software or spreadsheets, make sure you keep income and expenses separate and well-organised.

5. Claiming the wrong expenses

This can go wrong in two ways. Some people underclaim and miss out on legitimate deductions. Others claim things that aren’t allowed and risk penalties.

Tip:
Only claim expenses that are “wholly and exclusively” for your business. That might include travel costs, office supplies, or a portion of your home bills if you work from home. Always keep receipts and records in case HMRC asks for evidence later.

6. Forgetting about payments on account

If your tax bill was more than £1,000 last year, you might need to make advance payments for the next year. These are called payments on account.

Why it matters:
 If you forget to factor in payments already made, you could end up overpaying or underpaying.

Tip:
Log into your HMRC account to check your payment history. Make sure any payments on account are reflected in your calculations.

7. Poor record-keeping

HMRC doesn’t ask for your receipts and bank statements when you file, but they do expect you to keep records in case they ever carry out a review.

Tip:
Keep digital or physical copies of everything for at least five years. That includes invoices, mileage logs, bank records, and receipts for anything you’ve claimed.

8. Not submitting your return properly

Believe it or not, some people fill in their tax return, think they’re done, and forget to actually hit submit.

Tip:
Once you’ve completed everything, make sure you receive confirmation that your return has been submitted. Save the email or screenshot the confirmation page. It’s proof that you met the deadline.

So, should you file it yourself or get help?

There’s no rule that says you must use an accountant. If your tax affairs are simple, doing it yourself can work fine. But as things get more complex, professional advice becomes a lot more valuable.

If you have multiple income streams, tricky expenses, or just don’t want the stress, an accountant can make the whole process faster and more accurate. They might also spot tax-saving opportunities you’d otherwise miss.

Final thoughts

Submitting a self-assessment tax return doesn’t have to be stressful. The key is to start early, stay organised, and avoid common pitfalls like missing income or claiming the wrong expenses.

If you’re ever in doubt, it’s worth getting advice. A bit of help now can save you a lot of hassle later on.