Blog: The most common Self Assessment mistakes

You can make your Self Assessment tax filings much easier by avoiding these common mistakes:

1. Forgetting about deadlines

You’ll need to register for as self employed to receive a UTR number. It can takes weeks to get one, so its best not to wait until the last minute as you’ll need this for your Self Assessment filing. If you’re not sure about the deadlines, here’s a quick recap:

  • 31st October for paper submissions
  • 31st January for online submission

2. Maximising tax reliefs

There are a number of ways to reduce your tax bill.

You should:

  • keep a record of all the business related expenses (especially if you’re self-employed or a landlord)
  • make full use of your tax-free allowances
  • keep a list of your donations to charities
  • claim your private pension contributions.

3. Payments on account

HMRC may ask you to pay this year’s tax bill plus an additional “payment on account for next year. This means you first tax payment in January could be 50% larger than expected. If you’re self-employed, remember that payments on account are charged at the end of January and the end of July and offset against next years tax bill.

Read our guide to Payments on Account.

4. Getting your tax code wrong

If you are classified under the wrong tax code, you could be paying too much in tax. Check out our post on tax codes to find out what yours means and where to find it. If there’s a mistake, call HMRC as soon as possible to get it sorted.

5. Remembering to declare salary and benefits from PAYE jobs

If you work in full or part-time employment besides being self-employed, you need to let HMRC know to prevent paying extra tax. Request a P60 and P11D from your employer: this will show you how much you earned for the year and how much tax was deducted/paid on your behalf.

This will need to be recorded on the Employment pages of your tax return.


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