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The most common Self Assessment mistakes

  • 2 min read
  • Last updated 23 Oct 2023

You can make filing your taxes much easier by avoiding these common Self Assessment mistakes:

1. Forgetting about deadlines

You need a UTR number for your Self Assessment.

It takes weeks to get it, so don’t wait until the last minute.

If you’re not sure about the deadlines, check our post on the UK tax year dates here.

2. Forgetting about tax reliefs

There are many ways to reduce your tax bill.

You should:

3. Forgetting about payments on account

HMRC may ask you to pay this year’s bill and also a Payment on Account.”

Your first January payment could be 50% bigger than you think.

If you’re self-employed, remember that payments on account are charged at the end of January and the end of July.

Read our guide to Payments on Account here.

4. Getting your tax code wrong

If you are under the wrong tax code, you could be paying too much tax.

Check our post on what your tax code means and how to find it.

If there’s a mistake, call HMRC as soon as possible.

Your situation

Outlined number oneImage of an arrow
Tax code
Annual salary


Outlined number two
  • PAYE earnings

How your tax code is calculated

There are three main components that determine your tax code:

  • What you earn
  • Allowances you’re claiming
  • Your tax-free allowances

5. Forgetting to declare salary and benefits from PAYE jobs

If you have a full-time job besides being self-employed, you need to let HMRC know so you don’t pay extra tax.

Ask your employer for a P60 and P11D: this will show you how much you earned and how much tax was paid.

You need to record this on the Employment pages of your tax return.

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