Most people in the UK will pay their tax ‘at source’, for example, through PAYE if they’re employed, and will not be required to file a tax return. Therefore, self assessment does not affect all UK taxpayers and you will usually only need to complete a tax return if one or more of the following apply to you:
You also may need to complete a self assessment tax return if:
HMRC may also want you to complete a return for other reasons.
Note that if HMRC have sent you a tax return or a notice to complete one, then you must fill it in and return it by the due date unless there is a good reason you do not need to be in self assessment and HMRC agree to cancel it.