SA100 is a tax return form used by individuals in the UK to report their income and claim any tax allowances or reliefs they may be entitled to. The SA100 form is used for self-assessment tax returns and is submitted to HM Revenue and Customs (HMRC) each year.

The SA100 form requires the individual to provide details of their income from various sources, including employment, self-employment, rental income, and investment income. The form also asks for information about any tax allowances or reliefs the individual may be entitled to, such as the personal allowance, pension contributions, or charitable donations.

In addition to the SA100 form, individuals may need to submit additional forms or schedules depending on their personal circumstances. For example, individuals who are self-employed may need to complete the SA103 form, while those with rental income may need to complete the SA105 form.

Once the SA100 form has been completed, it must be submitted to HMRC by the deadline. The deadline for submitting a paper tax return is 31 October following the end of the tax year, while the deadline for submitting an online tax return is 31 January.

Overall, the SA100 form is an important part of the self-assessment system in the UK and helps to ensure that individuals are correctly reporting their income and paying the correct amount of tax each year.