SA1 is a form used by individuals in the UK to register for self-assessment with HM Revenue and Customs (HMRC). Self-assessment is a system used in the UK to calculate and pay income tax and National Insurance contributions.

The SA1 form is used to provide HMRC with basic information about the individual, including their name, address, date of birth, and National Insurance number. The form also asks for information about the individual's income sources, such as self-employment, rental income, or investment income.

Once the SA1 form has been submitted to HMRC, the individual will be registered for self-assessment and will receive a unique taxpayer reference (UTR) number. The UTR number is used to identify the individual's tax records and must be included on all self-assessment tax returns.

It's important to note that not everyone needs to register for self-assessment. Individuals who are employed and have their tax and National Insurance deducted automatically through the PAYE system usually do not need to complete a self-assessment tax return. However, individuals with more complex tax affairs, such as self-employed individuals or those with rental income, may need to register for self-assessment and complete a tax return each year.

Overall, the SA1 form is an important part of the self-assessment system in the UK and helps to ensure that individuals are correctly registered with HMRC and paying the correct amount of tax and National Insurance contributions.