The SA104 form is a form used by individuals in the United Kingdom to report their income and tax information to HM Revenue and Customs (HMRC). The form is part of the Self Assessment tax return process.

The SA104 form is used by individuals who are self-employed, or who have other sources of income that are not taxed under the Pay As You Earn (PAYE) system. This includes income from renting out a property, savings and investments, and certain types of foreign income.

The form is divided into several sections, each of which asks for specific information about the individual's income and expenses. In the form, the individual needs to provide information about their business, partnerships, property income and gains and foreign income, among others. The individual also needs to provide a calculation of their taxable income and the amount of tax they owe, based on the information provided in the form.

Once the form is completed, it must be submitted to HMRC along with any required supporting documentation and the tax payment. The deadline for submitting the form and making the payment is typically the 31st of January following the end of the tax year. It's worth noting that form SA104 is one of several forms that may be required as part of the Self Assessment tax return process and that different forms may be needed depending on the individual's specific circumstances. It's always recommended to consult a tax professional or HMRC guidance before submitting the form, to ensure that it is completed correctly and that all required information is included.