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The SA100 is the main Self Assessment form and it allows individuals to submit their personal income tax return. It is normally filed alongside another form.

More detail:

The SA100 is a type of tax form used in the United Kingdom by individuals to file their personal income tax returns. It is the main Self Assessment form.

Normally, another form is required for submission alongside a SA100 but that depends on who is submitting the form and why they are required to do so.

A SA100 form can be filed online or via post. For people that filed a tax return in the previous year, HMRC automatically sends them a form via email that they simply have to fill in and return to them.

It must be submitted by 31st October if you are filing via post but you have until 31st January of the following year if you are submitting the form online.

An example page of the SA100 from 2022/23 is shown below:

A photo of blank SA1 form from 2022/23
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