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The SA100 form is the main Self Assessment form you need to file to do your tax return, if you choose to file it by post.
Usually, depending on why you need to file a tax return, you need to attach another form to it (for example, if you’re self-employed, you also need to fill in and send a SA103 besides the SA100).
You can find the SA100 on HMRC’s website, where you can download it, print it, fill it, and submit it by post. You have until October 31st to do this.
If you’ve done a tax return in a previous year, HMRC may also make it easier for you by sending you an SA100 by mail. All you need to do is to fill it and send it back.
If you decide to file your tax return online, however, you don’t need to worry about printing and mailing the SA100 form – plus you get an extended deadline, January 31st of the following year.