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The SA100 form is simply the main document you need to fill when doing your Self Assessment tax return.
Most UK taxpayers need to fill it in – here is the list of who needs to file a tax return.
First, go to this HMRC page:
If you usually file a Self Assessment tax return every year, then HMRC might also send it to you by post.
In any case, make sure you first read the “How to fill your tax return” pdf for that tax year – it’s on the same HMRC page.
That pdf will explain what each item from the SA100 form means.
You’ll have to send it by post to HMRC, so allow some time for it to arrive there – HMRC will fine you if you miss the deadline.
In most cases you’ll also need to download, print, fill, and submit additional forms – depending on your tax situation:
Whichever you use, attach it to your main SA100 form.
You will only need to file it if you submit your Self Assessment in paper format.
We recommend you file your tax return online instead: the deadline for the paper tax return is October 31st – for online tax returns is January 31st.
In other words, if you do it online, you gain 3 months.