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The SA370 is a form that you can use if you filed your Self Assessment tax return late and want to appeal against the late filing and payment penalties that HMRC asked you to pay.
You should only file one if you believe you have a “reasonable excuse” to file or pay late.
You can download the SA370 on HMRC’s website – you’ll have to print it then send it by post.
If you were less than 3 months late in filing your tax return and just want to appeal the £100 penalty, you can just use HMRC’s online service – no need to send a SA370.
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