The short version: SA100 is the main Self Assessment tax return form. If you file by paper, this is the core form you fill in. Most people file online now and don’t need to touch the physical form.
What’s on it?
Your personal details, total income from various sources, tax reliefs you’re claiming, and calculations of what you owe. Depending on your situation, you might need to attach supplementary pages too.
Do you need to use it?
If you file online, the system handles everything digitally and you never see an SA100. If you prefer paper, you download it from HMRC, fill it in, and post it. Paper deadline is 31st October. Online deadline is 31st January.
Why file online?
Three extra months to file. Instant calculation of what you owe. Confirmation that HMRC received it. Less chance of errors. Unless you have a specific reason to use paper, online is the way to go.
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