What is an SA302 and how do I get one?
An SA302 is evidence of your earnings that you can get for the last 4 years once you have sent your Self Assessment return. Sign in to your HMRC online account, go to Self Assessment, then More details about your Self Assessment returns and payments. You cannot print it until 72 hours after you filed.
Figures checked against gov.uk on . General information, not tax advice.
Why people ask for it
gov.uk's example is a self-employed person applying for a mortgage: a lender may ask for the SA302 and a tax year overview as evidence of income. The tax year overview is available for any year, and lenders may want both documents.
Check that your provider accepts documents you have printed yourself.
If you used software
If you filed through commercial software, you print your tax calculation, sometimes called a tax computation, through that software. gov.uk says you can still print a tax year overview from your HMRC online account either way.
Allow for the wait: if you file just before a lender's deadline, the printed copy is not available for 72 hours.