An SA302 form is a document issued by HMRC summarising your total earnings and income tax payable for a particular tax year.
If you are self employed and applying for a mortgage or loan, your lender may request a copy of your SA302 form because it acts as evidence of your income as part of their application process. If you were employed in a job you would have payslips to evidence your earnings, however when you are self employed you may not have payslips. Therefore the SA302 is issued by HMRC as third party confirmation that you earn what you say you earn, summarising the information submitted in your personal tax return.
If you are applying for a mortgage, you lender or mortgage broker will probably request evidence of your earnings for the past 3 years. In this case you would need to provide three SA302s for each of the three tax years they wish to confirm.
If you were employed in a job as well as being self employed over the period of time being assessed for your mortgage application, you may need to provide a P60 or P45 documents issued by your employed evidencing your earnings. Your mortgage advisor, bank or building society will talk you through the necessary paperwork needed according to your specific circumstances.