If you are paid in cash, I am assuming that your employer gives you a 1099 and you use that as evidence of your gross income when you file your tax return. Often, the bank will use alternative methods to analyze your income. This includes examining tax returns, bank statements showing deposits into your account and possible independent verification from those that are paying you.
Mortgage companies understand when someone is self-employed and you should discuss with them how you should document your income.
This response does not create an attorney client relationship nor should the advice be relied upon because it is not specific to your legal situation. Before you depend on legal advice, you should retain competent counsel.
Getting paid in cash is a little bit of a problem nowadays. As the previous attorney stated, your 1099 or W-2 will have to be the basis for this. If you are saying that you are paid under the table in cash, and this isn't reported, you have a different problem and will likely not be able to substantiate your income to the satisfaction of the bank.