For the sake of mortgages, lenders only use the income you report on your tax returns. They’ll want the two most recently filed returns. So, if you have a side hustle house sitting for friends and get paid cash, but do not report that cash as income on your tax returns, your lender cannot use it to help you qualify for a home loan.
There are a couple of exceptions to this rule. They are very rare and require a lot of documentation to prove and accept.