John, I would have to research the IRS rules to give you an "informed" opinion but I don't think their rules would come into play in determining if the loan is a consumer or business purpose for the purposes of Reg. Z. I think they would be more concerned with the borrower being able to claim certain business and rental deductions on their return.
FWIW, anytime we have a situation where a relative is purchasing a home for another relative we treat it as a loan for family, household or personal use and disclose it as a consumer loan unless there is signed lease agreement between the parties, these are few and far between. In those rare cases we will treat it as a rental. We had one situation where there was a contract for deed between the parties. In this case we classified it as a personal investment for the borrower and treated it as a consumer loan.
There are several different scenarios that could be discussed but in the long run if the parents are purchasing the property for their kid then I would opine 9 out 10 times it's going to be a consumer purpose loan.
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The opinions expressed are mine and they are not to be taken as legal advice.