When a house is purchased and is not the primary residence of the owner, it is considered investment property. Many U.S. citizens own rental property without the need to form a business. You might contact an accountant who is familiar with U.S. income tax law to help you through the process. If you plan to regularly purchase houses and flip them or hold them as rental property, then you would want to register as a business.

Hi I was advised that because I had had work done on the house and was going to be gaining monies  either renting or by selling (Flipping). ..that is why I would have to do it?  . The house was not my home.. I didnt understand why either...I assumed that as long as I told the IRS that would be fine..

I'm not sure that I understand your question. It sounds like you purchased a home in the US and want to sell it. If that's the case, I don't see why you would need to register a business.