I just bought a house a couple of months ago, and the previous owners planted 4 possumhaw holly trees right in front of the house. They are planted only 12 to 21 inches away from the front wall of the house. The largest of the 4 trees is around 8 feet tall currently. Is having possumhaw trees that close to the house a danger for my house's foundation?

If your foundation is in good condition, it shouldn't be that much of a problem. The plant in question doesn't look destined to mature into a monster either if Google Images is to be believed, so no huge roots either.

If you're really unsure, ask a professional (engineer) for his opinion or perhaps an inspection - it won't likely be free though.