It is just EXT. BRETT'S HOUSE - DAY
You are over thinking it. It doesn't make a difference if he owns it, rents it, whatever. He lives there. Nobody gets confused. Who cares if he owns it or not? If ownership of the house is immaterial to the plot, then do not specify. Never write ANYTHING that does not make a difference in the story on the screen.
Further, always consider the reader of the screenplay to be a professional reader. No screenplay is ever given a dime until it has been read many times by many people. This is not a novel! The readers are not amateurs. If Brett's parents are found living in the house, they make the most common assumption possible: Brett's house belongs to his parents. And again, if ownership is not a plot element, they won't even think about THAT, because all they care about is if the story will be entertaining on the screen, and that isn't going to have a label on the house at all. It is just going to be a house, and a car pulls up, and a clown gets out with an AK47, then cut to INT and a living room. The DOORBELL rings.