It's under my house.

The first one is not because it has to correspond to the previous subject, but because above has to be followed by an object, and mine is the accusative case

Example: she lend a book to me.

The second one, below, means directly below, and under is also possible

The third one, I just stand/sit at your downstairs

I just stand/sit at your downstairs

The fourth one, I just passed by from your home / house with you

I just came from your downstairs / house with you. I just stand/sit at your downstairs

Fourth, I just passed by your home / house with you

I just came from your home / house with you

Third, I just stood / sat at your downstairs.