"I am at my house."
Translation:Ich bin in meinem Haus.
Are you sure that you aren't mixing it up with "bei mir zuhause"? That is common and normal, but "bei meinem Haus" you would, at least to my knowledge, use in other contexts. Not implying that you are at your house/home, but that for example something happened at (or in the vicinity of) your house (like an accident on your driveway etc.).
Well how would you explain why you can't say in English "I'm on my house"? You just don't say it like that. ;) There are certain prepositions that just work the way the do, with some similar things they wrk and with others they are totally wrong. You just need to memorize them.