I don't know if street art ever really works indoors. If you domesticate an animal, it goes from being wild and free to sterile, fat and sleepy. So maybe the art should stay outside.
BanksyGraffiti is one of the few tools you have if you have almost nothing. And even if you don't come up with a picture to cure world poverty you can make someone smile while they're having a piss.
BanksyTraditional graffiti writers have a bunch of rules they like to stick to, and good luck to them, but I didn't become a graffiti artist so I could have somebody else tell me what to do. If you're the type who gets sentimental about people scribbling over your stuff, I suggest graffiti is probably not the right hobby for you.
Banksy