I was sarcastic. But it's vandalism when you don't ask for permission. If you decided that it's better to ask for forgiveness than ask for permission, take responsibility. If the owner of the building likes it, you're good. If not, paint it back. There are other rules, like don't fuck up with monuments, don't break other laws while doing it (trespassing,...) and so on. Have a talk with a genuine street artist, they actually have some unwritten rules. And some are idiots, of course.