Originally a method of descending by rope used in mountaineering, spelunking and also in a modern context by military and police in urban warfare. Abseiling is the term for this technique of almost 200 years use.
I do not know all d methods of rappelling, but i know few styles.. (1) Geneva-style rappelling- the rappeller faces upward and his backside is facing towards ground...it is a famous method and safest...
You need permission, at least from the owner of the building and maybe the police, but if you get that, no, it's not a crime. Check it out before you do anything.