Perhaps it doesn't have to be a permanent part of thehouse? Just take a room in the basement or a part of the garage and make that into a "hidden" studio.
I've turned one room in my basement into a photography studio and you would NEVER know it until I pulled things out and installed them.
I have a formal dining room that doesn't get used..I plan on using that as an informal studio. Has a nice window right behind where I would put a white sheet for some nice natural diffused light portraits.