I think your best bet might be to keep the idea of "studio" firmly in quotes. Think of it as building a spare bedroom that you can use as a studio. There are some building techniques using ordinary materials that can improve acoustic isolation somewhat.
The thing is, building a proper "studio" is probably going to be a lot more expensive and incovenient than simply hiring a studio when you need to record, say, a drum kit or whatever. I'm also doubtful that you're going to pull off something that could plausibly be both a studio and saleble as a Mother-in-law quarters-- is the space going to have windows?
The best place to build a studio is in an industrial space, or in a barn in the middle of nowhere-- big, empty, high ceilings, no worries about keeping it "homey," easy to float walls, floors, ceilings, access for big, heavy things to move in and out, etc. Converting part of a suburban home into a full-blown recording studio is the kind of project that often ends up with the "studio" being a big space that takes up several rooms of the house, none of which are completely useable for the purpose intended, and the studio owner/operator spending most of his time sitting in there at the computer, playing through headphones.
Think carefully about what you actually expect to get out of this. An actual full-blown recording studio capable of recording piano, horn sections, drumkits, live bands, etc is hard to do in a residential neighborhood.
Cheers.