I have no construction advice, but some additional information would likely be helpful, such as:
- Will this structure be connected to or separate from another structure, such as a house?
- Can it piggy back on any existing heating/cooling/water, or be independent?
- Underground, above ground, above another structure (like 2nd floor of a garage)
From what I've seen people do, you'll need to think about everything from flooring to where you place you outlets to sound proofing. Want a bright area or dark and cavernous.
Anything is awesome, be creative, and post pics!