How do I sell the apartment attached to my house as a condo?

OK so it's really my parents apartment but I have been renting it for years and they suggested that I think about buying it. I have put a lot of work into it and it may be cheaper to buy it as a condo than to keep renting it. Plus they could use the extra money to put in geothermal heating. That would save them more than my rent. (The house is huge.) So how do I get it turned into a condo and appraised so I can buy? I live in NH if this helps. Thanks for any input.