You would want to build it to code.Try http://www.ez-stairs.com/stair_calculator/ this to see how much room it would take up for standard stairs. There's probably a circular staircase calculator and other references out there also, especially if you are going to have a landing to break up the stairs. Good luck.
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same vertical height as any other room
can be a bit less when you make it with high slope (near vertical up)