Replacing normal staircase with spiral?

I'm contemplating buying a duplex condo that I'll likely wind up sharing with housemates (them downstairs, me upstairs). Ideally, I'd like to remove the staircase altogether to recover the space for both floors (each has a door to the common stairwell and a window to the fire escape, so I imagine egress requirements would be fine), but I'm guessing that'd be a hard sell to whoever approves such things. Instead, it seems that a spiral staircase could reduce the current 8' x 6' footprint to something more like 3' x 3'. Has anyone done something like this? I'm wondering what the permit requirements and typical cost might be.