That's a small house.

Normally shafts and stairs are not accounted for on non-ground levels.

no - but make sure you look up who you are defining it for and what their definition is.

LOL small house indeed, was trying to reach the architects for their definition but to no response, what if the wording is: gross conditioned floor area?

Conditioned space is usually considered as a volume, not an area.

If you're dealing with an occupancy situation, then it depends on the type of occupancy.  There are net calculations, and gross calculations. 

Chigurh is right--you need to find out the definition.

I would say conditioned AREA is 175 sf.  Conditioned  VOLUME would be (essentially) 2 times 100sf times the height from finished first floor to second floor ceiling.  You would leave the two-story space in since you are conditioning it even though you can't walk on it.  Interesting question whether the floor structure should be subtracted.

Area is square feet; volume is cubic feet.

as others have pointed out, I've seen this definition vary from jurisdiction to jurisdiction. check the local zoning ordinance for the town on which the property is located and look for a chapter titled 'defininitions.'

i imagine it would be 175sqft as others have mentioned, but if the definition requires you to measure to the exterior face it might be a grey area.