Since you allready asked about NetArea and Gross Area and got a full answer from Julien, I will asume your are looking for the Volume of the space.
You can find all the relevant methods for space object in the SDK docs.
To answer what I think you asked: the void in the roof has no impact on the volume calculation nor on the floor area calculation. To prove it I created the following space:
Calculating its Volume and floor area using space.volume
and space.floorArea
yield the same outcome when the void was present, without void and with skylight window.
Can you specify what software you are using. Also are you looking for floor area, or net/gross area of surface with sub-surfaces?