How to get net and gross area of a space.?

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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.