How to get the sill height (height above the floor) of a Ifcwindow in ifc file
There is no solution to get the height above the floor directly. This is because the height above for depends on several factors, like how the wall is created in which the window resides, etc.
It could be that the sillHeight is exported by the original modelling software to a custom IFC property. You could check for that, but since there is no common standard for it, it's risky.
Your best bet is to look into the ObjectPlacement property which IfcWindow inherits from IfcProduct. The ObjectPlacement defines how a product is placed either in world space or relative to its host. See https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2/HTML/schema/templates/product-local-placement.htm for details.
You need to read the ObjectPlacement property, and check if there is a RelativeTo property, if so, you need to fill into that property as well, and check if it's the placement of a floor. If so, you can stop the looping, and perform a matrix calculation on all the placements you harvested to calculate the placement of window relative to floor.
(Maybe even more simple: calculate world placement of window and floor separately, than subtract the two vector z values to get the height of window from floor)