How can i fill an object that consists of 2D panes into a completely solid (water tight) model? I do apologize in advance as i am very new to blender and this forum. I will completely understand if i have not explained myself the best and encourage people willing to understand to ask further questions. I tried my best.

I have a .FBX file of a building that has an open bottom, the building consists entirely of 2D panes stitched together and i would like to fill the interior only without loosing any detail on the outside. This would be so that the bottom is flat of course. The end goal is to export this as an .stl file and 3d print the building. I think this process is along the lines of making your model 'water tight' however ive seen people do this but it ends up with rounded randomized bridges between gaps. This is a building with very blocky and has straight edges i would like to keep the accuracy and only fill the inside.

Im not the greatest at explaining and constructing questions but if anyone was willing to understand and help out it would be much appreciated.

I have attached images to help further anyone's understanding.



Cheers :)
Looking at the image, the wall seems to have some thickness at the base of the building, but further up inside I can see darker faces indicating planes with no thickness.  I think you will need to manually work through the model to delete any internal faces in edit mode, leaving only external faces. then ensuring all these planes are joined by merging vertexes then finally creating a new face on the bottom to seal it off. 

I am not aware of a simple shortcut.  There is a few hours work needed to convert this into a printable shell.