The occlusion tests still consume fill rate even for invisible objects. They just don’t consume as much pixel shader power as the fully-shaded thing, so if lots of things with complex shaders are hidden, they’re a win. If the occluded things use simple shaders, or are not fully occluded (visible through windows, leaves, fences, etc) then it’s actually a loss compared to “just draw it.”
Building modular pieces that have both front- and back-sides “feels” natural, but isn’t the most efficient way to build the geometry. On the other hand, if the player can look out the window, and is supposed to see an external part of the mansion, then making the pieces “thick” is the easiest way of making sure you don’t forget any spots 