I have an entire (empty) house already built, do I need to export all the walls / doors / floor as one big single fbx or is better to rebuilt it in ue4 using the modules (ie wall 400x300 etc etc)
Import your fbx file. iirc those premade props are brushes and are limited anyway. Always make your geometry in a 3d modelling software.
When you import your fbx file, in ue4, just don’t tick ‘‘combine meshes’’. That way all pieces in your fbx file are going to be separated in ue4. Just select them all at once and drop them in the scene and they’ll retain their original position!
Well, modeling walls, floors and ceilings with BSPs you can change lightmaps and textures resolutions for every face. Plus, better control for light leaks, textures placement and no unwrapping needed. Doors, window, furniture, etc. using third party. I think this is a way to take into account.