3D Builder is Saying "One or more objects is invalidly defined" Currently, 3D Builder is telling me "one or more objects are invalidly defined. Click here to repair." What is this and what does it mean? When I click it, it totally messes up my model (I can't tell what it even did due to the orthogonal camera). Model stats: * Made with Sketchup * Used STL Exporter to export it * Last I knew what the repairing did to it was to remove my sinks and reduced them to holes in the floor * I used ASCII encoding for my STL (I tried using Binary, but it didn't help) * I tried subdividing it but it didn't help. Screenshots: This is a picture of the model before repairing: 

How thick are those walls? 3D printers cannot print zero-width surfaces (and slicers cannot slice them). 3D Builder has tried to make your model printable, but obviously not in the way that you might have hoped for. You need to go back to Sketchup (or some other CAD package) and give the walls a definite thickness (and one big enough for a 3D printer to handle).