Most common query I get is objects showing up in 3D from a 2D AutoCAD plan. This is realized when it is imported into Trimble SketchUp or other modeling applications. This can be quite an irritant to the user. So here is one of the checks you can do to correct the AutoCAD drawing. The reasons for unnecessary 3D objects created can be many, but I address in this one of them that could be troubling you.
Check the command “THICKNESS”, if this is having any value other than zero, it will result in all lines to take the value in elevation. Hence ensure THICKESS=0. For those lines that already have taken the thickness, go to properties and use quickselect methods to filter such lines and change in properties to zero.