The most obvious choice for skp to dwg conversion, well any kind of SketchUp to AutoCAD conversion actually, would be SketchUp simply because not many programs actually support SKP format. You can usually achieve this through File ► Export ► 2D Graphics function found in the SketchUp program. The result will be a 2D vector file in DWG format (version 13 or 14) that you can open in any CAD program. However, you can also export it as 3D DWG file, in which case you need to select File ► Export ► 3D Model function. More about this can be learnt from SketchUp Help Center.
If you do not have access to the commercial version of SketchUp, you will have to use some of the third party alternatives or dedicated 3D model / CAD converter that will allow you to convert skp to dwg.Open SKP file Open DWG file
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