Import Sketchup Model to Unity 3D

You can use Sketchup to build your model then export them to DAE format.

You can then use the free FBX converter from Autodesk to convert the DAE file to FBX and load it to Unity 3D

Autodesk stand alone FBX converter:

http://usa.autodesk.com/adsk/servlet/pc/item?siteID=123112&id=10775855