How can I extract a Digital Elevation Model from a 3D model of a building, made with SketchUp, using Blender,qgis or any other software?
Export your terrain object from SketchUp as a .dae file (or .kmz if you're using the free version - rename the .kmz extension to .zip and extract it find the .dae file inside)
Open the .dae mesh in Meshlab (free) and export mesh to .xyz points format, ignoring normals. Now, you can add the .xyz as a layer using 'add delimited text layer'. Specify your x and y column, 'space' as custom delimiter, and that you have no header row. Looking at the data table, column 3 will contain your Z values.
Remember that unless your model was already georefenced, you will probably need to translate, rotate and scale the resulting layer, as required.
You can now use the raster > interpolation module to create a raster based DEM.