Hello Ceciliabr,
thank you for this explanation. I think I understood that I can not assign textures to an object with various areas, because they don't have the texture coordinates.
The workaround you proposed by building the object of many samll boxes and exporting to obj-file and importing again works if the material change option is used. But I have to live with a scale effect and need rescaling (in the case of the red bricks I need to rescale from 100% to 1000%). I do not understand where this is coming from, as the texture when imported was scaled to certain size in "meters". Maybe you have an answer to this as well.
What secondly I don't understand is the following. When I do the object in Sketch up and assigning a color to each of the areas the object should have the texture coordinates. Importing this object as collada file and change (see picture) the texture results still in distorted textures.
This is the object imported from Sketch up as Collada-file:
That's what I get opening the material editor:
then change texture 
and I am getting this
Why??? I really would like to understand.
