I wish to add different textures on a wall (textures plus a covering with wood) DO I need to make another wall , very thin ?
An easier way could be to add baseboards to the wall with its modification dialog box, or place a thin box along it.When I will build walls , will they be made always from the basement walls?
Walls at upper levels are built from the elevation of their level minus the level floor thickness to the height set in their modification dialog box, and not the the height of the level. for instance the basement wall have a certain height, the first floor is declared at a certain heigh, with a certain floor thickness, is that floor thickness to be added to my walls height computation ?
ExactlyAre the walls put automatically at the good dimension with the levels menus ?
No. This is on purpose in case you want to create some intermediate levels. The height of the level is used for the ceiling.Is it better to have independent walls for each level ? I wish so as my wall is thick at its bottom then smaller.
It's up to you. You can create walls with a height covering 2, 3 or more levels, or create smaller walls at each level.excuse my english
No problem, your English is clear, and most of us are not English here 
What may I enter as an "elevation" for my street level ? (I did tup - 1" and 1" ceiling thickness, is it correct ?)
If I follow correctly, the floor thickness of the street level should be 2". If you set an elevation of 1" for that level, then the floor will be half buried.Is it possible to have a floor thickness in the basement (only part of it have a concrete floor, with some thickness, but the level 0 have the thickness grayed, I cannot enter anything there)
The floor of the lowest level doesn't have any thickness as you noticed, but you can create a dummy lower level to be able to enter that thickness.Note that you could also set walls (and rooms) half transparent in the 3D view modification dialog box to better view your tests on levels.
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Emmanuel Puybaret, Sweet Home 3D creator
