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A bit of both - Extrude the walls and then extract the faces so the walls become seperate to the floor. Its easier to assign different shaders that way ie, a wet floor vs dry walls for example.
Also, it means all fits correctly
Cheers Alan, I extruded the walls up any way just to get the size of the room for the props, but i didnt think i would have to just extract them. Nice and quick :)