A newly generated planet of mine contains a few slums (German "Armutsviertel") They can be "rebuilt" in the editor and demolished, but I find no way to actually place new ones. The building menu doesn't contain an entry for them.
(P.S. I though I saw a red entry at the very bottom of the building menu flashing for a split second when I first checked, but anyway, there is now no entry for them.)

Are there other types of land coverage that you currently cannot explicitly place?