This might be already known but you can lower land under houses at certain points so that the house ends up on a "steep slope". The game draws it incorrectly and worst is that you can't raise the land back again, so you have to demolish the houses.
And by the way, it doesn't work with airports.
Off topic a bit, but @ Patchman: remember when I posted about the problem with trains not loading cargo? I realized it was caused by using unifiedmaglev so I just used cht:tracks 1 which fixed it. You might want to remember that if others have the same problem in the future

(or it would be safer to save the unifiedmaglev setting with the savegame)...