Thank you kindly, Solver. I took your advice re signature--see below.
I'm a newbie, but not that new. I spent two days messing around with the help and support features--I should have stopped on the first day and asked then for help, yes?
If I hear you correctly, it is not possible to rescale a terrain; we must get it right the first time.
However, I continued to work with the program after I posted. I selected the terrain and successfully managed to enlarge it. In doing so, the landscape became distorted. I attach photos of the correct terrain and of the distorted version with the house in place. The design is for an underground house, so I'm not bothered about the house being set down at a lower elevation, but I do mind the whorls that the program introduced into the terrain, clearly visible in the 2D sketch.
I am grateful for your help, and I have no wish to waste your time. I write and send photos only so that you can see what I'm trying to do. You need not reply to this message. I'll work more on the terrain today, removing the current terrain and replacing it with a newly drawn one according to your instructions. If I have I have any more difficulty, I'll get back to you soon.
Thank you, and have a lovely day!
Home Designer Suite 2018 on MacBook Pro Sierra 10.12.6