When you import a Lot from say the Library, and the ground tiles are just way too low or high to fit with surrounding terrain, so that there is an abrupt edge.
In Edit World mode:
Test: place an empty lot where you want it, if there isn't one there already. Import the Lot.
Now, look carefully at the edges, particularly the edge closest to the street. Too low or too high?
Back to Edit World mode, bulldoze the Lot.
Bring up the cheat window and enter SetImportedTerrainOffset #,
for example SetImportedTerrainOffset 3
- add a space and a number. Negative numbers work too.
If the terrain is too low add a positive number, if it's too high then enter a negative number; hit Enter to accept the cheat.
Now try placing your Lot again. It will be slightly higher or lower. Play with this, repeating the above actions, until the height is as you want it.
Just keep rentering with a different number and re-placing the Lot.
When you're happy with it, redo the cheat, setting it back to 0 for default thus: SetImportedTerrainOffset 0
You probably only need to use numbers in the range -5 to 5. Beyond that it gets a little silly - furniture, etc, gets buried in the ground or floats in the air...
(Thanks to jenie31 of The Sims Daily forum for explaining this.)