I remember reading about that lot depth problem. I THINK it is caused by the builder choosing "flatten terrain" to level a lot. Any way, here's a link to Crinrict's suggestion for a fix: https://sims3.crinrict.com/en/2011/06/bug-imported-house-raiseslowers-the-terrain.html
The cheat you type in:
SetImportedTerrainOffset (value)
(value) Is a positive or negative number depending on whether the lot created a hole or a mountain.
After you placed the lot, check if the elevation is level before pressing ok. If not, cancel and change the value you put in. Positive numbers raise the lot and negative numbers lower the lot. You can even use decimal values for fine tuning.