This is the second time I've encountered this anomaly - the ground is perfectly flat, I put a wall there ...and cannot build further because there is a small kink on the top of that wall preventing a second story wall being built. I tear out that entire end of the structure, flatten the ground again, put up the first floor wall, and there is the kink again.

The first time this happened it was only a single floor house with a flat roof, and it had a kink right over the front door; demolished that corner of the house and rebuilt twice, still had the kink at that spot (must have been fun for residents when it rained).

This is the current situation -

The missing grid there means you have to try to flatten the floor again using the elevation cheat. When this cheat is activated, even the smallest change in elevation could cause this. Even if the ground looks flat, it could have small depressions or bumps on it that appeared before or during construction.

"elevation cheat"?

The elevation cheat I'm referring to is "constrainfloorelevation off" command which turns off the elevation lock and allows you to change the height of floors, foundations and ground which has flooring placed on it.

Thanks, more things to learn ...and play with. Found this tutorial also, quite informative - https://www.carls-sims-4-guide.com/...hp?topic=9065.0