So when I built a smaller settlement building, 3x3 foundation layout, I used 3x3 concrete foundations. However, for larger ones - like say 4x3 buildings, I would save a bit on polygon count for settlement budget - despite using budget mod that makes size not an issue - by using regular floors for the middle positions.

e.g.

C C C C

C F F C

C C C C

This way I used lower polygon count for the floors no one would see had no concrete underneath anyways. But I noticed that many times when coming to my settlement, or sleeping there and settlers spawn into positions, there would be settlers annoying trapped under the floors, with just their heads sticking out.

Apparently, in my case at least, the collision detect didn't prevent settlers spawning there on loading settlement grid. But I noticed a lot of those were under floors where there was no concrete, so I replaced the floors with foundations and now I just have the usual every once in a blue moon settler spawning somewhere weird, not the content settlers sunk below floors.

I guess you get what you pay for is the adage, and going cheap on the construction materials wasn't a good idea.