Problem

My Sim sinks into the ground and can’t climb stairs anymore. Sometimes he’s visible through the ceiling, blocking objects below.

Cause

This bug is caused by the cheat moveObjects on. Only use the cheat to remove stuck items (not Sims) or to place items in build mode.

Moving a Sim with this cheat can have this effect even though the cheat is turned off again. Never move a Sims across multiple floors with the cheat.

If every single Sim sinks into the ground in a certain house, it’s not this cheat but the house. Sometimes it’s an unfinished foundation but there are other reasons.

Solution

Only turn on moveObjects temporarily and reset your sim
.
If you have unfinished foundations (the have to go all around), finish them

For all other problems:

1. Use the cheat constrainfloorelevation false.
2. Use the  level tool on every floor. Starting with the roof and going down to the basement.

Not this issue

If you have a Sim stuck in the ground, this is NOT caused by this but has something to do with the whole Stuck/Frozen/Invisible Sim problem.