Basements will not create irremovable flooring between levels.
Fixed in Patch 1.55.105
@plsnt_nightmares I have just heard from a fellow helper that this appears to be a bug/glitch. Please see this bug report, and most specifically the information/description from Hyperion_Rail.
I am currently asking if this post can/should just be merged on to that bug report, or if you should add your information to it yourself. If it's merged my shared experience/test will move into the post. Otherwise we can both 'me too' and add our information to the bug report, as this helps EA to figure out what conditions may be creating the issue - and to see that many folks are having the issue, which can increase priority. https://answers.ea.com/t5/Bug-Reports/OPEN-Basement-Split-Level-Rooms-Irremovable-Basement-Flooring/m-p/7956203#M92057
Oh so it's not just me or something i did wrong heh good to know. well then i'll provide a bit more information and things i tried yesterday. destroyed everything and tried rebuilding both basement levels to the same size, with no furniture, removed floor and ceiling on both did not work. also during previous attempts as i mentioned before, it acted like there was nothing there, sims could still walk through it to go to the bottom level, and objects could not be placed on the dirt. so i thought perhaps it was a camera glitch. eyedropper still worked to show it was basement floor though.
Then in one of many times I tried rebuilding it yesterday, there was an instance where the basement floor wasn't just a camera issue and i could place objects on it. despite having the floor and ceilings deleted. unfortunately this was before i knew it was a bug, so I didn't take note/don't remember what circumstances may have been different as to why this happened.
