Cannot build below fountain on the 2nd basement floor

[SassisSass]

I was trying to edit a basement floor on my older build but I got error "Blocks On Adjacent Levels Must Touch" when trying to build walls etc in certain areas. Turns out that the fountains I've built won't let me build anything under them anymore. I tried it both with and without mods and also in new save. Is this a bug or was it changed on purpose at some point? Not sure since when this is a thing as I probably built the basement around 2-3 years ago already. I've added pictures to illustrate where I can build now around the fountains, what error I get when trying to place a room 1 floor below the fountain, and a proof that I used to be able to build below fountains before. 

[SheriGR]

@SassisSass Basements are a bit funny/temperamental in several ways. I will add a bit of information that I found that may or may not be relevant to your build. 

• Please look at the info below it about the 'Basement & Foundation Tip, as the combo can be a bit funky to deal with: https://sherigr.com/sims-4-building-tutorials/wall-and-basement-tools/#basement-tool)
• I did try to build a basement in this test lot that had several platforms with one that is a platform I dragged downward to make a 'sunken' area on the main floor. I found that using the basement tool did work properly to make the basement under my platform areas on the main floor, however the area where I made the sunken platform area did not let me make a basement in that area. Please see screenshots:
  Basement area created using basement tool from main floor:
  
  I was also able to make another section to create an 'L' shaped basement... however...
  I was then unable to create a basement area under the sunken platform area:
  
  And this is the error I got when attempting to do so:
  

You said that you tried with and without Mods/CC. Please try the entirety of following again if there is any part of it that you didn't already do, also making sure you've deleted the mentioned cache file (click spoiler tag): 

Spoiler

Move your Mods folder from your "The Sims 4" folder [Documents > Electronic Arts > The Sims 4] to your desktop (save it for reference) and delete the localthumbcache.package file in your “The Sims 4” folder. Then check in a new game save to see if this issue is still occurring. If the issue is gone, please re-add the Mods/CC one at a time until you find the one causing the issue. If you have a lot of Mods/CC you can try the 50/50 method.

Is this happening in all of your lots, or just this one?

If it's just this lot and the basement behaviors I described above are different then yours, would you be willing to share it to the gallery so I could test in my game? Please name it something like 'Foundation and Basement Test' and please let me know your User ID in the gallery if it's not the same as it is here. Thanks!

[Trismagistos]

@SassisSass Presumably this issue started with the new patchupdate of the base game and the introduction of the building platforms. Because of the possibility of building platforms some of the other ground level works had to give way. Fountains have always been shallow parts and were not compatible with building basements on the same level. On the screenshot you can see that the grid below the fountain gave way on the basement level. So the only option to build a basement below it was, to make sure these parts would not be attached.

Now with the new platforms, the error you see has to do with the fact that your fountain is perceived as a platformlevel. You could try to work around this by creating one or more platformlevels at the basement level below your fountain. Then you also have to create new wall around this platform and finally create your fountain on top of it. The highest platform level stays he same. But you might create a walkable space below your fountain.

I hope this clear the situation for you.

Have fun playing. 😉