How do you get rid of a wall covering that's not visible?

@ScottB9 

That gap is normal for any raised foundation because there is no texture for the inside of any of the foundations.  The only way to get rid of it is using a flat foundation instead of a raised foundation.  I think part of the reason that it is that way is that in the original base game released in 2014, basements were not included, they were added later in an update.  So, a texture was not needed for the inside of the foundation because it could not be seen in the original base game because there were no basements.  Then they added basements a few months later, but I guess, forgot to add an inside texture to the foundations and it has been that way ever since.  That gap only occurs for raised foundations.  When you look at how a basement wall meets the underside of a raised foundation, you see it too.  If you use a flat foundation, there is no gap and the wall along a stair going down to the basement looks solid.

I use CC too and I found that it can complicate building and create problems.  So, I created another user folder and do all my building there in a mod and CC free save.  I also clear everything out of the library in that user folder/save by deleting it.  Once I have the new build done, I save it to the library and then save and quit.  Then you have a tray folder that only contains the build you just saved.  Next, all you have to do is copy all those files to the tray folder in your regular user folder.  Then start the game and place it wherever it was built to go via the Library.  Then, from the map, enter the lot in build mod and set lot traits and replace the game items that you want to replace with CC and add any other CC you want in the build.  And you are done, brand new lot installed. The nice thing about building in a mod and CC free save is that when you save it, it will not get the CC tag if you want to upload it to the Gallery.  And I have found I have a lot less trouble with my lots than I did when I put a lot with CC on it, saved it to my library and tried to place it elsewhere or in another save.  

Hope this helps.