Build Mode Floor-Installing Glitch? (Sims 4)

Bit of a late reply, sorry for that. I just found your question when searching aswers for it myself.

I had this same problem, although sometimes when I added a second floor it actually gave me the base-floor and sometimes not. When trying to make a sort of inside-the-house balcony (don't know what they're called, blast this language!) and I figured out you can add as much floor and in whatever shape you like with the sort of porch/foundation tool you can find in the same place as the wall tools. If you drag that to cover the whole room you want floored it actually doesn't make a porch but instead a normal floor base. Hope this helps!