@MadameNordica Hi! Unfortunately, the most you'd be able to do is add a platform trim, many of which resemble the foundations. I suspect they designed the trims this way so that exterior platforms would match foundations. They did not design platforms to have a foundation.
I think using the wall/room tools instead to build your foundation, will probably be better for you in the long term, especially if you ever want to make changes to the layout of your house. The platforms cause a lot of issues in general. One user posted their doors and stairs kept deleting when they tried to add rooms on top of their platforms. I was able to replicate that in my game as well. Anyway, I think having a standard foundation will make it a bit more stable.
Edit: I did some more testing and noticed clicking the platform to select it gave me the big center raise/lower arrows. Raising the platform this way added a foundation for the whole platform.
